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April 15

Why is DC voltage used in public transportation

Help my boyfriend sleep better at night. He's currently tormented by the idea that today's worldwide subway, tramway, and electric train system use brutally inefficient DC Voltage rails to power motors. Since voltage must be supplied over a long distance (a whole rail, or at least have a long cable going up to the rail in question) there would be a lot of energy lost even when there is no train going on the rail (leaking capacitor).

So why does subway, maglev, tramways, and train use DC third rail exclusively? Esurnir (talk) 02:40, 15 April 2011 (UTC)

Electric motors operating on direct current (DC) are readily controlled to vary the speed. This allows the vehicle driver to choose the speed at which the vehicle is traveling. Electric motors operating on alternating current (AC) rotate at synchronous speed, or some fraction of synchronous speed, such as 3600 revolutions per minute (or 3000 or 1800 or 1500 rpm etc.) It would be feasible to have the distribution system supplying alternating current, and for each vehicle to have an AC motor coupled to a DC generator, and then have one or more DC motors driving the traction wheels, but that would double the cost, double the weight, and double the electrical losses.

Variable-speed AC motors are now available, but in the past they were not, and they are more temperamental than DC motors. Very-long-range power distribution is often done with direct current because of lower losses. Dolphin (t) 02:51, 15 April 2011 (UTC)

(ec) DC power lines are "leaky capacitors"; AC power lines are "giant antennas"; both suffer losses due to physical limitations. But, it's rarely a good idea to try to apply "first principles" physics to complicated, sophisticated engineering projects; estimating the losses requires detailed analysis of specific technologies and parameters for any particular project. While AC power has certain theoretical advantages, it also has certain practical disadvantages. In today's technology, high-voltage DC is probably more efficient than AC for long-distance power transmission, though in any particular instance, specific engineering details may sway the balance one way or the other. DC systems are usually supplied by AC from a power plant, so there is a conversion loss to worry about; but you never have to worry about phase matching, nor radiative losses. We have numerous articles on electric train topics; the most helpful will be Railway electrification system, which discusses AC and DC systems. Nimur (talk) 02:53, 15 April 2011 (UTC)

I don't get it - if HVDC is so efficient, why has everyone been drilled into thinking that long-range transmission requires AC? Wnt (talk) 05:09, 15 April 2011 (UTC)

High-voltage AC is easy to convert to low-voltage AC. It's harder with DC. --Trovatore (talk) 05:18, 15 April 2011 (UTC)

Yes, in a word transformers. These are only available for AC (except in network theory books). High voltage (sometimes as high as 400 kV) gives less loss over long distances but is unsuitably high for most power stations to generate directly (around 25kV is normal for anything with a turbine) and is way too high to be safe in a factory or a home. SpinningSpark 11:30, 15 April 2011 (UTC)

The AC vs. DC dispute is old, see War of Currents. Cuddlyable3 (talk) 12:18, 15 April 2011 (UTC)

The article link that Nimur provided in his comment provides a good discussion, but briefly, HVDC transmission makes economic sense only for long runs with few 'taps' off them. First, there is a small loss incurred each time current is converted from AC to DC or back again, so the line has to be long enough that the increased efficiency on the line more than makes up for the bigger losses in AC/DC conversions at the ends. Our article puts the break-even distance as about 50 km for undersea cables and 600-800 km for overhead cables, but it doesn't have a supporting reference. Second, each tap off the HVDC line needs to have an installed high-voltage inverter (to convert DC to AC) before the transformer, this increases the cost of each tap. TenOfAllTrades(talk) 16:33, 15 April 2011 (UTC)

Diagnosing an electrical problem

In my house all the lights on the even circuits are blinking, computers are rebooting on their own, etc., while the odd circuit numbers are fine. There are two "phases" coming into the house, one which feeds the even circuits, and one which feeds the odd circuits. So:

A) This indicates to me that the source of the problem is external to the house. Is this correct ? In this case, how can I convince the power company it's their problem ? (The power company is Detroit Edison.)

B) Is there any danger to continuing to use the even circuits until it's fixed ? (I installed an uninterruptable power supply for the computer, to stop it from rebooting when the voltage drops.) The other approach is to use extension cords from the odd circuits, but I doubt if the whole house can be run on half power like that, especially if this isn't fixed by the time we hit A/C weather (we use window air conditioner units). StuRat (talk) 03:59, 15 April 2011 (UTC)

No, the problem can be at the transformer, or it can be in the house. If you have a high-resistance arcing connection in the panel, disconnect or meter socket, you could have a fire. This should be checked out immediately. If you run any 240V equipment, it won't work efficiently, and if it's a 240V motor, like a heat pump or high-output air conditioner, you'll burn out the unit. Intermittent loss of one leg (it's technically not a phase) is often a sign of a failing circuit breaker - in this case, the main breaker. Have it checked as soon as possible by an electrician. Acroterion (talk) 04:17, 15 April 2011 (UTC)

There's no sound or smell of arcing, so it doesn't seem likely that's happening within the house. Nothing is running 240 in the house (we have gas heat, water heater, and dryer, and small window A/C units). StuRat (talk) 05:59, 15 April 2011 (UTC)

If the problem is at a transformer, neighboring homes will also be affected (unless yours is the only one on that transformer.) We had a situation years ago where some circuits were fine; others had no power. They had to replace the transformer to fix that.

I get the impression this situation has gone on for some time. If the power company won't check the situation, perhaps the only way to get their attention is to have an electrician tell them the fault is in their equipment. I'm quite sure our power company would be out quickly to check the transformer if I reported such a problem.

The above is just speculation. As Acroterion says, Have it checked as soon as possible by an electrician. Wanderer57 (talk) 05:28, 15 April 2011 (UTC)

You can have high-resistance connections and arcing without sounds or smells. My money would be on either a transformer fault or a breaker fault (I had a similar issue with a 50A 240V breaker for a range - the oven wouldn't heat all the way and half the burners didn't work - replacing the breaker fixed it), but the possibility of a faulty connection is sufficiently dangerous to warrant immediate investigation and action. An electrician can provide ammunition if it's the power company's fault; I got my power company to add a transformer for my house and my neighbors after convincing them that six houses on a transformer was too many - we had serious voltage drops every time somebody turned on a load. Get it checked out immediately: it's potentially dangerous. Acroterion (talk) 13:06, 15 April 2011 (UTC)

Thanks so far. How would an electrician determine if it's the main breaker or the transformer ? StuRat (talk) 18:06, 15 April 2011 (UTC)

Measure the voltage before the breaker with a graphing multimeter. You could probably do it yourself if you like to take risks, the wires are accessible in the box. You could probably attach a light before the breaker, and one after and watch them. It's risky though - you have no protection against shorts while working on it. Also, ask your neighbors, if it's only you it's your breaker, or meter, or wires, or junctions. Ariel. (talk) 19:33, 15 April 2011 (UTC)

What Ariel said; the main feed lugs are normally shielded, but can be accessed by a very careful person, ideally one who is used to doing this. Since it's intermittent, some time will be needed, which are further grounds for leaving to an electrician, who will have the right clamps. If there's a load on the leg and the problem is internal, there may be visible damage or heat somewhere in the panel - the main breaker may be hot.. Bear in mind that conductive tools inside a live panel can be dangerous for the uninitiated; an arc flash (vaporized copper) in your face can blind you. Acroterion (talk) 19:41, 15 April 2011 (UTC)

Damn Detroit Edison! Perhaps they should never have been granted a franchise. Better to have a DC generating plant every mile or so. Edison (talk) 03:51, 16 April 2011 (UTC)

UPDATE: We inspected the wires outside the house. Where the service drop connects to the house via 3 insulators, the top connector is melted, and the tape around it has burnt off. The neutral and other leg look fine. I will call the power company tomorrow and hopefully they will agree that this is the problem and fix it. StuRat (talk) 21:03, 17 April 2011 (UTC)

Since it's on the power company's side of the meter socket, they'll have to take care of it. It might be intermittently grounding itself to the house, which would be bad, or the conductor might have been so damaged in the event (lightning strike?) that it has poor conductivity. Glad you've figured it out. Acroterion (talk) 01:56, 18 April 2011 (UTC)

It looks like they just twisted two wires together in the hopes that they would have enough contact to make a good electrical connection, then wrapped them in insulating tape. This is common practice in a light switch, but I'm surprised they would think this was good enough for the mains connection. Since it's exposed to the elements, probably water got at it and froze and forced the wires apart until we got to the intermittent connection we now have. The resulting arcing likely burnt off the remainder of the tape and melted the connector. StuRat (talk) 02:15, 18 April 2011 (UTC)

That certainly explains the problem. The appropriate repair, short of a new drop (which you should encourage them to install if the insulation on the hot conductors is doubtful) would be a clamp connection with the wire ends laid parallel - they have those clamps on the truck by the bucketload. If you have three separate wires running to individual insulators, the drop's so old that they would probably want to replace it with cable and make the connections in the drip loops. Acroterion (talk) 14:37, 18 April 2011 (UTC)

Yes, we would like a new drop, mainly because the old one is so low we are in danger of striking the wires. However, it seems they would want us to get permits and pay for that. Our current drop is a single cable, that splits into 2 legs and the neutral, right before they attach to the house. StuRat (talk) 23:43, 19 April 2011 (UTC)

Relativistic mass

Hello. Mass in special relativity#Controversy explains that some researchers have rejected the concept of "relativistic mass", but apart from saying that it is "a troublesome and dubious concept", the article doesn't go into any depth regarding why they reject relativistic mass. Could someone please tell me what is fundamentally wrong with this concept? Thank you. Leptictidium (mt) 06:17, 15 April 2011 (UTC)

It just seems like a fudge factor to me. That is, when the numbers didn't add up, they just decided to say that the mass was changing. Imagine if your tax accountant could do that: "well, the balance sheet doesn't balance, so I will say that this is 'dynamic cash' and changes quantity as needed to make everything balance". StuRat (talk) 06:40, 15 April 2011 (UTC)

Isn't this just what the central banks do all over the world? 95.112.143.65 (talk) 09:02, 15 April 2011 (UTC)

It's not a fudge factor and it is a occasionally useful concept. For instance, if you put a block on a scale to measure its mass, then you heat it up to a higher temperature increasing its internal energy, its relativistic mass also increases and that should be measurable by the scale in principle (In practice the effect is too small to be measured). The problem is that the concept causes more confusion than it's worth and the modern convention is to reserve the word mass for the rest mass as BenRG points out below. Dauto (talk) 15:08, 15 April 2011 (UTC)

What's troublesome and dubious is thinking that you can plug relativistic mass into equations that have an m in them, such as F=ma, and get something that makes sense. Generally, you can't. To the extent that relativistic mass is just energy divided by c², it's a well-defined concept, but there's no point having two names (energy and relativistic mass) for the same thing. The modern convention is to call it energy, and reserve the word "mass" for rest mass (which is often written in units of energy as well). -- BenRG (talk) 09:23, 15 April 2011 (UTC)

I think it's quite the opposite - if you don't include relativistic energy in equations such as f=ma you will get incorrect results. This energy has inertia and momentum and causes gravity. Which is why I prefer to call it mass. But the hard part is that the mass of an object is relative, it's not fixed. This can make calculations all but impossible. For example, what is the mass of a magnet? If I turn on an electromagnet on the other side of the planet, the magnet in my hand is now heavier (relative to that electromagnet anyway). Ariel. (talk) 19:30, 15 April 2011 (UTC)

If you use the proper form $F={\dot {p}}$ with the three-momentum $p=\gamma mv$, you can easily dispense with "relativistic mass". Here's a quote from a book by Taylor and Wheeler: "Our viewpoint ... is that mass is an invariant, the same for all free-float observers... In relativity, invariants are diamonds. Do not throw away diamonds!" Modern physics understands relativity as a geometrical theory, a theory of the structure of space-time, not as a dynamical theory as suggested by the concept of relativistic mass. In relativity, energy and mass remain distinct physical quantities; "relativistic mass" obfuscates that distinction. Having said that, if the energy is in internal degrees of freedom of a composite body (not center-of-mass motion), then this energy does indeed show up in the effective mass of that body. That's where the mass deficit of, say, the helium nucleus comes from. --Wrongfilter (talk) 20:38, 15 April 2011 (UTC)

Ariel, the point BenRG was making is not that The formula F=ma is correct if you use the rest mass. The point is that even if you use the relativistic mass the formula is still wrong which seems to have gone right over your head. Dauto (talk) 03:15, 16 April 2011 (UTC)

Triplet v.s. Singlet Oxygen

According to the formula for calculating the bond order for diatomic molecular species, both triplet and singlet oxygen species have bond orders of 2. This does not make sense to me - triplet oxygen cannot have a double bond and concurrently be a diradical. I think the reason for this is that the formula ignores electron spin direction. Is this true, and does triplet oxygen have a bond order of one? Or, do I have it wrong? Plasmic Physics (talk) 08:17, 15 April 2011 (UTC)

Sure it can. You cannot draw a proper lewis structure for O₂ with a bond order of two and still have it be a diradical, but that's just a limitation of lewis structures. The molecular orbital diagram at the lower right corner of triplet oxygen shows how it works. The diradical occurs in the two degenerate π* antibonding orbitals. The bond order is calculated as: (bonding electrons - antibonding electrons)/2, which (8-4)/2 = 4/2 = 2. The reason that the Lewis Structure doesn't work out is that the geometry of the molecular orbitals does not work easily in a 2D representation, and lewis structures ignore the whole "antibonding" thing all together. --Jayron32 12:17, 15 April 2011 (UTC)

I was not using the Lewis model approach. Each oxygen has 6 valence electrons, if it has a bond order of two, that means that each atoms makes a net contribution of two electrons to the bonding orbital. This means that each oxygen has four remaining valence electrons. If triplet dioxygen is a diradical which I believe it is, then each atom has only one electron pair and two unpaired elecrons, not just one (according to a two bond order system).

I used the MO diagram for my argument, from it I used two factors - the total number of valence electrons, and the total number of unpaired electrons. Plasmic Physics (talk) 12:50, 15 April 2011 (UTC)

You can't consider the electrons of each atom separately when you're looking at their arrangement in the combined molecule. Electrons that are unpaired for the independent atoms are not required to remain unpaired in the final molecule. Do you understand how the orbitals are filled in the MO diagram at triplet oxygen, and how to calculate bond order based on filling of bonding and antibonding orbitals? TenOfAllTrades(talk) 13:51, 15 April 2011 (UTC)

@ Plasma Physics: To expand on what TOAT said above, when considering the organization of the electrons in the O₂ molecule, you cannot consider each atom as retaining any individual characteristic. The whole thing with Molecular orbital theory is that you treat the entire molecule as a single entity, and calculate the quantum states of the electrons based on that presumption. The system cannot be accurately modeled (for these purposes) as individual atoms which are merely sharing a few electrons (which is how both valence bond theory and hybridization theory model bonding). Molecular orbital theory models the O₂ molecule as a 16-electron system with two nuclear charges of 8+, and calculates the shapes and populations of the various orbitals that way. For convenience, the molecular orbital diagram referenced above only looks at the valence electrons, but the actual calculations are based on all of the electrons. Its the organization and shapes of these molecular orbitals that gives rise to the particular properties of triplet oxygen, that being that it has a second order bond (or "double bond") and is diradical. You can easily arrange all of the electrons in orbitals to get this result; in the case of triplet oxygen you have a total of 8 valence orbitals: five of them have 2 electrons in them, two have one electron each, and the last one is empty. That's 8 orbitals, a double bond, and a diradical. This is empircally confirmed by things like the bond strength and length of the O=O bond in O₂ (compare the bond lengths of O=O with the average peroxide bond length of O-O here, which gives O=O a bond length of 121 picometers, and O-O a bond length of 148 picometers. This is on par with the 20 picometer difference between C-C and C=C), and with the magnetic properties of ground state(triplet) O₂, which is experimentally confirmed to be paramagnetic, as would be expected of a diradical. To sum up; the experimental evidence indicates that oxygen is BOTH second bond order (double bond) and a diradical. Molecular orbital theory predicts both facts about oxygen (again, check the MOT diagram at triplet oxygen), so it is the model which is perhaps best in describe the organization of the oxygen electron cloud. There's actually even MORE evidence that confirms this model of triplet oxygen (vis-a-vis chemical reactivity and spectroscopy), which I'll not go into in the interest of not extending this discussion to the TLDR point. If you are using a model that leads you to a structure which contradicts the empirical, observed properties of oxygen, then simply put, the model is inadequate and needs to be discarded for this purpose. --Jayron32 14:33, 15 April 2011 (UTC)

Well, it's just that the molecular modelling program I'm using does not like the idea of a double bonded dioxygen diradical. It seems to have the least problems when the unpaired electrons are on the same atom. Yes, I have to specify where the unpaired electrons are. One more thing, why do I find lewis structures of single bonded triplet oxygen on google images? Plasmic Physics (talk) 20:47, 15 April 2011 (UTC)

The usual skeletal diagram notation doesn't work so well for complex MO situations. The line between two atoms represents a covalent bond, which means a shared pair of electrons. But the diradical isn't a shared pair making one bond--that's impossible if they have the same spin. Instead (again, exactly as the MO diagram illustrates) it's two shared lone electrons each in separate orbitals that are orthogonal to each other. The limitation is in your modelling program and meaning implied by the diagram style (or at least in the the modelling program's interpretation of the diagram style). Drawing it as a single-bond with a single electron on each atom at least gets the idea of "electrons are not paired, and therefore easily have same spin" correct, at the cost of error in the estimated bond-length. Drawing it as a double bond would imply (to those that don't know the details) that ³O₂ would undergo reactions characteristic of other pi bonds, which is not true. DMacks (talk) 21:03, 15 April 2011 (UTC)

OK, helpful. What do you think, should there be two bond order formulae, one for calculating the total net bond order, and one for calculating spin matched bond order? In the spin matched bond order, only electrons of the same spin cancel, this kind of formula can distinguish between a true bond, and a quasi-bond, which is really what triplet oxygen has. Triplet oxygen has an effective bond order of one and two halves. Plasmic Physics (talk) 21:34, 15 April 2011 (UTC)

"One and two halves" is a pretty good description! One way might be to have the "second" bond written as two dots (but in the bonding position) rather than as an actual connecting line (like the sigma bond is)? The minimum technical notation is to include a superscript "3" to the left of the whole diagram to indicate the spin (see my last sentence in my previous comment), just like you can write the net charge of a structure as a superscript to the right.^† I would like a more detailed way (analogous to "formal charge" on atoms vs "net charge" on a structure) to indicate where the unpaired spins are rather than just the fact that there are two of them, but I don't know a notation for that. But using the "individual dots" vs whole bonds comes close perhaps. A more technical way is to write the spin at each electron (up/down vector), not just its location (dot) or bonded/nonbonded state. DMacks (talk) 15:31, 16 April 2011 (UTC)

^†(later/expanded comment on this point). That's actually the standard way for quantum modelling programs: you declare the spin of the system not of specific electrons or parts of the model. The whole Lewis/skeletal-diagram idea of covalent bonds as specific pairs of electrons at specific atom-pair locations is of a bunch of crap (to use the scientific terminology) at that level anyway. So ³[O=O] vs ¹[O=O] would be fairly accurate for bond-lengths, bond-order, and spin, at the cost of the less important (for many readers) idea that one of those lines is "two half-bonds" rather than a normal covalent pair. But it also leads the reader to think the only difference is one of an overall or hidden detail, even though it really is a major difference in the whole nature of the bonding and physical and chemical properties. DMacks (talk) 18:27, 16 April 2011 (UTC)

While I'm on topic, while I studied the consequences of particle spin at Univeristy, I have no idead what spin actually is. All I know about it, is that it is some kind of description of a particle's kinetics. Does an electron cycle around an actual locus? If it does, what are the constraints? Does the locus describe a point, line, or a surface? Does the locus lie within the electron? Plasmic Physics (talk) 00:07, 16 April 2011 (UTC)

If a charged particle (such as an electron) were to spin, that would cause it to have a magnetic moment and other magnetic effects. These particles do have the properties that result from spinning, therefore they are described as having "spin". But it's an intrinsic property itself, not quite that the particle really "spins". To my mind, since a quantum particle doesn't have a definite position, I can't see how that would be consistent with it having a definite physical rotation as the actual underlying property. See spin (physics) for the gory details. DMacks (talk) 15:31, 16 April 2011 (UTC)

Period food

Are there any studies into what foods women particularly crave on their periods? I know sugary food is pretty common, and iron-rich food, as well as the more specific chocolate, but I'm interested in if anything has looked in more detail or more broadly. I don't have access to most of the paid-for literature at the moment, so public-domain or summaries would be appreciated. 86.164.75.102 (talk) 09:48, 15 April 2011 (UTC) Brain-fart, I meant freely available, not actually released into the public domain. I think. 86.164.75.102 (talk) 15:22, 15 April 2011 (UTC)

Binge eating occurs in a minority of menstruating women. This may be due to fluctuation in beta-endorphin levels. Source: Price WA, Giannini AJ (November 1983). "Binge eating during menstruation". J Clin Psychiatry 44 (11): 431. PMID 6580290. See also Premenstrual dysphoric disorder.

Thanks, that's an interesting start, although I cannot read the paper (even its summary). The Wikipedia article is an interesting read, as the more recent findings on variable response to hormones fits with some of the stuff I've been reading about different women's responses to different versions of the pill, suggesting years more profitable research to be done. So thanks. For this question, I suppose I'm looking more for research into food cravings experienced by women in the 'normal' range, rather than eating associated with various interesting disorders. 86.164.75.102 (talk) 15:22, 15 April 2011 (UTC)

Pinion?

what is racken pinion —Preceding unsigned comment added by 175.157.66.79 (talk) 14:09, 15 April 2011 (UTC)

Did you mean to start a new question? (I've reformatted this, because I think you did..) Are you perhaps thinking of Rack and pinion steering in an automobile? SemanticMantis (talk) 14:54, 15 April 2011 (UTC)

See also rack railway.--Shantavira|^{feed me} 16:57, 15 April 2011 (UTC)

Design pattern: both for software and art

Is there a design pattern which can be applied both to works of art and software? Quest09 (talk) 15:40, 15 April 2011 (UTC)

You may be interested to read the works of Lawrence Lessig et al. The so-called "free culture movement" inherited its philosophical inspiration from the free software movement. Nimur (talk) 17:18, 15 April 2011 (UTC)

I'm not sure that I necessarily understand your question correctly but you may also be interested in Carlos Amorales' modular "Liquid Archive" approach to making his art. Sean.hoyland - talk 17:46, 15 April 2011 (UTC)

I also am not sure what your question means. could you be thinking of fractal geometry? as in the Mandelbrot set.190.148.136.166 (talk) 19:32, 15 April 2011 (UTC)

Software is often created by iterating the cycle Run-Crash-Debug. Most artists and composers follow a similar cycle of successive improvement e.g. Compose-Listen-Make better. Cuddlyable3 (talk) 12:19, 16 April 2011 (UTC)

Cuddlyable3 has the precise answer: something that can be applied both by an artist and by a software developer. The rest is also interesting, in a more broad context.Quest09 (talk) 12:34, 16 April 2011 (UTC)

Live vs. recorded sound

Why do they sound so different? Quest09 (talk) 15:42, 15 April 2011 (UTC)

You mean Acoustic music vs. electronically recorded music played back through electronic amplifiers and loudspeakers? Pfly (talk) 15:50, 15 April 2011 (UTC)

Live sound reaches your ears as each sound source sends out waves which bounce around rooms and off surfaces and arrive at your ears. Recorded sound can only approximate all of these relationships, and generally (unless you are an audiophile with a shitload of cash and time) most people listen to recorded sound from a set of earphones/headphones or speakers which are at a fixed location, and so do not accurately model the actual source sounds. They do a passable job for most people who just want to listen to the latest Lady Gaga song, but there is a noticable difference in the sound of hearing her come from the speakers on the stereo in your living room, and hearing her sing in your living room. --Jayron32 16:01, 15 April 2011 (UTC)

Also, consider a piano, for example, vs. a recording of a piano played back through loudspeakers. There's a huge difference between what is making the sound. Loudspeakers are nothing like pianos. A piano's sound board alone is a resonating surface far larger and different in design and function than a loudspeaker. The amazing thing is that loudspeakers can even come close to sounding like so many different things, from pianos to people singing to cymbals and so on. Still, loudspeakers do a poor job of reproducing the experience of sitting close to the front of an orchestra, or listening to a powerful pipe organ playing full bore. Pfly (talk) 16:14, 15 April 2011 (UTC)

But if the question is about live, as in performed in "real time" vs. recorded, as in created in a studio, then the answer is something to do with live performance vs. the process of building something slowly and in pieces. Something like theater vs. film. Pfly (talk) 16:12, 15 April 2011 (UTC)

No, I asked in the sense that you pointed to at your first reply. But, what's the difference in the wave that hits my ears when coming from a recording or coming directly from an instrument? Which properties are different? Quest09 (talk) 16:33, 15 April 2011 (UTC)

The waveform is different; and to be strictly accurate, the only real reason is because the wave field is different. It is difficult (but not impossible) to completely re-synthesize the entire wavefield; instead, electronic recording only resynthesizes a sampled waveform. For most purposes, your human ears are a stationary set of two single points that sample the pressure level of a multidimensional acoustic wave field; a more complete description of the total wavefield must account for its extent in 3 spatial dimensions, plus the pressure, velocity, and other non-linear acoustic properties of sound waves in air. Most electronically recorded sound waveforms only seek to sample the pressure as a function of time at one (or maybe two or more locations). (They accomplish this by recording pressure fields, placing a microphone as a pressure transducer at one or more fixed locations). For most purposes, this totally replicates the "99th-percentile" of the audible experience; but it is a known fact that the perception of sound waveforms is actually more complicated. A good friend of mine worked on a project to synthesize a multidimensional surround-sound experience; you can read "artistic" and technical descriptions at the technical specifications page. Nimur (talk) 17:27, 15 April 2011 (UTC)

From the list of sound wave properties, frequency, wavelength, and wavenumber are basically the same in this context, as are amplitude, sound pressure, and sound intensity. "Speed of sound" doesn't seem relevant in this context. So that leaves three basic properties, frequency, amplitude, and direction (Nimur addressed wave shape issues above I see, which curiously isn't included in that properties list). A few quick comments:

• Frequency: typical consumer-grade loudspeakers have trouble with very low and very high frequencies. Not uncommonly they drop off below 40-60 Hz and fall off somewhere around 15,000 Hz or perhaps around 20,000 Hz, depending on quality. Subwoofers can handle very low frequencies, but typical consumer-grade, not-super-expensive subwoofers are not very good at producing strong stable pitches, tending more toward booms and rumbles. Compare pipe organs, which can produce extremely low pitches--sometimes so low you feel them more than hear them. No subwoofer can make a sound like a big 32' pipe on a pipe organ. I was lucky enough to get to watch this pipe organ being installed. The sound of the 32' pipes was intense. If I recall right, that beast can accurately and loudly produce pitches down to 8 Hz or so. There's further issues regarding the recording medium, which limit and distort the frequencies in various ways (see Sound recording and reproduction).
• Amplitude: With electronic amplification you can make acoustic instruments much much louder than they could ever be themselves. It is common, I think, to listen to recorded music at louder volumes than would be normal for acoustic instruments (at least the quieter ones like guitar). Plus, when music is put together in a studio it is typical to mix the amplitudes of different instruments in "unnatural" ways--a quiet singer overpowering a drum kit, for example. In olden times you needed a operatic voice to overpower loud instruments. Today on a recording a whisper can overpower a marching band, if desired.
• Direction: The sound from stereo loudspeakers comes from two relatively small, usually non-moving places. Surround sound systems up that to five or so. Compare a live modern orchestra, where the sound comes from well over 100 sources. Even a single acoustic instrument, say a piano, produces sound from multiple sources—the strings, the sound board, etc. Pfly (talk) 18:05, 15 April 2011 (UTC)

To clarify: if you are only modeling the sound pressure level, you are only modeling p wave; this is the straightforward "acoustic wave equation" that is suitable for describing sound in a gas (like air). But in reality, air is not ideal; it has some viscous properties; and you should use a suitable elastic wave equation, such that the velocity of any individual particle is not equal to the velocity of the wavefront. It is unlikely that most human ears can perceive this difference, but we absolutely can measure this phenomenon using sophisticated equipment. A typical condenser microphone only responds to pressure, not to velocity; but a scientific-grade acoustic transducer will be able to record pressure and at least one component of the 3-dimensional acoustic velocity vector. Sampling and recording this information would be critical to a total and exact re-synthesis of the wavefield. And as always, I will re-emphasize the caveat; the goal is to mathematically model the physical effect to some specified level of accuracy/detail. We can always make a more complicated model to account for the 99.999999...th percent effects. Most human ears only "hear" or perceive stereo sound (i.e., two pressure-waveforms) sampled at 40 kHz, so it's not necessary to record more data. Nimur (talk) 18:12, 15 April 2011 (UTC)

Nimur, your technical understanding is way beyond mine. This brings up a question I've wondered about. Human hearing range doesn't normally reach much beyond 20 kHz (if that), and one often hears how it is therefore unnecessary to record or reproduce frequencies above 20 kHz or so. But I wonder--is it possible, or even common, for frequencies above 20 kHz bouncing around a room after, say, a gong is struck, to interact with other waves bouncing around, with the room itself, etc, such that lower, audible frequencies are produced and heard? Something akin to a resultant tone (or better, combination tone), but at the high end? I've long wondered if this is possible and if so, how common it is. And if so, whether it would have an effect on music with lots of very high overtones/harmonics--making it sounding richer live, with all those ultrasonic waves existing and bouncing around, than recorded, with no ultrasonic frequencies in the first place. ? Pfly (talk) 18:55, 15 April 2011 (UTC)

Certainly I feel that way, but my preference is actually for the recording. The clash of real cymbals in the highest frequencies is just so painfully loud that it cuts up the rest of the song. Wnt (talk) 23:37, 15 April 2011 (UTC)

Regarding technical understanding... Anyone can understand mathematical descriptions of physical phenomena if they spend enough time analyzing them. I have spent, and continue to spend, a lot of time thinking quantitatively about daily mundane things. It also helps to have academic or formal training in physics or mathematics. Anyway - regarding wave mixing. It is a fundamental assumption of linear system theory that frequencies are preserved by systems. In other words, the simplest model forbids a high frequency from "reflecting" and producing a lower frequency. On the other hand, a nonlinear wave model (such as the elastic wave model) does permit nonlinear, frequency-altering interactions. For most purposes, the amplitude of such an effect is very small, so we safely ignore it for day to day ordinary sound waves. I can think of at least one case where we cannot ignore these effects! A broken loudspeaker results in a nasty rattle that is nothing like the intended waveform; what is happening is that energy is coupling nonlinearly into the torn paper cone of the speaker, which is then buzzing and sounding awful! Nonlinear acoustics are therefore not merely a theoretical concoction of bored physicists! Nimur (talk) 02:25, 16 April 2011 (UTC)

I question the premise that they sound different, if the sound is recorded and reproduced by high-fidelity media. If they are all that different, then perhaps you need to buy better microphones, recorders, amplifiers, or speakers. Edison (talk) 03:46, 16 April 2011 (UTC)

They certainly aren't exactly the same. The question is if they are enough different that you can tell. StuRat (talk) 03:49, 16 April 2011 (UTC)

And that's the entire premise behind sampled audio and lossy sound compression technology - when recorded properly, essentially no human can perceive the difference between the live and the recorded sound. Nimur (talk) 05:59, 16 April 2011 (UTC)

...except for those blessed with one or more Golden ears, such as everyone who writes in an Audiophile magazine. Years ago a loudspeaker amnufacturer (was it Quad Electroacoustics?) gave a stage demonstration showing a string quartet. Midway through a music piece the musicians got up and left the stage, leaving only the loudspeaker that had really been producing the music all the time. Cuddlyable3 (talk) 12:13, 16 April 2011 (UTC)

The "live vs recorded" demo would be more impressive were it not for the fact that Thomas Edison did the same thing with acoustically recorded "Diamond Disc" records and his phonograph 1915-1925. It was reported that the audience could not tell when the musician was performing and when the phonograph was playing[1], [2]. The repertoire was generally limited to solo voice or solo instrument, so there was no need to reproduce the high sound pressure level of a brass band or orchestra, and the soloists may have avoided loud sounds. For an overview of "live vs recorded" tests over the years, see [3]. Edison (talk) 19:50, 16 April 2011 (UTC)

My impression is that a lot of people who think/claim they have golden ears don't, they just don't ever actually test their alleged ability to hear differences properly (e.g. with an ABX) but often still make the claim they can hear these differences. I can't say if this applies to audiophile magazine writers in particular, but again my impression is a lot of the magazines don't publish or use ABX or other more scientific testing methods and are willing to promote things like Monster cables usually with questionable technical advantages (worse of course are those that promote cables and other things that are supposed to provide better output despite the fact both are capable of providing the exact same digital signal). I'm not of course denying there are people who can hear differences that many others can't and there are of course audiophiles who do approach their hobby/whatever from a more scientific viewpoint, e.g. those at Hydrogenaudio Nil Einne (talk) 21:40, 16 April 2011 (UTC)

A string quartet plays quite low frequency sound - only a few sorts of things go over 20,000 Hz. One thing about very high pitched sound is that it's almost line of sight. Back in the 1980s it was easy for people to tell because all the TVs and monitors and videocameras consistently made a lot of this sound when on (sometimes uncomfortably loud in the case of "hidden" anti shoplifting cameras) - I don't think they do this nowadays, though I'm not sure because I'm also losing high-pitched hearing with age. See ultrasound. Wnt (talk) 04:08, 17 April 2011 (UTC)

A cathode ray tube screen still makes the high-pitched hum, but these are becoming rarer. Flat-screen TVs make a lower-pitched whirring sound, as far as I can tell, but I may be picking up the sound from the digibox that switches on at the same time. 86.164.75.102 (talk) 13:23, 17 April 2011 (UTC)

Even before flat screens the monitors seemed to get progressively better about the noise. I think the scan rate, especially frames per second was related to it, but I don't think it was the only factor. Wnt (talk) 00:55, 19 April 2011 (UTC)

two questions about electromagnetic waves

1.I have no imagination of photons as particles. I mean we can think of electrons as particles moving (or present) in regions called orbitals. do photons have a certain measurable "area of influence" or something like that?

2.In the emission or absorbtion spectrum of different elements,there are lines that show different transitions in energy levels.well, according to the formalae, each kinds of transition must produce a single kind of electromagnetive wave(E(n2)-E(n1)=hf). but when we look at the spectrums there are neighbours of wavelengths, not "exactly one" wavelength.I mean although the neighbourhood may be small, but it is still a neighborhood, otherwise we couldn't see it.

It's REALY hard for me to ask such questions in English, so I'm sorry and I hope you understand what I mean.Thanks! —Preceding unsigned comment added by 178.63.158.171 (talk) 16:55, 15 April 2011 (UTC)

For question #1, a single photon's "area of influence" is probably best thought of by defining a particular fall-off in the electric field intensity (or, for one single photon, the probability of measuring an electric-field at a particular intensity). There's going to be a "soft edge" for the photon; for any radius r from the photon's "center location," there's going to be a lower probability that the photon can interact with anything. This is described mathematically as a wave packet, which has finite extent in space and time. (We also have wellenpaket in German).

For question #2, see hyperfine structure (also available in German at Hyperfeinstruktur). Nimur (talk) 17:57, 15 April 2011 (UTC)

(EC) The answers to both of your questions involve the Heisenberg uncertainty principle.

Similar to how you can't say with certainty exactly where within an orbital an electron is, there is also a "fuzziness" as to where exactly a photon is. For example, if you shine a monochromatic light on a circular aperture, the light intensity behind the aperture will form a diffraction pattern known as an Airy disk, rather than a solid circle as one would expect if each photon traveled along an infinitely thin line.

Spectral lines not having zero width is due in part to excited states having a finite lifetime, which causes an uncertainty in the emitted photon's energy, as explained in Uncertainty principle#Energy-time uncertainty principle. Red Act (talk) 18:18, 15 April 2011 (UTC)

The German articles on the topics I mentioned are Heisenbergsche Unschärferelation, Beugung (Physik), Beugungsscheibchen, Spektrallinie and Linienbreite. Red Act (talk) 18:34, 15 April 2011 (UTC)

An other reason for widening of spectral lines is the Doppler effect due to the relative motion of the atoms that emit the photons due to thermal motion. Dauto (talk) 03:24, 16 April 2011 (UTC)

Discovery of rimantadine and references

In the past few days a history section was added to the rimantadine article. An anon-IP editor has added several references to verify the claims, but I am having difficulties verifying the claims, mainly of two reasons: 1) Chemistry is not something I am familiar with. 2) English is not my native language. Can someone with knowledge in chemistry please have a look at the references, and explain whether the references really verifies the claims or not, and perhaps explain why – since I, as a layman am not able to understand the references given at first sight. Thanks in advance. ^Talk/♥фĩłдωəß♥\^Work 18:02, 15 April 2011 (UTC)

Head rotation

Is it theoretically possible by surgical means to make the human neck safely rotate the head at 360 degrees or so (bearing in mind that it's not fatal in some birds)?--89.76.224.253 (talk) 18:16, 15 April 2011 (UTC)

No, this is not possible. The structure of our spine and neck is different from birds. Nimur (talk) 18:20, 15 April 2011 (UTC)

I'm pretty sure birds don't rotate 360 - they probably do 180. Ariel. (talk) 19:23, 15 April 2011 (UTC)

OP, what birds are you referring to? The usual example of a bird rotating their head to an extreme is the owl. And according to our article on the owl, "Owls can rotate their heads and necks as much as 270 degrees in either direction". Or do you mean 360 from one extreme to the other? Dismas|^(talk) 19:41, 15 April 2011 (UTC)

Screen-width Problem in computer

The ratio of width-to-height in new laptops is considerably different than traditional desktops. Does that mean that everything will appear deformed i.e. stretched along horizontal axis ? —Preceding unsigned comment added by 124.253.130.232 (talk) 21:52, 15 April 2011 (UTC)

Please don't ask the same question on multiple ref desks, especially when it has already been answered on one of them (Computing). Looie496 (talk) 22:16, 15 April 2011 (UTC)

Seafloor river bed in Toyama Bay, Honshu from Google Earth

Hi. On Google Earth, a long river valley appears carved onto the seafloor leading out from Toyama Bay in Japan, extending toward the bottom of the Sea of Japan. It is most likely not volcanic or tectonic in origin, and also does not extend from the mouth of the Shinano River, which lies closer to Niigata farther northeast. It appears to wind itself around the eastern edge of some continental shelf and steadily drops off in elevation on the seafloor, tracing a path from a reverse-delta toward the north. In fact, the main undersea channel appears to be a merger of the Shō River and the Jinzū River. The channel appears to have a lower sea bottom elevation on Google Earth than its surroundings and stretches for about 590 km before ending at the edge of a deep basin, at which point it is closer to Sapporo than to Tokyo. At the point equidistant between the two cities, the elevation at the bottom of the channel is about 2800 metres below sea level, It is noticeably longer than any other apparent seafloor river channel in the area and may be one of the longest in the world. Also, the channel appears to bypass the edges of a few underwater volcanoes, suggesting that sea levels were lower or the river has enough power to carve this deep channel even when underwater. At various points, the channel itself lies an average of 145 metres lower than the surrounding ocean floor. Any information on what was responsible for creating this channel, whether it was an ice age event or a phenomenon similar to the conditions today, and how this particular locale compares to any similar channels worldwide? Thanks. ~AH1 ^(discuss!) 22:59, 15 April 2011 (UTC)

Apparently it's called the Toyama Deep-Sea Channel.^[4] Wnt (talk) 00:02, 16 April 2011 (UTC)

I don't know about that one specifically, but our article on submarine canyons give information (or at least speculation) about how such things form. Looie496 (talk) 00:05, 16 April 2011 (UTC)

"Submarine canyons are well developed around the Japanese Islands. Three major large-scale submarine canyons are the Kushiro Canyon, the Toyama Deep-sea Channel, and the Boso Canyon. The Kushiro Canyon greatly encroached on the continental shelf and deeply eroded the continental slope. This characteristic is markedly different from other canyons. The Toyama Deep-Sea Channel is characterized by the length of over 500 km, considerably meandering, a vast submarine fan, and well-developed submarine natural levees. The Boso Canyon has significantly incised meander 100 km in length."^[5] Looks like 3.6 million articles just aren't nearly enough. ;) Wnt (talk) 00:10, 16 April 2011 (UTC)

April 16

Question about gravity and absolute zero temperature?

As both [Einstein’s and Newtonian] models of gravity are independent of temperature therefore “Does gravity cease and clocks stop or slow down at Absolute zero temperature?” Also this might help: http://www.jupiterscientific.org/sciinfo… 68.147.41.231 (talk) 02:47, 16 April 2011 (UTC)Eclectic Eccentric Khattak#1

Seeing that we've gotten much closer to absolute zero than we have to the speed of light, with no strange things happening (EDIT: with gravity and time; some other strange things happen, for example see superconductor), I think it's safe to say "no". 72.128.95.0 (talk) 02:52, 16 April 2011 (UTC)

Your link is broken, but I'm guessing you might be indirectly referring to a Big Freeze, which is one form of a heat death of the universe scenario. In any form of a heat death of the universe scenario, there will no longer exist any kind of astronomical objects, or even any macroscopic objects, that would produce anything significantly different from a constant curvature of spacetime, so you could kind of in a limited sense say that gravity at that point has ceased. And in any heat death of the universe scenario, entropy comes arbitrarily close to its asymptotic limit (in particular, a temperature of absolute zero is approached in a Big Freeze scenario), so at that point there will be essentially no thermodynamic arrow of time, so you could kind of in a limited sense say that at that point "time has ceased". But the same laws of physics will still apply, it's just that there won't happen to be any astronomical objects around for gravity to be meaningful anymore except for describing the universe as a whole, and there will no longer be any physical processes going on such that "forward in time" looks noticeably different from "backwards in time" locally, so the second law of thermodynamics will lose its significance.

At any rate, it's not like a temperature of absolute zero causes gravity to become insignificant. If you managed to create a macroscopic object with a temperature of absolute zero, it would still produce a gravitational attraction, to use the Newtonian description. Basically, any nonzero component of the stress-energy tensor causes a curvature in spacetime, to use the general relativity description, and thermal energy is in general only a minor contributor to the stress-energy tensor, so zero thermal energy does not at all imply a zero curvature of spacetime.

And a constant temperature of absolute zero doesn't really cause time to cease, either, even if there is no longer a thermodynamic arrow of time, and even though a clock can't actually function at a temperature of absolute zero. At the very least, even in a Big Freeze scenario, there will still exist a cosmological arrow of time. Red Act (talk) 06:28, 16 April 2011 (UTC)

Here is the link "http://news.harvard.edu/gazette/1999/02.18/light.html" Sorry about the broken one. —Preceding unsigned comment added by 68.147.41.231 (talk) 04:19, 17 April 2011 (UTC)

Einsteinian gravity is not independent of temperature. Temperature is energy and therefore has mass. 83.134.138.215 (talk) 06:05, 17 April 2011 (UTC)

Oh, your question actually has nothing to do with the Big Freeze scenario; I guessed wrong. Your question is really about slow light travelling through a Bose-Einstein condensate. Slow light just involves a quantum mechanical interaction between the photons and the atoms in the medium they're travelling through, and doesn't involve time itself slowing down, or involve gravity in any unusual way. Spacetime within and around the medium through which the slow light travels is still just a normal, unremarkable inertial frame of reference as in special relativity. For example, the decay rate of any radioactive nuclei within the medium will be the same as it always is. It's just that the group velocity of light through the medium in the frequency range of interest is less than c. Red Act (talk) 07:14, 17 April 2011 (UTC)

I was just thinking about

1. Einstein’s theoretical light clock but at microscopic level [either stationary or travelling] through a state of matter of Bose-Einstein condensate [c reduced to 38 miles/hr]

2. Thermal energy or heat can’t be transferred further [can become no colder] at absolute zero temperature and thus circumlocutory it cease if not at instantaneous motion. Thus as per above comment E=mc^2; so does mass remains constant with changing in thermal energy to a minimal motion of matter through a Bose-Einstein condensate in which c to reduce to 38 miles/hr OR at absolute zero temperature? EEK#1 —Preceding unsigned comment added by 68.147.41.231 (talk) 03:49, 18 April 2011 (UTC)

c isn't reduced to 38 miles/hr. There is a fundamental physical relationship between space and time, such that 299,792,458 meters of space is equivalent, except for a change of sign in a component of the metric tensor, to 1 second of time. c is that ratio between equivalent amounts of space and time, and is always exactly 299,792,458 m/s. One of the most important and easy-to-understand ways that c shows up in nature is that it's the speed at which light travels in a vacuum, so c is commonly referred to as "the speed of light". But referring to c as just "the speed of light" comes with the implicit understanding that that name is only actually accurate macroscopically when discussing light that's traveling through a vacuum.

Although slow light is a dramatic example, you don't need an exotic phase of matter, or an extremely cold temperature, to illustrate that light travels at a speed lower than c when it's not in a vacuum. Light travels at a speed significantly lower than c when traveling through ordinary materials at ordinary temperatures. For example, at room temperature, visible light travels through water at about 0.75 c, through glass at about 0.66 c, and through air at about 0.9997 c. The inverse of those numbers is called the refractive index of the material the light is going through.

Light traveling slower than c in a medium does not affect the value of c within that medium, and hence does not affect the ways other than the macroscopic speed of light that c shows up in physics, such as in the equation E=mc².

You could do calculations involving light clocks in which there's something other than a vacuum between the mirrors, although the equations would get a little messier than in the vacuum case. It's only within a vacuum that how fast light travels is the same for all inertial frames of reference. If the light is traveling through some medium other than a vacuum, you'd have to take into account not only how the speed at which light travels varies with what the medium is, but how it also varies with how fast the medium is moving. But it'd be conceptually straight-forward; since the light is traveling at a speed less than c, you can treat the light basically the same as a normal massive object that's traveling at that lower-than-c speed. You'd just need to mess around with things like the velocity addition formula that don't come up when analyzing the simpler case of there being a vacuum between the mirrors. And in the end, you'd wind up concluding that how space and time in one inertial frame of reference relate to space and time in another inertial frame of reference doesn't depend on what medium light happens to be traveling though in that vicinity. Red Act (talk) 21:28, 18 April 2011 (UTC)

Help! I have some questions about Venus, Uranus and the moon

1. "Does the planet Uranus get hit by earthquakes? I read this off of a website: "In the course of a study of tidal effects on earthquakes2, the astronomical positions of the planets have also been taken into account and a remarkable correlation between the positions of Uranus and the moment of great earthquakes has been established for a certain period. Gutenberg and Richter's data of all earthquakes equal or greater than magnitude 7; have been used." Could someone explain this to me?

2. A long-standing mystery of Venus observations is the so-called 'ashen light', an apparent weak illumination of the dark side of the planet, seen when the planet is in the crescent phase. The first claimed observation of ashen light was made as long ago as 1643, but the existence of the illumination has never been reliably confirmed. Observers have speculated that it may result from electrical activity in the Venusian atmosphere, but it may be illusory, resulting from the physiological effect of observing a very bright crescent-shaped object." In astronomy, Venus has been called the twin sister of Earth. Both planets are about the same size. If another planet could support life, it was once thought to be Venus (of course, now it's realized that the heat and sulfur make that impossible). What association do the Moon and Venus have in common in astronomy?

3. Does the planet Venus get hit by meteor showers? I read this information of a website: Venus is a Crescent V when it is most visible twice a day and for a long period. Also during this time it becomes much easier to monitor the ever going meteor storms ranging around and into Venus. Venus is known well for being the planet most hit by Meteor Showers. Go outside before sunrise, around 5:30 a.m. is best, and look East. The brightest object in that direction is the planet Venus. It looks like a star going supernova. Above Venus lies Saturn, and below, near the horizon, is Jupiter. Every 10 minutes or so you'll see a meteor streak among these planets. The meteors are pieces of Halley's Comet.

"Every year around this time Earth glides through a cloud of dusty debris from Halley's Comet," explains Bill Cooke of the NASA Marshall Space Flight Center. "Bits of dust, most no larger than grains of sand, disintegrate in Earth's atmosphere and become shooting stars."

"It's not an intense shower," he says, "but it is a pretty one."

Astronomers call it the "Orionid meteor shower," because the meteors appear to stream out of a point (called "the radiant") in the constellation Orion. The radiant is near Orion's left shoulder. But don't stare at that spot, advises Cooke. Meteors near the radiant seem short and stubby, a result of foreshortening. Instead, look toward any dark region of the sky about 90 degrees away. The vicinity of Venus or Jupiter is good. You'll see just as many Orionids there, but they will seem longer and more dramatic." Please give me more information. Neptunekh2 (talk)

While we wait for experts to answer your first two questions (and give a better answer to the last), I can answer the bit about meteor showers on Venus. Yes, all the planets get hit by meteor showers, but we can't see these from Earth, or at least not with the naked eye. The shooting stars mentioned are meteors in Earth's atmosphere that just happen to be highly visible near to the horizon where Venus can often be seen most clearly. There is no connection between the planet Venus and the Orionids except this visibility factor. I would like to see some evidence for the correlation that you mention between the position of Uranus and the time of earthquakes. I can think of no logical reason for any causality here. Dbfirs 07:02, 16 April 2011 (UTC)

1) If they are talking about the position of Uranus causing quakes on Earth, that's absurd. At that distance there's no way it could have an effect. The Moon could contribute to quakes, due to it's proximity, and possibly the Sun, due to it's size. StuRat (talk) 08:48, 16 April 2011 (UTC)

2. The OP mentioned but didn't ask about the Ashen light of Venus. An early astronomer believed that it was from the fires from celebration of a new Venusian emperor. This purports to be a picture of it] but I can't make out the emperor nor even his clothes. Cuddlyable3 (talk) 11:57, 16 April 2011 (UTC)

To me the "ashen light" looks pretty clear there - I can quite easily see the dark side of Venus, if that's what it is, against the background. If it's due to electrical activity it must be something pretty homogeneous throughout the dark side. But Earth has all sorts of weird upper atmosphere activity that people are discovering around all the time - and Venus has more sun and more atmosphere. Wnt (talk) 01:44, 17 April 2011 (UTC)

The OP will be aware that Venus has no moon for moonshine on the surface, though one was announced in 1686 and the object 2002 VE68 is in a 1:1 orbital resonance with Venus, so is considered a quasi-moon, but only for the next 500 years or so. Dbfirs 12:04, 16 April 2011 (UTC)

As for question 3, all planets are hit by meteorites, but the OP seems to be confused as to how meteors are observed. They are strictly atmospheric phenonomea and the ones the OP has seen are certainly in the Earth's atmosphere. The apparent streaking among the planets is merely the coincidence of alignment. Astronaut (talk) 12:44, 16 April 2011 (UTC)

This is the letter you're quoting: R. Tomaschek. "Great Earthquakes and the Astronomical Positions of Uranus." Nature 184:177-178 (18 July 1959). (To see more than the abstract, one needs an appropriate subscription.) The author found a fairly strong correlation between the location (the right ascension, actually) of Uranus in the sky and the occurrence of earthquakes of magnitude 7.75 and greater, during the years 1904-1906. It is not explained why he chose that particular cutoff for earthquake strength. The author fails to discuss in any detail the rest of the earthquake record; the source he was working from contained data up to at least 1950, but all we hear is that the correlation after 1906 "becomes less significant". Hm.

Among other issues, his significance testing fails to account for the effect of multiple comparisons. If one takes a three-year window and slides it across five or six decades' worth of the earthquake record, one will find clusters of earthquakes that correlate with other totally irrelevant measures purely by coincidence. Besides cherry-picking the six or seven percent of the earthquake dataset which best fits his thesis, we also don't have any information about what other phenomena were examined. (Did he look at the right ascension of a bunch of other astronomical objects and find no correlations? Weather conditions: rainfall, snow, temperature? The stock market?) The larger the set of data he was trying to match the earthquakes against, the greater the likelihood that such a match will occur somewhere by coincidence. If we don't know how many such comparisons were made, then we can't appropriately account for this source of false significance. There is, as always, an appropriate xkcd: [6]. TenOfAllTrades(talk) 14:35, 16 April 2011 (UTC)

This response to some criticism suggests that it is the hour angle of Uranus, not the right ascension that correlated with earthquakes (with an orbital period of 84 years for Uranus, RA doesn't change very much over the course of two or three years). Which doesn't change the fact that this was bull from the start... --Wrongfilter (talk) 16:00, 16 April 2011 (UTC)

I'd say that the immediately preceding letter (a response by a legitimate mathematician to the original paper, and which Tomaschek poorly and incompletely attempts to rebut in the abstract you've linked) is probably the most illuminating comment: Burr, E.J. "Earthquakes and Uranus: Misuse of a Statistical Test of Significance" Nature 186:336-337 (23 April 1960). He makes essentially the same points that I do, but does so much more eloquently. (This isn't the first time I've been scooped at the Ref Desk, but it may be the first time I've been fifty years late.)

I will note that Tomaschek's rebuttal is silent on the most important question—that is, how many different sets of astronomical or other data did he examine in searching for correlations with earthquakes?

Tomaschek's rebuttal also asserts – in defence of his silly claim – that the apparent effect of Uranus is significant (though smaller) when the entire fifty-year earthquake record is considered. This is unsurprising, since we already know that a portion of that record contains a small region with a (likely spuriously) high correlation. Tomaschek errs in including the 1904-1906 data when he asserts that his correlation exists across all of the data; you can't reuse data when you're trying to confirm a correlation. If one only looks at the remaining data covering the years 1907-1952(?), then one finds 111 earthquakes, 24 of which occurred within one hour of Uranus being in the desired location. The expected value is 1/6 of 111, or 18.5, so there were slightly more quakes than the mean we would expect by chance. If we assume that earthquakes are instead randomly distributed in time (and follow a Poisson distribution), that expectation value carries with it a standard deviation of 4.3. Our measurement is a trifling 1.28 standard deviations from the mean, something that we would expect to occur by chance about 20% of the time.

A more subtle point is that the earthquake events listed aren't actually all independent. In Tomascheck's Table I, there are several quakes that appear to be clustered in time and space and which I would expect are quakes followed by aftershocks. Between June 25 and June 27, 1904, there are three earthquakes at (nearly) the same location in Russia; the initial temblor and first aftershock falling outside Tomaschek's Uranus rule, and only the second aftershock fitting within. Similarly, there are a pair of earthquakes in or near Mongolia in July of 1905; the first quake fails the Uranus test but the aftershock squeaks in. Should initial quakes and aftershocks be treated the same way? It's just sloppy analysis and wishful thinking. TenOfAllTrades(talk) 19:05, 16 April 2011 (UTC)

Old Balls

This came up in a question on the Miscellaneous Desk. Someone found what is probably an American Civil War cannon ball which has a diameter of 3 inches and is covered in rust. Using a table found on Field artillery in the American Civil War, it ought to weigh 9lb 8oz. My thought was that oxidation would make it a bit lighter now; was I right or just talking balls? Alansplodge (talk) 08:18, 16 April 2011 (UTC)

The rust ought to make it heavier, not lighter. Rust is iron + oxygen + water, so it's heavier than just iron. None of the original iron atoms have escaped unless the rust has flaked off. On the other hand, if by 'lighter' you meant less dense, then you would be right - rust is probably less dense than iron. --Heron (talk) 10:23, 16 April 2011 (UTC)

A common high-school or freshman-college chemistry experiment is to take a piece of metal of known mass and oxidize it. From the mass increase, students then determine how much oxygen was added, and from that, various details about the metal and its oxididation properties. DMacks (talk) 14:58, 16 April 2011 (UTC)

The rust might make it heavier if it were all there, but for an iron ball that has been sitting around in the rain since the Civil War, a good bit of rust is likely to have flaked off and washed away. Looie496 (talk) 16:22, 16 April 2011 (UTC)

I suppose that was my chain of thought, but there wasn't much scientific enquiry behind it. Thank you for your answers all. Alansplodge (talk) 16:59, 16 April 2011 (UTC)

Translational acceleration

I am looking for a definition of translational acceleration, preferably one that is easy to understand. Lova Falk talk 09:09, 16 April 2011 (UTC)

It means acceleration (getting faster) in a straight line, as opposed to rotational acceleration, which means acceleration in a circle. See Translation (physics). --Heron (talk) 10:26, 16 April 2011 (UTC)

I think the exact meaning may depend on context. In Newton–Euler equations, "translational acceleration" is used to mean the instantaneous acceleration of the centre of mass of a body. In rotation around a fixed axis it is used to mean the tangential component of the acceleration of a general element of a body. Gandalf61 (talk) 10:42, 16 April 2011 (UTC)

Thank you! Something that confuses me is that the Thesaurus in the free dictionnary says that translational is: "of or relating to uniform movement without rotation". As far as I understand, uniform movement is the same as constant movement, and in that case there cannot be any acceleration. Lova Falk talk 11:11, 16 April 2011 (UTC)

A thesaurus is a reference work that lists words grouped together according to similarity of meaning (containing synonyms and sometimes antonyms), in contrast to a dictionary, which contains definitions and pronunciations. Never rely on a thesaurus for definitions! Cuddlyable3 (talk) 11:43, 16 April 2011 (UTC)

Always nice to learn more than I asked for. Thank you! But to return to my original question, is translational acceleration basically the same thing as linear acceleration? Lova Falk talk 14:02, 16 April 2011 (UTC)

Gandalf61's answer was better than mine, so the answer to your question is no. Linear acceleration is a special case of translational acceleration in which the object follows a straight line while its speed varies. If the path is not a straight line then the acceleration is still translational but not linear. --Heron (talk) 17:15, 16 April 2011 (UTC)

Strange archive search message

Whenever I search the reference desk archives I am told You may create the page "Your search terms prefix:Wikipedia:Reference desk/Archives", but consider checking the search results below to see whether it is already covered. Surely I may not do this; maybe someone could tweak the appropriate template.--Shantavira|^{feed me} 09:42, 16 April 2011 (UTC)

You used the Search box at top right of the page, which is for searching ARTICLES. When you want to want to search the ref. desk archives, use the Search archives button (in the header on this page). Cuddlyable3 (talk) 11:37, 16 April 2011 (UTC)

But I did use the search archives box. Try it. (I meant to put this question on the Help page but never mind.)--Shantavira|^{feed me} 12:43, 16 April 2011 (UTC)

I will echo this. This is the current response from the "search archives" box at the top of the page. I'm not familiar with the reference desk, but Cuddlyable3's answer suggests that what we're seeing is not what's intended. As Shantavira points out, we should not be offered the ability to create a page called Fred prefix:Wikipedia:Reference desk/Archives (which is what is presented in response to a search for 'Fred'). -- EdJogg (talk) 14:52, 16 April 2011 (UTC)

Yes, this seems to be a bug in the archive search engine. Fortunately it still gives sensible search results. I struck out my incorrect assumption above. Cuddlyable3 (talk) 22:47, 16 April 2011 (UTC)

An archive search box is just a convenient way to produce a search with a prefix operator you could have entered manually in the normal search box. bugzilla:21102 is: "don't propose to create the page when using prefix:, intitle: or +incategory:". PrimeHunter (talk) 01:15, 17 April 2011 (UTC)

Sensations and the vocabulary to describe them

In elementary school, children are taught that humans have five senses (sight, hearing, taste, smell, and touch.) These senses convert physical stimuli into signals that go through multiple layers of processing before they eventually reach consciousness. There are other signals (feelings? sensations?) processed by the brain that are not linked to simulation of the five traditional senses, at least not clearly so. Some examples:

• hunger
• nausea
• vertigo
• the feedback that you get about the tension in a muscle
• the feeling that you experience when you are in a free fall
• the feeling that you experience when you exercise at a level that your body cannot comfortable handle
• the feeling of disgust when you think of something unpleasant

These examples are not all of the same nature, especially the last one. However, at some level they are "signals" that rise to the level of consciousness.

Is there is systematic categorization of these "signals"? If so, where can I find a categorized list of terms for referring to the different signals/feelings/sensations? —Preceding unsigned comment added by 173.49.79.68 (talk) 12:36, 16 April 2011 (UTC)

Not very systematic, but our sense article lists quite a few more than the "traditional" senses, plus a few more in the see-also list. Part of the problem is that many of these senses do not have specific names.--Shantavira|^{feed me} 14:15, 16 April 2011 (UTC)

Proprioception! Lova Falk talk 14:17, 16 April 2011 (UTC)

Most of these can be caused by vestibular system, so it could be related to sense of balance, disgust however is an emotion - you perceive something, largely with the five traditional senses, and only then realize that it is disgusting ~~Xil (talk) 17:46, 17 April 2011 (UTC)

I think qualia is the word we're looking for to describe these sensations. 50.92.126.63 (talk) 22:25, 17 April 2011 (UTC)

I was just browsing list of common misconceptions and found five senses also listed as such - according to the list depending on how it is defined humans have 9 to 20 senses, including (in adition to 5 commonly known) balance, acceleration, pain, body and limb position, hunger, thirst, pressure, temperature, itching, full stomach, need to urinate or defacate and carbon dioxide level in blood ~~Xil (talk) 00:36, 18 April 2011 (UTC)

We just had a question where it came up that oxygen is also sensed. And does "taste" or "smell" cover the 'taste' sensation that seems to come from the artery at the back of the throat, the awful smell when a few microliters of DMSO gets on the skin or the bitter 'taste' when Neosporin is put on an open wound? What do you define it as when a particularly intense shade of red seems to flicker constantly, or a pattern of blue stripes creates spurious yellow diagonals, or any other sort of optical illusion one hasn't heard of? What name would I use to call the unpleasant sensation, neither itching nor nausea nor pain, of handling an erection before puberty, or using a finger to apply suction to the navel before about 25, or getting a hair or dust up into the urethra even long into adulthood? And what would I call the dizziness produced by the emotion produced by the sight of terribly fake things (like seeing a small motorized water fountain with a hyper-colored plastic shade sold as a "tranquility fountain" in a mall, to drown out the noise that too many people have resigned to), or the entrancing, encroaching feeling of the laughter of flowers, when vegetables stand by mutely in witness to horrors? The array of nerve receptors is likely even wider than we imagine, the transitions probably continuous, their targets spread throughout many different parts of the brain. The world is full of mysterious feelings and sensations that at least English has never bothered to put a name to, and I have no idea how much of them is shared by all, or some, or few, or none. Wnt (talk) 07:09, 18 April 2011 (UTC)

What do glucocorticoids do?

My 9-year old son has an Intelliquest Quiz Book "Gross and Yucky Human Body". The Intelliquest system uses an electronic wand to check the answers of a series of multiple choice questions. For the question What do glucocorticoids do? (no 156 in the book) the correct answer is (apparently):

D) Prevent you doing your homework

My son wanted to know why this was the correct answer (the others being: Prevent bleeding, Prevent infection, Prevent swelling.)

We did have a look at the glucocorticoid article, but not being degree level chemists we could not work out an answer for ourselves. Hence we come here to consult the collective wisdom of the reference desk! -- EdJogg (talk) 14:45, 16 April 2011 (UTC)

Judging by this line in the article, "glucocorticoid turns immune activity (inflammation) down", I'd say "prevent swelling" is a better answer. SemanticMantis (talk) 15:13, 16 April 2011 (UTC)

Our article on glucocorticoids is pretty sucky when it comes to conveying useful information. I think the information the question was trying to evoke is that glucocorticoids, especially cortisol, are activated by stress, especially chronic stress, and if elevated for a long time can have a variety of negative effects on brain function. Looie496 (talk) 16:18, 16 April 2011 (UTC)

I think glucocorticoids also have a vasoconstrictive effect that may counter bleeding, but their use on a bleeding animal is apparently debatable.^[7] But I think any book entitled "Gross and Yucky Human Body" aimed at kids should not be asking such a question unless it gives the answer somewhere inside! Wnt (talk) 20:18, 16 April 2011 (UTC)

To answer Wnt, it is a rather curious way of learning. The book contains a series of multiple choice questions, and the wand (which is pre-programmed with the book number, and hence the answers) keeps track of how many the child gets right. I think the idea is that through repeated attempts to get all the questions right, the facts are memorised. One reason for giving all the book details was on the off-chance someone knew if the answers were on-line!

I would guess that Looie496's answer is the most likely (assuming that the wand has been programmed correctly!) but it is a surprising question to find in a kid's book. Thank you all for your help. -- EdJogg (talk) 22:13, 16 April 2011 (UTC)

A dumb kid would get the answer faster, but even a genius should eventually figure out the right place to tell the author to stick that wand... =) Wnt (talk) 01:47, 17 April 2011 (UTC)

Infinite re-reflection

I remember a ,probably, childhood experience about mirrors. If you stand between two mirrors and position yourself so that you can see your rear reflection reflected in the mirror in front of you, and you look closer you will see that the same reflection back and forth, smaller each time is repeated apparently ad infinitum. Not something I ever worried about very much, but I did cogitate a bit about the reflections continuing down to the micro limits of light particles/waves, and wondered if this had anything to do with infinity. Recently I saw something on a N.Geo. documentary about 'fractal geometry'. This triggered the mirror memory, and I've tried several guesses at artical headings but can't find anything that mentions the mirror thing or anything about a connection with infiniy or fractal geometry. Am I seeing something that's not there? Layman. No advanced math please.Phalcor (talk) 18:51, 16 April 2011 (UTC)

Some thoughts:

1) The reflection isn't really infinite, since mirrors don't reflect 100% of the light that hits them, so get darker each time, eventually fading to black. I wonder what the maximum number of reflections ever recorded is ?

2) I see what you mean about the fractal nature of the reflections, each being the same as the original but slightly smaller (depending on the curvature of the mirror, of course).

3) Light has a dual wave/particle nature. It's an interesting thought problem to consider when the reflection stops, under both the wave and particle models. In the case of waves, there's no reason they can't continue to reduce in magnitude infinitely, although at some point they become indistinguishable from "noise". (Think of ocean waves below a certain size being impossible to tell apart from the random splashes and ripples on the surface.) But, as for the particle nature of light, each photon either is or isn't reflected, so there should be a precise time when the last photon fails to reflect, but instead turns into heat, I suppose. StuRat (talk) 19:01, 16 April 2011 (UTC)

Essentially I think you are talking about recursion. I remember Douglas Hofstadter's book Gödel, Escher, Bach has some pictures showing and discussing the effect you describe (except using cameras and monitors) to illustrate the various optical feedback strangeness that can occur. The "In science" section of the optical feedback article is likely to be of interest to you. Sean.hoyland - talk 19:45, 16 April 2011 (UTC)

I'm not sure if an infinite regress of mirrors is a fractal - I think a fractal branches at each increasing level of detail (like tree branches or snowflakes, but infinite). Anyone want to venture an opinion on the Hausdorff dimension of the image? Wnt (talk) 20:09, 16 April 2011 (UTC)

Relevant is our short Perfect mirror article. Comet Tuttle (talk) 20:21, 16 April 2011 (UTC)

Here [8] an article that shows (with photographs) how you can generate a fractal (similar to the lakes of wada) via the reflections produced by four colored, reflecting spheres (e.g. Xmas ornamnents). Enjoy! SemanticMantis (talk) 18:05, 20 April 2011 (UTC)

Brain scan lie detection

I was recently fascinated to read this paper about modern lie detection (available here temporarily) from the U.K. Is there a review of this technology that talks about accuracy rates for incarcerated convicts, captured belligerents, and whether it is possible to discern subjects' answers to questions when they refuse to speak? How much more accurate are these techniques compared to a typical polygraph? 99.39.5.103 (talk) 19:09, 16 April 2011 (UTC)

I suspect it would have the same shortcomings as regular lie detectors:

1) People are different, and can react differently when lying or telling the truth. Hopefully each person reacts consistently when either lying or telling the truth, but there's no guarantee.

2) Probably wouldn't work on people who don't know what the truth is, like those with mental diseases.

3) Still sounds like it has the same problem as the current test, that the results are highly subjective, and different experts may read the results differently (possibly depending on who is paying them).

4) Not very good info for a jury to base decisions on. That is, even if a certain portion of the brain "glowing" absolutely meant they were lying, this isn't obvious to the jury, they just have to take the word of an "expert". For juries that distrust so-called experts, this doesn't work.

It might have the advantage of stopping one method of fooling the test, by causing pain (say by stepping on a tack), to produce stress on truthful answers. Presumably the pain area of the brain would be distinct. However, a brain scan that involves swallowing a radioactive tracer sounds like it might violate the "first, do no harm" law. Doing that to diagnose a medical problem is one thing, but doing that for no medical benefit whatsoever sounds like iffy medical ethics. StuRat (talk) 19:26, 16 April 2011 (UTC)

I have to wonder if you read the paper. It was about (2) entirely. 99.39.5.103 (talk) 22:09, 16 April 2011 (UTC)

The use of fMRI for lie detection is still in early stages of being developed, and the technique has not been validated for use in a court of law, at least in the US. (Some strange things have happened in India, as I recall.) Our article on lie detection gives a brief description of it, with references. In my view this is not likely to become a widely used technique even if it can be validated, because it is very expensive, and because the procedure is pretty unpleasant, especially for people who are claustrophobic. (You're crammed into a narrow tube surrounded on every side by a huge mass of metal, and while the scan is running the machine makes a hell of a racket.) Looie496 (talk) 19:55, 16 April 2011 (UTC)

Thanks! I was amazed to read in the article that there are already two commercial vendors and a Mythbusters segment where 1 out of 3 were able to fool the test. 99.39.5.103 (talk) 22:09, 16 April 2011 (UTC)

(ec) I would be wary of assuming all such techniques will go away. See P300 or "brain fingerprinting", which sounds intimidating - but which is aimed specifically at recognition. I can think of very sinister uses.

The data here sounds significant-ish, but it's not entirely clear to me what data that Student's t-test was done on, and p<0.05 studies come up fairly often (someone posted a cartoon about that recently...) That 74% +- 17.5% consistency (50% baseline) sounds like something to watch.

I find myself suspicious of an ulterior motive with this study. It says the sample group included one murderer and one rapist, and I can't get over the feeling that somebody was interested in some single particular truth in the study. I also wonder if their conclusion - that schizophrenics make an active effort to lie, involving "executive decision" might potentially be used to make an argument that they know what they're doing is wrong, and hence are not legally insane. I'm likely wrong about where or what, but I smell something funny here. Wnt (talk) 19:59, 16 April 2011 (UTC)

Nationwide ground stop effect

After the September 11 attacks, a nationwide ground stop was ordered at all US airports, which lasted for several days. Our article about this is at Security Control of Air Traffic and Air Navigation Aids. At one point, I read that after a couple of days of no commercial (nor GA) air traffic, there had been a small average temperature increase that was attributed to the nationwide ground stop.

Has anyone compiled other studies or analyses, of atmospheric effects or any other effects, of the nationwide ground stop? Comet Tuttle (talk) 19:32, 16 April 2011 (UTC)

There is often a temptation to wrongly infer causation from correlation. Edison (talk) 19:39, 16 April 2011 (UTC)

Sounds like the sample size might be too small to determine if there is even a real correlation. StuRat (talk) 19:43, 16 April 2011 (UTC)

Christ, you guys, I wasn't born yesterday. I'm under the impression the claimants of the above weren't, either, though I could be wrong and it could be some news reporters speculating or something. But I seem to remember that atmospheric scientists were said to be excited about getting data from this event that would be unlikely to ever recur, and a definite temperature increase attributable to the stop was said to be one of the findings. Comet Tuttle (talk) 20:19, 16 April 2011 (UTC)

I heard that they were interested in how jet exhaust contributed to cloud formation, and a few days without jets gave them a nice "control", which they lacked before. But extrapolating from that to it having an effect on the global temperature is new to me. StuRat (talk) 02:50, 17 April 2011 (UTC)

You are looking for this or this. There may be other papers too, but that was what I found first. People have definitely looked at this issue. Dragons flight (talk) 21:05, 16 April 2011 (UTC)

I'd go with the 2nd paper, which says that it's difficult to draw conclusions over such a short time period, since local weather patterns have far more effect. StuRat (talk) 03:11, 17 April 2011 (UTC)

Antarctic ice sheet

Hi. What are the values of pressure, temperature and ice density over the rock of Antarctica beneath the ice, assuming 2.2 km of ice depth and bedrock elevation of 150 metres above sea level? Thanks. ~AH1 ^(discuss!) 20:41, 16 April 2011 (UTC)

Roughly 0.92 g / cm³ and 20 MPa (200 atm). Temperature will depend strongly on local conditions (e.g. surface temperature, geothermal flux, ice convection), but somewhere between -30 C and 0 C typically. Dragons flight (talk) 22:29, 16 April 2011 (UTC)

Citation needed! If you used hand-calculations to obtain those numbers, the least you can do is explain what type of calculation you used, so we can validate the assumptions you're making! Ice dynamics in Antarctica have been very heavily studied; and, as with all things scientific, the more you study it, the more you see it deviate from the ideal! Here's a few books: The West Antarctic Ice Sheet: Behavior and Environment; and more general books, Deformation of glacial materials and The Physics of Glaciers, that should help get you pointed in the right direction.

As an example of how complicated it can be, here's a NASA brief on active lakes underneath the ice - liquid water can form, flow, and re-freeze under there! Nimur (talk) 15:43, 17 April 2011 (UTC)

3 science questions

What is the bond that holds salt molecules together with water molecules and what is the most efficient way to break these bonds leaving the water by itself? Would bombarding the nucleus with alpha particles move the element up the table of elements and would high voltage electricity have a effect on this? Could waste materials be heated up to a high enough temperature leaving all the elements of the waste by themselves then could these be filtered by their density? —Preceding unsigned comment added by 82.38.96.241 (talk) 21:39, 16 April 2011 (UTC)

I'm thinking that moving up the table would involve cold fusion. No?190.56.105.59 (talk) 23:51, 16 April 2011 (UTC)

I believe that sodium chloride has an ionic bond, but I guess that's not what you were asking. As for how to separate water and salt, heat the water until it evaporates, using the Sun, leaving behind the salt. Of course, if it's the water you're after, then other desalinization methods work better. StuRat (talk) 02:40, 17 April 2011 (UTC)

For the first question, see solvation. In the case of polar solvents (like water) dissolving charged particles (like salt), the ion-water bonding is ofter thought about in terms of dipole moments and electrostatic interactions (like the above mentioned ionic bond, but much weaker). More detailed treatment would also consider the covalent nature of the solvent-ion bond.

For your second question: One typical way to remove charged particles is through ion exchange. Distillation is another, which requires not just breaking the water-salt bonds, but also the water-water bonds. Finally, in reverse osmosis, a pressure gradient across a semipermeable membrane is used to remove dissolved substances. Distillation (particularly Multi-stage flash distillation) is currently the most used method in the desalination of water, with reverse osmosis the second preferred method. Both methods require large amounts of energy to purify water: distillation takes about 23-27 kWh/m³ of water [9], while reverse osmosis systems are probably only somewhat more energy efficient. Buddy431 (talk) 03:56, 17 April 2011 (UTC)

If this article on reverse osmosis is to believed (and the calculations at least look plausible) the theoretical maximum efficiency in getting pure water from sea water through reverse osmosis is less than 1 kWh/m³, considerably more efficient than the 23-27 used in flash distillation. However, real reverse osmosis desalinization plants apparently operate at close to 2-4 kWh/m³ [10] [11]. These values are still much more energy efficient than the distillation plants, though I think distillation plants are easier to scale up to large sizes, and are liked due to their "simplicity, robustness and inherent reliability" (over reverse osmosis plants) [12]. The total volume of desalinated water from distillation is greater than from reverse osmosis, even though there are more reverse osmosis plants, if our article is to be believed. Buddy431 (talk) 04:15, 17 April 2011 (UTC)

If you have more than one completely unrelated questions like that you should open more than one inquires and title them something more relevant than "science question" which is as vague as it gets. Dauto (talk) 13:26, 17 April 2011 (UTC)

Thanks every one and will do dauto —Preceding unsigned comment added by 82.38.96.241 (talk) 20:12, 17 April 2011 (UTC)

Freedom of navigation in the Earth's outer core

Is there any way to predict whether the Earth's outer core is free of strands of solid material? In particular, the Earth's inner core may rotate at 1 revolution per 400 years. Is there any chance that some "mountain" poking out from the inner core might ram into some crystalline strand poking in from the mantle and suddenly the whole inner core is stopped or pushed off center? If so, would this have noticeable effects for those on the surface? Wnt (talk) 22:58, 16 April 2011 (UTC)

Not sure about the Earth's core, but there is some evidence that giant impacts may have perturbed the Earth's interior, causing giant mantle plume eruptions on the opposite side of the impact point, see e.g. here.

Hmmm, the formation of antipodal craters involves the coalescence of waves travelling along the surface of the Earth which comes together. But do these also affect the deep mantle? Wnt (talk) 01:04, 17 April 2011 (UTC)

The deep mantle would have to be affected, there is no other way for the antipodal volcanic effects to be so-precisely "antipodal", millions of years after the impact. The full article can be downloaded free of charge here It is made clear that the mechanism for antipodal volcanism is not known. Perhaps an oscillation mode where the core ocillates relative to the mantle can be excited by very large impacts that puncture the Earth's crust and give rise to large impact basins. You can then imagine that the oscillation preserves the information about the precise direction from where the impact was coming. Count Iblis (talk) 14:59, 17 April 2011 (UTC)

Also if this actually occurs then it could cause not only plume erruptions but also magnetic reversals. Phalcor (talk) 23:34, 16 April 2011 (UTC)

Any body remember any periodicity info on magnetic reversals.Phalcor (talk) 23:42, 16 April 2011 (UTC)

Our article magnetic reversal talks about the frequency and timing of them. DMacks (talk) 23:48, 16 April 2011 (UTC)

I think there is persuasive modeling of magnetic reversals with a simple liquid outer core model - though the disruption I'm asking about, if plausible, could cause all sorts of weird phenomena. Wnt (talk) 01:01, 17 April 2011 (UTC)

Sounds like you're concerned about navigation. Are you planning to take a magma proof submarine down there? A little early I think caus. nobody knows yet.190.148.132.157 (talk) 03:49, 17 April 2011 (UTC)

Sorry, I meant that just in the sense of the core rotating without bumping into things. I mean, it's not like land vs. air - it's solid iron in liquid iron. I wonder how regular it really is. And likewise, so far as I know, the mantle could have some kind of projecting webwork of odd high melting point minerals filling all the space in the outer core wherever the core hasn't been sweeping them away. Of course, probably not - I just don't know if it's known. Wnt (talk) 04:42, 17 April 2011 (UTC)

I see what you're getting at. I did not intend to be flippant. Well, not very anyway. Does'nt seem to be much info coming about that. In Hawaii some magma samples were very high in He3 which is thought to come from very close to the iron core. Let's face it. It's right under our feet but absolutely inaccessible. Much more inaccessible than space.Phalcor (talk) 05:42, 17 April 2011 (UTC)

Don't forget the reason why there are no mountains higher then nine kilometers on the surface of Earth: such a mountain would just collapse under its own weight. Something similar must be true to the inner core: it has to be spherical with only very small bumps because gravity forces it to that shape. (Okay, not really sphere but somewhat squashed because of the rotation of earth, but that's irrelevant now.) – b_jonas 10:16, 17 April 2011 (UTC)

On such time and length scales, if two "rigid structures" collide, they will elastically deform and/or melt/liquify. Energy might be lost to viscous damping, but it's unlikely it could cause any sort of total halting of the bulk motion of the inner structure of the Earth. Even though we call some parts "liquid" and some parts "solid", everything down there is convecting if you consider a long enough time-scale. A quick web-search turned up Turbulence and Small Scale Dynamics in the Core and Inner Core Dynamics; as you can see, a dynamic model (read: fluid model) exists even for the so-called "solid" inner core. Nimur (talk) 15:51, 17 April 2011 (UTC)

I should thank you for those references, which are interesting. One of them come close to saying that the outer core is homogeneous and well-mixed, though not literally swearing to it, and the various plumes and currents they described didn't seem very solid. They put an upper limit to the viscosity of the core, though I wasn't exactly sure what it meant. For now I'm going to assume that the scenario I pictured, which even I thought was likely outlandish, is truly not a factor. Even someday in the future, when the Earth's core nears freezing solid, they made the good point that gravitational coupling could stop it from spinning, so no collision need actually happen. Wnt (talk) 19:49, 19 April 2011 (UTC)

April 17

Does FRET work on IGE?

Does Förster resonance energy transfer work on induced gamma emission? Specifically, for purposes of sci-fi, I'd like to rely on "metatope batteries" in which covalent or ionic compounds of elements with nuclear isomers are used to break up the energy from one into another in diminishing steps until fissionable energy is broken down to a usable chemical scale. (I think that the range, which varies by the sixth power of distance, is a function of the virtual photons, rather than the size of the emitting/receiving particles, but I'm not 100% sure) Wnt (talk) 03:27, 17 April 2011 (UTC)

How to cheat reading eye chart?

I'll start with lenghtly explenation, if you don't want to read it all just cut to the question in the end :) I have nystagmus, I did some research on it and noticed that many people with it can't drive just because they need to be able to read number plate from 20 meters away. Now I don't intend to get drivers licence any time soon, but I don't think my vision is so bad that I wouldn't be able to drive. I also believe that eye charts are crapy way to test visual acuity. So I did some research - in my country you need acuity if no less than 0.5 to be able to drive, pretty sure they use eye chart for testing. I recently got myself contact lenses (I also got astigmatic myopia), so I developed a theory on how I could cheat the eye test. I imagine nobody would ask to check the power of my contact lenses and you can buy any contact lenses you want online. The big idea is that using viciously owerpowered lenses for duration of the test would turn my eyes into binoculars. The problem, besides widely known tale that wrong glasses can make you ill, is that I noticed that my current contact lenses make things look slightly smaler, I'm not sure if it is from blurrines reduced or because of lenses themselves - you know how things look smaler when you look trough magnifying glass from distance - which is making me question my theory - maybe overpowered lenses would instead make chart appear mycroscopic. The question - is there any way to shortly improve vision for sole purpose of reading as much of the eye chart as possible? —Preceding unsigned comment added by 46.109.116.140 (talk) 03:23, 17 April 2011 (UTC)

Overpowered lenses will give you blurry vision, not zoomed in vision. Lenses adjust where the focus of light is inside your eye. For most people, it does not fall directly on the retina. Your prescription will be the focus in or out to place the focus directly on the retina. If you increase the power of your lens, you will move the focus too far and end up with blurry vision. -- kainaw™ 03:26, 17 April 2011 (UTC)

Hear's an idea. Why don't you take a real pair of binoculas with you to the eye test. probably nobody would notice.190.148.132.157 (talk) 04:10, 17 April 2011 (UTC)

The point is that you have to wear corrective eyewear you had for test at all times when driving. There is a piece of equpiment for testing the power of glasses, but they wouldn't ask to remove contacts for testing (probably wouldn't even notice they are present). So I need binoculars in form of contacts 46.109.116.140 (talk) 04:47, 17 April 2011 (UTC)

Wikipedia has an article about pathologic nystagmus that describes actual therapies - I'd say I hope you've explored those, but actually, I suppose it would be better if you haven't since then you have an obvious alternative to cheating the test. Best to be able to read the speed limit signs, road construction, etc. I don't know if there are specific stimuli that trigger the vibration for you - might there be some alteration to the test environment you might be able to request, so that you can pass the test that way? Wnt (talk) 03:36, 17 April 2011 (UTC)

I don't know the laws in Latvia (which is what the bot says your IP is from), but in the U.S. either a license requires lenses or not. It doesn't say you have to use glasses rather than contacts, even if you tested with glasses. With 20/40 vision and nystagmus, do you really want to drive around without contacts at all, when you're wearing them now? Wnt (talk) 06:19, 17 April 2011 (UTC)

Generations of people have passed some eye tests just by remembering

E

FP

TOZ

LPED

PECFD

• Just saying. The doofuses do not make sufficient variation in the tests. Edison (talk) 04:19, 17 April 2011 (UTC)

Doofi ? :-) StuRat (talk) 05:59, 17 April 2011 (UTC)

When I took my drivers's eye test here in USA, the letters were on mechanical wheels like a mechanical digital clock. Everyone got a different sequence of letters to read. APL (talk) 07:32, 17 April 2011 (UTC)

There are various eye charts, I'm not sure what they are using here (and some places could still have ancient C charts). I tested at home from 3m distance, which is kind of hard to interpet, but it appears my acuity could be half the normal, which is 0.5. I might need about one extra line. The nystagmus mostly isn't issue, except it acts up in stresfull situations.

I never seen stronger power lens giving me blur, which is why I think that even if it doesn't work infinite, there still could be room for improvement, especialy when it doesn't have to be long term improvement 46.109.116.140 (talk) 04:32, 17 April 2011 (UTC)

Well, it wouldn't be the first time, or the tenth, that I've heard people complaining about an incompetent optometrist. Spherical contact lenses aren't perfect, but you should be able to walk out of an office with a pair of eyeglasses that lets you read much BETTER than 20/20, at least if the nystagmus isn't really so big of an issue as you say and there's nothing else the matter. Wnt (talk) 05:05, 17 April 2011 (UTC)

Maybe a friend could go into the office and check out the chart for you.190.148.132.157 (talk) 05:01, 17 April 2011 (UTC)

For reference, Visual acuity and Snellen chart describe the above. But everyone deserves to actually have high quality visual correction available to them, not just make-believe. Wnt (talk) 06:00, 17 April 2011 (UTC)

Lenses for myopia do not magnify. They actually reduce. If you want to make things bigger you actually need reading glasses. But those will make the chart blurry. There's no way to put binoculars into contact lens form.

Anyway, I would not want anyone who couldn't pass the test to drive. Even if, (like you seem to incorrectly believe) the test only tested your ability to read it would not be safe for a driver to not be able to see the signs. Speed limit signs alone should be enough to convince you of this. (Also : "BRIDGE OUT" ) APL (talk) 07:32, 17 April 2011 (UTC)

Most people who need glasses here are allowed to drive only if they wear glasses or lenses (and even carry a spare pair of glasses). The police, however, rarely checks whether they really wear glasses or lenses, and they have no way to check whether the glasses they wear are actually suitable for them, so many people get away without effectively seeing well while driving. (Of course, people also cheat to get a driving license in other ways, like paying to a doctor.) – b_jonas 10:11, 17 April 2011 (UTC)

The road signs are hudge and easy to grasp, the only problem I have is indeed reading - I've tried, but on moving vehicles, I can't read the direction signs and such, not because I don't see them, but because they drive past too soon (probably I myself could slow down). You all seem to be ignoring the fact that I said that I don't intend to get drivers license any time soon. I just want to explore posibilities in theory. Now how about glases+contacts? Binoculars seem to only have two (times two) lenses 46.109.116.140 (talk) 12:12, 17 April 2011 (UTC)

That is a very interesting question. Theoretically, could glasses and contacts be used together to form binoculars? -- 110.49.248.2 (talk) 15:41, 17 April 2011 (UTC)

I suspect this might be possible. Contact lenses don't seem like they make the world seem larger or smaller, but eyeglasses change the apparent size of the area they cover. I think you could get a quite modest magnification using contact lenses for an excessive amount of nearsightedness and eyeglasses for farsightedness. But contact lenses only go so high, and you don't have control over the distance between the lenses ... I suspect you'd get a rather modest effect. Wnt (talk) 16:42, 17 April 2011 (UTC)

You're fooling yourself if you think your eyes make it impossible to read, but do not impact your performance otherwise. Picking out details out in the distance, and in your mirrors, is an important part of driving. For example, you wouldn't spot a bicycle, until you were much closer than a normal driver. You'd still have time to swerve by the time you saw it, but if you'd seen it sooner you could have gradually adjusted your speed so that you weren't next to a giant truck when you needed to swerve. For another example, you might miss hand-signals other drivers are giving you. You might not be able to pick out street name signs soon enough to be useful (So you'd drive dangerously slow, or make dangerous sudden turns.) Even something as simple as making sure no one's coming up on you in your mirror when you open your door would be difficult without good vision. I'm not saying that you'd crash your first time out, but you'd crash way more often than a properly sighted driver. APL (talk) 22:49, 17 April 2011 (UTC)

It may be easier to visit some third world country and get your driving licence from there. Count Iblis (talk) 14:45, 17 April 2011 (UTC)

We Latvians tend to think that we already live in some third world country ;) but indeed - simply put I just wish to know what 110.49.248.2 was saying (or any other optical/fun means to do it). I mentioned the reason I am asking just in case someone would come up with practical way to apply it to everyday life and because I want to know how bad could stronger lenses be (as I mentioned mine allready make things smaller) 46.109.116.140 (talk) 16:53, 17 April 2011 (UTC)

molten earth heaviest elements where

In a molten earth, shouldn't the heaviest elements have gravitated to the centre. Or would the centrifugal force of earth's rotation have thrown them out to the crust? — Preceding unsigned comment added by Phalcor (talk • contribs) 07:07, 17 April 2011 (UTC)

Of course there's no data that far back but if the earth was very fluid then convection currents could have carried heavier elements to the surface.190.148.135.18 (talk) 07:42, 17 April 2011 (UTC)

Some thoughts:

A) Gravity is far stronger, so heavier things should sink.

B) More dense elements, as opposed to more massive elements, should sink to the center. So, an atom that is twice as heavy but 3 times the volume wouldn't sink below the other.

C) Many elements form molecules, so the density of the entire molecule must be considered.

D) The Earth was only fully molten for a short period at the beginning, and significant mass was added later, in the form of meteorites, so much of that would have been blocked from dropping straight to the center of the Earth.

E) Plate tectonics provides a mechanism whereby things on the surface of the Earth may eventually be submerged to the molten portion. However, this depends greatly on the proximity to a subduction zone. StuRat (talk) 08:22, 17 April 2011 (UTC)

Thank's a bunch sturat. Several points of info. there.Phalcor (talk) 09:29, 17 April 2011 (UTC)

See planetary differentiation. Denser minerals do sink. Minerals can be broadly classified as lithophiles, chalcophiles, and siderophiles by the Goldschmidt classification. Lithophiles tend to form mineral complexes in association with oxygen and other light weight elements, and hence they would be relatively low-density as a group and tended to stay near the surface (on average). By contrast, siderophile elements tend to bond with iron and other metals, which leads to high density and tendancy to be depleted near the surface. The Earth's core is mostly iron with some nickel and other things. Dragons flight (talk) 17:55, 17 April 2011 (UTC)

Modern mammals nocturnal?

According to this, http://www.bbc.co.uk/news/science-environment-13083990 , modern mammals are typically nocturnal. Is that really true? Thanks 92.15.8.229 (talk) 09:52, 17 April 2011 (UTC)

It says they're predominantly (i.e. mostly) nocturnal, not "typically" nocturnal, which is rather different.--Shantavira|^{feed me} 13:10, 17 April 2011 (UTC)

Alright, is it really true that modern mammels are predominantly noturnal? 92.24.177.71 (talk) 15:49, 17 April 2011 (UTC)

What exactly makes you think otherwise? If you take a look at Mammal you'll see that largest orders of mammals are rodents, bats and moles, if you break it down to species in these orders you'll probably find that most are nocturnal and/or live in dark environment ~~Xil (talk) 17:19, 17 April 2011 (UTC)

When you see weird statements like that, often they refer to 'predominant' in terms of number of species, which says nothing about distribution, number of individuals, biomass, etc. Bats alone make up 20% of mammal species, and the vast majority of them are nocturnal. Rodents make up 40% of mammal species, and many (>60%?) of them are nocturnal too. Really, though large diurnal mammal species are salient to human culture, I see no reason to disbelieve that at least 51% of mammal species are nocturnal. SemanticMantis (talk) 17:24, 17 April 2011 (UTC)

Bats at 20% and 60% of rodents at 40% only makes up 44% of mammals in total. I would expect "predominantly" to mean 80% or more. So there is a large gap to fill before I could believe the BBC article assertion. 92.15.15.88 (talk) 08:37, 19 April 2011 (UTC)

Late-leafing trees in southern England

Why don't trees in southern England come into leaf earlier? (I can only speak from my personal experience, so I'm only commenting about southern England). Herbaceous plants have been in leaf months earlier. Why do trees not come into leaf in February or March rather than leaving it till late April? Wouldnt they benefit from growing more? In a wild woodland or scrub situation, would not the earlier-leafing young tree be able to out-grow its nearby rivals and thus caputure the sunlight at the canopy? Thanks 92.15.8.229 (talk) 09:59, 17 April 2011 (UTC)

I suspect that late-leafing has evolved to avoid damage from late frosts. However, why elder and hawthorn can leaf a couple of months earlier than oak and ash I don't know. This page has some information on the leafing dates of oak and how it relates to temperature; it seems to be not clearly understood. Alansplodge (talk) 12:02, 17 April 2011 (UTC)

I was actually thinking of asking about this myself. Has global warming actually created a new season? I always thought of spring as running from Easter to May Day, bud to flower. But it seemed like thirty years ago the winter held on more strongly, at least past the end of March, and that the tender young buds were always at real risk of frost; whereas nowadays it seems like there are two or three weeks of above-freezing weather before the plants respond. I assume some climate scientists looking at the global warming problem must have measured such a thing, if it's not just a trick of memory and time. Wnt (talk) 16:26, 17 April 2011 (UTC)

Easter (24 April) to May Day (1 May) is only six days this year! Alansplodge (talk) 17:09, 17 April 2011 (UTC)

I don't agree with the seasons being different in the past that what they are now. I can remember, for example, a winter without snow when I was a child; as well as a winter with lots of snow. I can also remember warm sunny days in November. I conclude that weather varies more from year to year than people suppose: when you get a random run of one extreme or the other, then people mistakenly believe that its due to changes in the seasons. If people move further south or to a lower altitude than in the past then they may also mistake the improvement in weather to climate change. 92.24.177.71 (talk) 17:00, 17 April 2011 (UTC)

I have a weeping ash that usually doesn't come into leaf until mid-June because we sometimes get sharp frosts here in northern England at the start of June. Spring certainly seems much warmer than usual this year, so I'll watch to see if it responds. I'm surprised that anyone thinks that spring can start as late as Easter, especially this year, but I suppose it depends on what you mean by "spring". Dbfirs 16:47, 17 April 2011 (UTC)

Since the date of Easter varies from year to year, then Wnt would have to be more precise than that. 92.24.177.71 (talk) 17:05, 17 April 2011 (UTC)

This thread suggests that oak is temperature responsive, whereas ash is photoperiodic; ie it responds to the change in daylight hours. Alansplodge (talk) 17:03, 17 April 2011 (UTC)

I think leaves appear in mid spring after ground plants are already out of ground and even after tree blossom. As far I remember in previous years all this happened much sooner, so it doesn't form a pattern, which could be interpreted as result of climate change, it is just that this year winter was longer ~~Xil (talk) 17:27, 17 April 2011 (UTC)

Failure due to mechanical vibrations

Hi,

I was wondering if anyone knew of any intersesting/dramatic examples (other than Tacoma Narrows!) of a device or structure failing due to mechanical vibration? Thanks for any suggestions! --58.175.32.140 (talk) 11:31, 17 April 2011 (UTC)

Many aircraft have been destroyed in flight, or seriously damaged, due to aeroelastic flutter, often simply called flutter. This is a form of mechanical vibration, just like the Tacoma Narrows Bridge. Dolphin (t) 12:20, 17 April 2011 (UTC)

You want dramatic? This is worth the wait.--Shantavira|^{feed me} 13:17, 17 April 2011 (UTC)

That seems incredibly dangerous. Look how much kinetic energy the motor imparts: enough to flip a two-hundred-pound washing machine. If the brick had been ejected through the open front-door and had hit the experimenter, it would certainly have been fatal. "Don't try at home." Things like this make me worry that regular people should not be permitted access to powerful machinery; they have a total lack of respect for the amount of power in the motors and engines that are readily available in today's vehicles and appliances. Nimur (talk) 15:58, 17 April 2011 (UTC)

What do you propose, robots? Anyway, it's hard to picture the brick gaining that kind of lateral momentum in the opposite direction. And the video is an important third-party test of what it takes for the washer to become dangerous during use (unless you trust the government to think of every possible failure?) Wnt (talk) 16:21, 17 April 2011 (UTC)

I suppose by that logic, we should encourage people to film themselves playing recklessly with other dangerous tools, like motor vehicles and firearms, to test the limits of government regulatory power? I can't see your statement as anything but a non-sequitur. Some activities are inherently dangerous and should be discouraged. Regarding robots, I have worked in several robotics laboratories; and lab-safety has always been one of the first things we had drilled into our heads. The alternative, of course, would be a 300-pound robot arm drilling into our heads, because our robots had motors that were strong enough to kill a human. Electric motors are very strong and very stupid; they require intelligent instructions from smart humans to operate safely. Nimur (talk) 16:35, 17 April 2011 (UTC)

Fire Marshall Bill was always willing to personally demonstrate how dangerous things were. StuRat (talk) 20:53, 17 April 2011 (UTC)

Spike TV isn't my favorite channel, but yes, it's good for a laugh. Regarding lab safety, well, I'm not saying you shouldn't be permitted to make your own safety decisions. I'm OK with it if we delay the beta release of SkyNet by a few months... Wnt (talk) 22:37, 17 April 2011 (UTC)

Regarding the original question: you might read our articles on earthquake engineering, earthquake simulation, etc. For the less dramatic, more common case - a lot of consumer electronics solder joints will crack or fail during vigorous vibration. Everyone worries about this: mobile phone manufacturers, aircraft electronics, even NASA analyzes vibrational failure. While this usually doesn't result in a catastrophic and dramatic collapse, it often causes "invisible" electronic problems, which are very bad. Nimur (talk) 16:51, 17 April 2011 (UTC)

Resonance can greatly magnify the damage caused by vibration. StuRat (talk) 20:53, 17 April 2011 (UTC)

@Nimur, Evolution works in mysterious ways ;-) Richard Avery (talk) 21:58, 17 April 2011 (UTC)

Not sure if it is exactly what you are after but helicopters can experience an effect called Ground resonance which can lead to catastrophic failure due to mechanical vibration. Vespine (talk) 05:32, 18 April 2011 (UTC)

Thanks everyone for your suggestions, they have been quite helpful. Just to clarify my question a bit in case I get some more suggestions, I'm after industrial examples so while the exploding washing machine was humourous it wasn't exactly what I was after! :) --58.175.32.140 (talk) 12:09, 18 April 2011 (UTC)

There is also the example that marching troops can supposedly cause a bridge to collapse if it's at the proper resonance frequency. Mythbusters did a segment on this (I forget if they were able to confirm it). So, troops may be told to walk across normally rather than marching in step, to prevent this. StuRat (talk) 17:24, 18 April 2011 (UTC)

Regarding which, see Broughton Suspension Bridge and, for a related problem, Millennium Bridge (London). {The poster formerly known as 87.81.230.195} 90.197.66.111 (talk) 19:04, 18 April 2011 (UTC)

Is there a theory about the stuff called "Significant Atomic Mass"?

Today I saw something called "Significant Atomic Mass" in this web archive, but I did not find any other references, so is it a well-known scientific concept? — Preceding unsigned comment added by Inspector (talk • contribs) 11:56, 17 April 2011 (UTC)

I have never heard of the term, and I can't find any respectable-looking sites that use it. The site you linked to seems to be quite poorly written: maybe it is a mistranslation from another language? 81.98.38.48 (talk) 14:00, 17 April 2011 (UTC)

I believe what they mean is that they're giving a round number (334) rather than an exact mass. Except for carbon-12, defined at 12, all isotopes are just a little bit off from an integer because of various bits of energy involved in their composition. These bits of energy, though small, turn out to be extremely meaningful during fusion and fission reactions and such - thus the need for an embarrassed disclaimer. With so little of this element available, an exact mass could only be simulated guesswork. Wnt (talk) 16:15, 17 April 2011 (UTC)

The technical term for what WNT has described is Mass number. Compare the following:

• atomic number, Z, an integer number specifying the number of protons,
• neutron number, N, an integer number specifying the number of neutrons,
• mass number, A ≡ Z + N, the integer number of baryons (neutrons + protons) in the atom
• atomic mass, an exact value which can be measured in kilograms (or eV or daltons or your favorite convenient mass unit), that accounts for the binding energy (mass defect) (and also accounts for the fact that neither one proton nor one neutron weigh exactly one atomic mass unit)
  • Standard atomic mass, (also called "atomic weight" for historical reasons), which averages over the natural abundance and individual atomic mass for each isotope.

That's basically the rundown of the standard terminology. Anything else is "non-standard" terminology and should be interpreted with caution. Nimur (talk) 16:25, 17 April 2011 (UTC)

Contrariwise. Standard atomic weight can't be suitable for this isotope. The problem with elements that don't occur naturally is that one can't calculate an average. --Aspro (talk) 19:17, 17 April 2011 (UTC)

• I know about the atomic number and binding energy already. But I am still curious about how they find the formula which was used to predict the mass of unknown elements(even up to 218).--Inspector (talk) 09:25, 18 April 2011 (UTC)

Lewis structure standard

Is there an IUPAC standard for what side of the element symbol to start on and draw up to two dots as the s orbital and in what order to place the dots for the other p orbitals? 76.27.175.80 (talk) 16:39, 17 April 2011 (UTC)

No, there is not. In fact, there are several "standards" for drawing lewis structures for single atoms; some of which have you place the "s" electrons on one side (usually the top) and some of which have you ignore the "s" and "p" nature and just draw the electrons individually around the atom one at a time on all 4 sides, and then double up after. People who espouse the first standard (first 2 "s" electrons, then one of each "p" around the other three sides, then double up the "p"s) claim that it better represents the electron configuration of the element. I happen to usually teach students the second method, as lewis structures are primarly about learning how atoms bond in molecules, and the second method (ignore s and p, and draw dots one at a time around the outside) is a better representation of the orbital hybridization in bond formation. The difference between the two methods shows up in the dot diagram for Carbon most starkly: If you use the "s" and "p" method, you get two dots on top, one on the right, and one on the bottom. That would seem to indicate that carbon can form 2 bonds; to match each of its unpaired electrons. In reality, carbon most readily forms 4 bonds; best explained by having 4 "hybrid" sp3 orbitals. As a teacher, being able to use dot diagrams as a bridge to drawing the lewis structure of molecules (i.e. connecting the dots around Carbon to the 4 bonds that Carbon makes in in a molecule) is more important than correctly modelling the "s" and "p" organization in unbonded carbon. Indeed, so many other models do that better and more obviously than the dot diagram that it doesn't serve much purpose to make the dot diagram do that as well. If you look at this google image search and this google image search and this one as well you'll see that most people use the "one dot per side" method. However, you should do whatever your teacher tells you to; they are grading the test. Understand that different teachers have different reasons (pedagogically) to teach one model or another, and that one or the other is not wrong (i.e., your teacher is not wrong because they favor a different model than I do). Understanding the limitations of each model is more important. --Jayron32 02:19, 18 April 2011 (UTC)

Most reflective mirror

What type of mirror is the most reflective? --75.40.204.106 (talk) 16:40, 17 April 2011 (UTC)

For most purposes, aluminum on silica glass has the peak reflectivity. If you have special needs, you should check with your optics vendor. Nimur (talk) 16:44, 17 April 2011 (UTC)

See dielectric mirror. Red Act (talk) 17:11, 17 April 2011 (UTC)

I don't know if you would call it a "mirror", but many optical devices like binoculars and periscopes use total internal reflection to redirect light with minimal losses. -- 174.21.254.3 (talk) 18:37, 17 April 2011 (UTC)

Found this link up the page a bit Perfect mirror. -- 174.21.254.3 (talk) 18:41, 17 April 2011 (UTC)

Prism as a mirror?--78.150.239.71 (talk) 09:17, 18 April 2011 (UTC)

Yup. While people are probably most familiar with dispersive prisms, which are the ones which separate white light into rainbows, there are also reflective prisms, such as the Porro prism. -- 174.21.254.3 (talk) 15:56, 18 April 2011 (UTC)

Antimatter explosion

What would an an antimatter explosion look like? --75.40.204.106 (talk) 16:40, 17 April 2011 (UTC)

See the answers from when the same question was asked last month.[13] Red Act (talk) 16:53, 17 April 2011 (UTC)

(ec) It depends entirely on how much antimatter is annihilated. In practice, usual lab experiments involve minuscule amounts of antimatter - so the actual annihilation looks like nothing at all. Photons are produced, and effects are measured; experimental high energy physicists analyze volumes of experiment data and determine a statistical presence of one or more particle trajectories that corresponds to a matter-antimatter annihilation energy. So, in practice, the "explosion" looks like numbers in a database. Nimur (talk) 16:55, 17 April 2011 (UTC)

It would look like gamma radiation. Photons. Light. Collect (talk) 17:40, 17 April 2011 (UTC)

Lots of discussion of this here. To the naked eye it would look much like any other explosion — it would generate a considerable amount of heat in a short amount of time, which is what most explosions "look like". --Mr.98 (talk) 17:52, 17 April 2011 (UTC)

It could range anywhere between a crackling flash over in less than a fraction of a second for a relatively small amount, to a brilliant, magnesium white, expanding ball of ionised air hot enough to vaporise everything in its path. The sky itself will burn from the radiation, and flare different colours as the components of the air would fluoresce, this is accompanied by aurora. Although anyone, who witnesses such a fireball will receive a deadly dose of gamma radiation, to kill in seconds. Of course, these are two extremes, depending on the amount of antimatter. Plasmic Physics (talk) 23:26, 17 April 2011 (UTC)

It would look like a nuclear explosion, really. Dauto (talk) 14:32, 18 April 2011 (UTC)

And those look pretty much like a very large conventional explosion. Googlemeister (talk) 19:02, 18 April 2011 (UTC)

Keystone species

Is the human a kind of keystone species? How about their pets? I see on the talk page for that article that somebody has removed the domestic cat from the list of examples, reasoning that they are artificially high in abundance and only harm other populations. Given that they are artificially high in abundance because of the actions of humans, which are a kind of animal, and given that all keystone species manage, that is to say, eat, other populations, and that there is no inherent balance of nature, does either argument for the exclusion of pet cats have merit? Is it merely that humans are not part of nature by definition, and therefore every human effect is termed an artificial effect?  Card Zero  (talk) 18:29, 17 April 2011 (UTC)

The notion of keystone species is somewhat nebulous and subjective by nature (as are many ecological concepts). Most ecological studies make a distinction between ecosystems with strong anthropogenic influence and more 'natural' communities. However, this distinction is basically a convenience, because we know that humans and all their their domesticated and commensal species radically change ecosystems. The human species is definitely 'keystone' if the definition is 'disproportionate effect on its environment relative to its biomass.' The same is true for our cats: they can radically effect the ecosystem, for instance when introduced to islands. That being said, the more common usage for keystone species is indicated by the examples in our our article. Other good examples of keystone species are ecosystem engineers such as many ant and termite species. SemanticMantis (talk) 19:21, 17 April 2011 (UTC)

Glasses that correct for higher order abberations

Are glasses that correct for higher order abberations (spherical aberration, trefoil etc.) for sale? I think that my night vision is not so sharp because my glasses only have a diopter and a cylindrical correction. Count Iblis (talk) 20:10, 17 April 2011 (UTC)

Yes [14], but why do you think it is caused by higher order aberration? It sounds more like Nyctalopia, which can have causes other than astigmatism. Besides visual acuity is reduced by about 90% in night, you should worry if you don't see anything at all or if it is not sharp in day ~~Xil (talk) 20:36, 17 April 2011 (UTC)

Well, I actualy have very good vision at night compared to most other people. E.g. if I go outside during half moon, after a while it looks like dusk to me, I can see everything around me, while the people I'm with are complaining about not being able to see where they step well. But what I see looks like dusk, but with me not wearing my glasses. Now, I like to do observing at night and I have read e.g. here, the last message by Brian Skiff that getting the eye correction sorted out precisely could make a difference. Count Iblis (talk) 21:06, 17 April 2011 (UTC)

Everybody's night vision is less sharp than their daylight vision. This is because the sharpness of the image that is formed on the retina is partly controlled by the diameter (or aperture) of the eye's pupil - the smaller this is, the sharper the image. At night (and in any other low-light situation) the pupil expands to its widest to allow in as much light as possible, in order to make the image as bright as possible, but this has the unavoidable effect of unsharpening the image.

If you have exceptionally good night vision in terms of brightness, your pupils are probably able to open wider than average (and than those of older people, since maximum pupil size declines with age), so the unsharpening you experience will also be greater than average.

Atropine used to be used to widen the pupil to improve night vision in critical situations (such as night combat in warfare). There are doubtless some drugs that would have the opposite side effect of shrinking the pupil, sharpening but dimming your night vision, but this marginal benefit would probably be outweighed by their less desirable main effects on your body and system (whatever those might be) so this would not be safe method to pursue. {The poster formerly known as 87.81.230.195 - who also has short sight and astigmatism, and used to practise astronomy} 90.197.66.111 (talk) 23:53, 17 April 2011 (UTC)

Because I've been wearing glasses for such a long time (since I was 7 years old), I don't remember the characteristics of normal vision anymore. So, whenever I am not seeing sharp, I tend to blame my glasses :) . Count Iblis (talk) 14:42, 18 April 2011 (UTC)

Heh, that sounds like a very screwed up understanding about vision :) First of all -1 astigmatism is not all that bad vision, it is fairly close to normal, the guy on yahoo groups is saying he needs much stronger correction than you. Secondly reffractive error occurs due to shape of the eye and how light is focused in the eye, I think it is something an eye care specialist should see when checking your eyes. Lastly - why don`t you do a vision test at home - take an eye chart, and write down what you see in it from distance (writing down will produce better results and ensure you don`t memorize the chart), then try to do the same in darker conditions - you`ll know what your vision is in normal conditions and how good it is in darkness (as I said 90% reduction is considered normal). I would say that ``normal`` is what you get removing glasses and allowing eyes to adjust for a while, technicaly ``normal`` is 20/20 for which you might need glasses ~~Xil (talk) 20:10, 18 April 2011 (UTC)

Getting cheese moldy?

I have an old cook book, it has a recipe for cheese, which pretty much is a way to utilize old, molded quark. I want to try it, but I only have fresh quark. A recipe for another somewhat similar cheese clarifies that it will get moldy if left in warm place for 3-4 days. Unfortunately I don't have the patience to look on ball of quark left on table, showing no sign of aging. Is there any way to speed this up? I was thinking I could find mold somewhere, but I figure I need some specific fungi, is there any way to identify suitable mold species? ~~Xil (talk) 20:11, 17 April 2011 (UTC)

Yes, but part of that would involve using a microscope and maybe growing cultures and exposing them to various agents to see if they promote or retard growth. The wrong kind of mold can certainly be dangerous. I suggest you buy a professionally made moldy cheese, like blue cheese, and use that instead. StuRat (talk) 20:58, 17 April 2011 (UTC)

But not this professionally made cheese, I presume? Count Iblis (talk) 21:14, 17 April 2011 (UTC)

Just don't forget your goggles ! StuRat (talk) 22:28, 17 April 2011 (UTC)

Proffesional, but illegal? :D Older members of my family know how this cheese should turn out, so I think I am not at risk. I don't know why, but it is absoultely imposible to get less known food of local origin in my country, so I have no choice, but to make it myself to get it. I had moldy philadelphia cheese, which is similar product, hence the idea to introduce it from some other food, I am not taking stuff that grows ln walls ofcourse :) What are good conditions for mold anyway? It says warm, but not exact temperature, besides it also suggests to cover it (but that appears to be for the part where you allready have mold) - wouldn't covering it keep mold away? ~~Xil (talk) 22:41, 17 April 2011 (UTC)

Yes, covering it is to keep other toxic molds away, once you have introduced the safe mold strain. Also, you need to keep the moisture in, and covering helps with that. As for temperature, you can certainly grow mold just find at room temp. Warmer might grow faster, though, as long as it's not so hot as to drive the moisture out or kill the mold. StuRat (talk) 00:57, 18 April 2011 (UTC)

Brie has an edible mold casing. Bbuy some of that and put it in the same container as the cheese you want to get moldy. —Preceding unsigned comment added by 165.212.189.187 (talk) 12:53, 18 April 2011 (UTC)

Are you sure you are not confusing blue cheese with moldy cheese? The former is safe to eat (or at least I assume so as it is sold in shops etc); but many molds are carcinogenic so the later should be avoided. Decaying cheese may also produce harmful bacteria such as listeria. 92.28.241.233 (talk) 18:11, 18 April 2011 (UTC)

Of course I am sure that a particular sort of cheese is not some other cheese, which is why I am not very sure about introducing mold from another cheese. On top of that - there is no particular strain of mold identified for use of this, I figure it is naturaly occuring in this region, brie or blue cheese strains would be imported. There is one other thing I am unsure about - the recipe discribes the mold as ``a slimy layer covering the cheese``, no particular color or anything, at the time the average man wouldn`t go check his cheese with microscope, so maybe it isn`t mold, but something else that happens with old quark (note that it essentialy is just milk gone sour and boiled, so maybe it is whey coming out of it or some other proccess that occurs in old milk). According to google the end result is this (the black dots are caraway seeds, not mould), there is also this though, which is from article giving recipes for two diffrent cheeses, it looks moldy, but it also looks much like the other cheese described in the article ~~Xil (talk) 19:35, 18 April 2011 (UTC)

There are a great many different species of mold, you are far more likely to obtain something harmful to eat than good to eat. 92.29.122.67 (talk) 20:40, 18 April 2011 (UTC)

Allright, but leave the decision to eat or not to up to me :) let`s consider this an experiment in culturing mold ~~Xil (talk) 21:11, 18 April 2011 (UTC)

Please read Mold_health_issues#Food etc. 92.15.15.88 (talk) 08:45, 19 April 2011 (UTC)

Thanks, that mostly answers my question about introduction - apparently most bread and fruit molds are harmfull, note that health concerns mostly are allergies and opportunistic infections, thus mostly dangerous for people who are allergic or have weak imune system. Plus I found a site clearly stating that the mold should be transperent and white ~~Xil (talk) 18:02, 19 April 2011 (UTC)

Brie has a white edible mold casing. Buy some of that and put it in the same container as the cheese you want to get moldy. —Preceding unsigned comment added by 165.212.189.187 (talk) 18:55, 19 April 2011 (UTC)

Spray to clean dog feces

Does it exist? I've got the idea from Envy_(2004_film). I've also found some poop freezing sprays, but they were more of a specific solution, for dogs with diarrhea, and not for daily use. Some chemicals can indeed process feces, so it's not so crazy to think that it exists. Quest09 (talk) 21:59, 17 April 2011 (UTC)

It's not clear what you want the spray to do. Some possibilities:

1) Sterilize the feces, so they don't spread disease.

2) Remove feces stains from carpet, etc.

3) Harden the feces for easier removal.

Of these, I don't know if 3 is feasible (fece-able ?). 1 and 2 certainly are, but only after the bulk of the feces has been removed. StuRat (talk) 22:27, 17 April 2011 (UTC)

None of the above. I want that they disappear, and that the owner does not have to collect them. Could a chemical simply decompose them into powder? Quest09 (talk) 23:43, 17 April 2011 (UTC)

No, not just powder, since they are mostly water. You could pour some kind of strong acid on them which might turn them into a sludge that sinks into grass, then add a base, like baking soda, to cancel the acidity, but this doesn't seem like a reasonable way to deal with them. StuRat (talk) 00:53, 18 April 2011 (UTC)

But, what is the residual remainder of a chemical toilet, after formaldehyde, glutaraldehyde or quaternary ammonium compounds are applied to our feces? Quest09 (talk)

The mass and volume will be the same as the original feces, plus the mass and volume of the chemicals which react with them. It could be turned into a liquid, or even a gas with the addition of lots of energy, but the mass never decreases. The volume wouldn't decrease, either, and would dramatically increase if you turned it into a gas. Also consider that chemical reactions are far simpler in a container than in the open, where the reactants all flow away. StuRat (talk) 01:21, 18 April 2011 (UTC)

Yes, the mass + volume won't change, but it could be in a form that's easier to handle, or even in a form that does not have to be treated. Quest09 (talk) 10:53, 18 April 2011 (UTC)

Assuming the poop is on the lawn,there was a time when I used to solve the problemm by simply squirting it with a heavy jet from a water hose. Of course it wasn,t really gone but it couldn't be seen and in such tiny particles that decomposition would be very rapid.Phalcor (talk) 01:39, 18 April 2011 (UTC)

Let's hope you didn't have kids or pets rolling on that grass right after. You could also get it splashed all over yourself, too. StuRat (talk) 02:07, 18 April 2011 (UTC)

Thank you sturat, no kids one dog and I stood well back. but if somebody did manage to touch those tiny particles hidden deep in the grass it might be less harmful than any chemical that could do the job.Phalcor (talk) 02:31, 18 April 2011 (UTC)

One thing to remember is that whatever chemicals you put on your lawn will be there in some form (reacted with the feces or not) for a time. There's a chance that the dog may go back to sniff whatever you've put on the feces and ingest it. Even if this isn't your dog (think pesky neighbor's dog), many including myself wouldn't think too highly of a dog being poisoned, accidentally or not.

If this is your dog, you may want to consider a raw diet. I feed all my dogs (5) raw chicken and their feces just crumbles into a fine powder once it dries. I think nothing of stepping on it while mowing the lawn since it just crushes into the soil. And since it's a powder, I don't really track it into the house especially after walking around finishing all the other lawn chores. Dismas|^(talk) 02:38, 18 April 2011 (UTC)

Yes. Plus automatic nutrients for a healthy lawn.Phalcor (talk) 03:00, 18 April 2011 (UTC)

April 18

Why isn't drinking coffee when tired considered negative feedback?

I was under the understanding that negative feedback could apply to both mechanical and voluntary actions to maintain homeostasis, such as a cold-blooded lizard regulating its temperature by crawling to a sunny rock. Why isn't drinking coffee when tired considered negative feedback? The person senses he is tired, this upsets homeostasis, then he drinks coffee, which contains caffeine, which restores the person to a normal level of energy. --Gary123 (talk) 00:25, 18 April 2011 (UTC)

~ It is a feedback loop, only one that is limited by it's positive and negative power rails, which limits the output swing. A person can drink a cup of coffee to counteract the feeling of tiredness. But can a person drink an unlimited amount of coffee? No. Therefor the output driver (coffee) is limited. It hits the supply rail (a limited number of cups of coffee). Once the driver has risen to the supply rail, the output driver can't rise any further, and if the input signal still goes negative (the person becomes more tired) then the negative feedback loop can't drive the output high enough (the person can't drink enough coffee) to balance the input (the tiredness). --InverseSubstance (talk) 00:37, 18 April 2011 (UTC)

It goes deeper than that. In the case of the lizard analogy, temperature is directly corrected by solar warming. In contrast, "tiredness" is a complex symptom that is only partially counteracted by caffeine, and the latter has a number of side effects. Therefore, tiredness and coffee consumption don't approximate a negative feedback loop very well. -- Scray (talk) 02:20, 18 April 2011 (UTC)

Drinking coffee to counteract fatigue is a type of negative feedback. Looie496 (talk) 03:00, 18 April 2011 (UTC)

It is, since fatigue reduces alertness, and caffeine can increase alertness. I think the responses above were highlighting the limitations - there are limits on caffeine's efficacy, and reduced alertness is only one part of fatigue. -- Scray (talk) 03:28, 18 April 2011 (UTC)

How are each Harley-heads better than each other?

How is a Twin Camhead better than an Evolutionhead better than a Shovelhead better than a Panhead better than a Knucklehead better than a Flathead better than an F-Head? --98.190.13.3 (talk) 00:40, 18 April 2011 (UTC)

I don't know anything about this, but might help others to note that this question is about Harley-Davidson motorcycle engines. DMacks (talk) 09:35, 18 April 2011 (UTC)

"Better" is very difficult to define, especially for a work like a Harley engine, which is both an engineering product and an aesthetic piece. We have articles about most of those; there are a few cases where the newer model has a clear, objective, measurably higher performance for particular specs (such as peak power, fuel efficiency, and so forth). You'll have to be more specific if you want a more precise answer - in the meantime, consider the links available from the Harley-Davidson engine timeline. Nimur (talk) 17:26, 18 April 2011 (UTC)

Deja vu - but without the sense of experiencing that exact event

I experienced the oddest thing today: I had a strong sense of deja vu, but it had nothing to do with the action I was committing. I was petting my cat, and suddenly, I had the strong impression I'd thought about doing this before. However, the memory was one of traveling in the car as a child (I actually remember which part of the country I was in, and the scenery). Simultaneously, I also had a memory come up of being in first grade, with each of us in the class sitting at a computer using the keyboard (I remember the teacher telling us to press "ESC").

Is there a name for this phenomenon? And for that matter, can I surmise that it's just a glitch in the way that my brain stores my memories - that it stored a few in a close location and that it improperly vibrated some neuron that brought the other two to bear? Magog the Ogre (talk) 01:21, 18 April 2011 (UTC)

Don't know about the rest but if you have improperly vibrating neurons that sounds like medical advice.190.56.16.200 (talk) 01:54, 18 April 2011 (UTC)

Perhaps you previously petted the cat when thinking about those things, so there's a link between memories of petting the cat and those things. StuRat (talk) 02:04, 18 April 2011 (UTC)

I absolutely cannot imagine sitting at a computer in the first grade. Edison (talk) 03:49, 18 April 2011 (UTC)

Yeah, i was thinking the same thing, i was actually thinking "wow, it's like computer usage somehow got mixed into his memory of being in 1st gradet" until i realized that had nothing to do with it and it was just a regular memory of using a computer in 1st grade. lol. Vespine (talk) 05:21, 18 April 2011 (UTC)

Yes, to clarify: me sitting at this desk in first grade was a real event. Me petting the cat was a real event. And me going down the interstate and looking at the landscape 20 years ago is a real event. But they have nothing to do with each other, on an intellectual level, but my brain couldn't shake the feeling they were related. Magog the Ogre (talk) 05:39, 18 April 2011 (UTC)

I don't think it is unusual for memories to be triggered in strange ways, and many of us have strange glitches in memory and recall, so I wouldn't regard the incident as one requiring a medical diagnosis, or even as a problem. The strongest memory links are those related to scents and emotions, so perhaps there was some subtle link that was not noticed by the conscious mind. Dbfirs 06:29, 18 April 2011 (UTC)

When I hear a recording of spoken word (I download a lot of podcasts, etc), and listen to it. I get a strong mental image of where I was last time I heard that segment of the recording. I can trace entire journeys by listening to the podcast again. This is the odd thing that my memory does.Zzubnik (talk) 14:16, 18 April 2011 (UTC)

See Involuntary memory. Alansplodge (talk) 22:11, 18 April 2011 (UTC)

UV and Air Disinfection

HI everyone, I understand UV disinfects air and clear solution such as water, but what if I have 5mL of water contained in a sealed 10mL vial. If the vial is exposed to germicidal UVC lamp for some time, does it mean that the air inside the vial and the water inside the vial become sterile. Considering the vial is clear and 1 ml thick. Thank you zak at <email removed> --Zkoulou (talk) 06:15, 18 April 2011 (UTC)

I assume you are talking about a vial you already have. Be aware that most glass is opaque to UV, so you will not be able to sterilize it that way. Ariel. (talk) 06:17, 18 April 2011 (UTC)

Transparent plastics are even better UV absorbers than the typical glass. Unless the vial was intentionally designed to be UV transparent, it is very unlikely that you can use UV light to sterilize the contents. (You can buy UV-transparent test tubes and cuvettes, if you are thinking about future applications.) If heating is okay, you might try placing the vial in a hot water bath (assuming the vial contains a water-based solution). If you do that though, you need to be sure it is only heated by the hot water to limit the temperature it is exposed to. If heated above its boiling point, a sealed vial may explode. Dragons flight (talk) 06:42, 18 April 2011 (UTC)

P.S. I removed the email address, per standard convention at the reference desk. Any replies will be posted here. Dragons flight (talk) 06:53, 18 April 2011 (UTC)

A few years ago I saw on a documentary that in India they put water in ordinary plastic botles and expose these to sunlight for some hours (I think more than 5 hours). The water was tested and it was found to be sterilized. So, perhaps, while the bottles are not very transparant to UV radiation, the small amount of UV radiation that still makes it into the bottle, is enough to sterilize the water over a period of several hours. Count Iblis (talk) 14:19, 18 April 2011 (UTC)

If exposure to solar UV were enough to sterilize things than the whole surface of the Earth ought to be sterile, including the rivers, oceans, grasslands, etc. whenever there is no cover from the sun. Perhaps sunlight was good enough to kill off certain pathogenic bacteria that they happened to be studying? I could imagine that might be plausible for many bacteria that thrive underground or inside hosts. However, many light-adapted bacteria actually benefit from such exposure. For the broader context of the question, I will note that near-UV, such as we receive from the sun, does partially penetrate plastic. By contrast, the hard-UV used in germicidal lamps will not penetrate plastic at all. Dragons flight (talk) 15:21, 18 April 2011 (UTC)

Yes, they were only looking at some specific pathogens, they grew some cultures using the original water and the water after it was exposed in that way. Count Iblis (talk) 15:36, 18 April 2011 (UTC)

I suspect that soda bottles left in the sun get very hot, which would go a long way towards sterilizing the contents. APL (talk) 19:00, 18 April 2011 (UTC)

Micro hydro and a home's power consumption

According to this, an average Vermont home uses 4,475 kWh of electricity per year. And according to our micro hydro article, these systems can generate up to 100 kW. So, how would I figure how much that 100 kW would cover of the needed 4,475 kWh? Dismas|^(talk) 10:07, 18 April 2011 (UTC)

You would multiply by the number of hours in a year:

100kW × 24h/day × 365 days = 876,000KWh

This should power 196 typical Vermont homes (876,000KWh/4,475 kWh). StuRat (talk) 10:25, 18 April 2011 (UTC)

Okay, that's what I thought but 196 homes doesn't seem very micro to me, so I thought I was doing something wrong. Thanks! Dismas|^(talk) 13:42, 18 April 2011 (UTC)

You might also be interested in peakers, which are also "micro-sized" power plants, for comparison. Peakers usually run on coal, diesel, or gas; they typically produce power ranging from hundreds of kilowatts to a few megawatts, and are considered "small" supplemental power stations to fill in the power supply/demand curve throughout a 24-hour period. Nimur (talk) 13:59, 18 April 2011 (UTC)

Peaker plants very seldom burn coal. Coal-fired plants generally take many hours to start up and shut down, and they don't scale down well. In some places you'll see coal plants that come on line to meet seasonal (often air conditioning) demand or to fill in when other (less smoggy) plants are down for maintenance, usually over many days, weeks, or months. In those circumstances, I'd consider them as serving to adjust the baseload supply, not acting as peaker plants. TenOfAllTrades(talk) 14:59, 18 April 2011 (UTC)

200 homes is definitely in the micro range. A typical hydro power plant can supply energy for several cities... Dauto (talk) 14:25, 18 April 2011 (UTC)

A limiting factor for microhydro is the elevation of the water source (termed the "head") and the flow. If you are in relatively flat land, there may not be enough head to generate much power. If the flow is not very much, ditto. If you wish to build a dam, there are extreme licensing requirements for the dam does not flood someone else's land, or bust and drown the people downstream. Hydro requires a great deal of maintenance to keep the trash rack clean. In extremely cold weather, there are problems with freezing. A dam also has environmental consequences. 100kw does not sound very "micro." Edison (talk) 19:35, 18 April 2011 (UTC)

Thanks all! I'm doing a small paper (2 pages, double spaced) for a quiz grade and was looking into alternative energy sources for houses. So, peaker plants are a bit out of the realm of what I was looking at. As is micro hydro now that I see it provides so much power! Dismas|^(talk) 03:37, 19 April 2011 (UTC)

galileo's theory

galileo was punished by the church for teaching that the sun is stationary and earth move around it. his opponents held the view that the earth is stationary and the sun moves around it. if the absolute motion has no meaning,are the two viewpoints not equally correct or equally wrong? — Preceding unsigned comment added by Himani gill (talk • contribs) 11:45, 18 April 2011 (UTC)

Himani gill (talk) 11:48, 18 April 2011 (UTC)helpme

Neither are truly stationary (which is a concept that has no real meaning in a relativistic universe), and either could be used as a reference point, yes. However the Copernican model was "less wrong" with regard to the relative motion of the other stars than the Tychonian model the Church preferred at that point (but there was no actual observational evidence of that until much later). But it should be noted that it was not just statements about nature that caused Galileo to be punished — it was more a question of philosophy and politics that led to his particular outcome. --Mr.98 (talk) 11:53, 18 April 2011 (UTC)

It should also be noted that the condemnation of Galileo was not universal even within the church; many important cardinals privately held that Galileo was probably correct technically, but felt that the information would "confuse" common believers as to whom to believe. Some cardinals were openly in support of Galileo, Cardinal Caesar Baronius is quoted as saying "The bible tells us how to go to heavan, not how the heavens go." Copernicus was also a priest, and so it should not be clear that every religious person rejected the notion of Heliocentrism. However, as with many such organizations with diverse opinions, the stodgy, established, conservatives won out, which is why Galileo was ultimately censored and persecuted. Within a generation or two after his death, however, few were seriously questioning his conclusions. It was just a case (as it usually is) for the stodgy old establishment to simply die off before the more revolutionary (but ultimately more correct) ideas become established thinking. --Jayron32 13:06, 18 April 2011 (UTC)

Again, it's not so much the ideas that were the real problem. It had to do more with 17th century politics than anything else. Galileo was purposefully trying to provoke the Vatican, purposefully trying to undermine their authority in a number of ways. I'm not defending the Vatican, obviously, but it's not the case that they were just being stodgy and mean or backwards. They had some very, very talented astronomers and mathematicians working for them. In any case, Galileo did not actually have any great reason to prefer the Copernican model over the Tychonian one — both accorded perfectly with observations at that time. It was, at its purest (e.g. least political), a philosophical dispute, not a scientific one. Giorgio de Santillana's The Crime of Galileo (1955) is the classic reference here; Rivka Feldhay's Galileo and the Church (1995) is a somewhat more recent spin on it. --Mr.98 (talk) 17:43, 18 April 2011 (UTC)

Also, motion is relative but acceleration isn't and if we forget about general relativity for a moment The sun's acceleration is much smaller than the earth's acceleration which means the heliocentric model is the correct one. Dauto (talk) 14:20, 18 April 2011 (UTC)

It is mathematically consistant to create a set of comoving coordinates which hold the earth to be stationary while the universe spins around us during the course of a day. It is also non-intuitive to do so; by convention it makes more sense to hold that, when two bodies are in relative motion, the smaller (the Earth) is moving while the larger (everything else in the universe) is stationary. This is also not true, but (for example) it isn't useful to consider the movement of the Milky Way galaxy relative to the Local Group when I am, say, calculating my driving time on my trip to the grocery store. We use whatever conventions regarding "moving" and "stationary" to make intuitive sense (i.e is most conventient), even if choosing what to make "moving" and what to make "stationary" is mathematically arbitrary. In other words, since a) it doesn't matter what we choose and b) we still have to choose something to be stationary, we choose that which makes the most sense for the particular situation we are working in. --Jayron32 14:28, 18 April 2011 (UTC)

Just to add that the Sun and Earth orbit around their common barycentre (ignoring the gravitational influence of other bodies). See orbit for a full explanation.--Shantavira|^{feed me} 15:12, 18 April 2011 (UTC)

And I believe that barycenter is actually beneath the surface of the Sun, so the "most correct" way to view it is that the Earth orbits the Sun, and causes it to wobble a bit as it does. (One two-body system not like this is Pluto and Charon, where the barycenter is between them, so Charon is not so much a moon, as a twin, of Pluto.) Now, if the Earth and Sun were the only objects in the universe, there would be virtually no difference in a model that had the Sun orbiting the Earth. However, the other objects lock us into the true model, since the orbital observations only make sense when all the planets, including Earth, are modeled as orbiting the Sun. The apparent retrograde motion of some planets is the relevant observation, in this case. StuRat (talk) 17:03, 18 April 2011 (UTC)

Not only is it beneath the surface of the sun, it is much closer to the absolute center of the sun than it is to the surface. --Jayron32 17:09, 18 April 2011 (UTC)

Note: You could theoretically have a model where the Sun still orbits the Earth and yet all the other planets do orbit the Sun, and this would closely match observed orbital motions. As far as I know this was never proposed. I wonder why not, as it would seem to both explain observations of the day and be more consistent with Church teachings. This model would have only broken down once the relative sizes and masses of the Sun and Earth were known, although, even then, I suppose you could just say "the Earth doesn't move because God holds it still". StuRat (talk) 17:15, 18 April 2011 (UTC)

It was proposed and it was the theory the Church was advocating at the point of the Galileo incident (not the Ptolemaic model, which was completely incompatible with the phase of Venus observation, as the Church's astronomers did admit fairly quickly). See: Tychonic system. --Mr.98 (talk) 17:43, 18 April 2011 (UTC)

Hand shaking

Hey all. I find that when I'm nervous, especially (oddly enough) when I'm playing chess, my hands become cold and shake. How can I prevent this? Thanks. 72.128.95.0 (talk) 16:35, 18 April 2011 (UTC)

Relaxation techniques ? You might want to cut back on caffeine or other stimulants you may be taking, such as some diet pills. Also, if chess makes you that nervous, you might want to find something more relaxing. (I find a short timer makes me nervous, so maybe you just need to play longer games, perhaps even correspondence chess.) StuRat (talk) 16:41, 18 April 2011 (UTC)

Probably you get overexcited, you need to find a way to calm down. Technicaly, I think it is tremor, maybe reading the article will give you some ideas (note though that unles your hands shake all the time, you probably don`t have any serious condition that article might mention as cause) ~~Xil (talk) 19:17, 18 April 2011 (UTC)

Stress balls can also work. Count Iblis (talk) 21:08, 18 April 2011 (UTC)

Shake your hands at the same frequency and amplitute, 180 degrees out of phase, the superimposition of the waves should cancel, leaving your hands motionless. Plasmic Physics (talk) 06:56, 19 April 2011 (UTC)

Yes, if the laws of physics don`t kick in, it might confuse brain and senses enough to stop, thinking that they are still shaking ;) alternatively - shaking hands on purpose acctualy seems to make blood flow to them, warming them up and probably will make your body preocupied with something other than the game for a while ~~Xil (talk) 08:18, 19 April 2011 (UTC)

UV TEA laser

In a uv TEA laser, what sort of electric discharge between the electrodes is best/necessary?. ie should it be a brush discharge, glow discharge or some other sort of discharge? How would the desired discharge be achieved practically with regard to shaping/placing of the electrodes? Not home work. Just interested in maybe making one.--92.28.80.252 (talk) 17:24, 18 April 2011 (UTC)

We have repeatedly had such questions about home-brew lasers on the desk. I will repeat verbatim a quote from last month: "Avoid using Class III and Class IV lasers in your home!" Guys, this is not something to play around with. Powerful lasers are very dangerous. UV laser light is invisible and can permanently injure and blind you. If you are interested in lasers, get in touch with an optics research laboratory and learn how to work with lasers under proper, controlled conditions. Unlike other dangerous hobbies, such as playing with firearms and explosives, UV laser damage is both invisible and instantaneous. Nimur (talk) 17:30, 18 April 2011 (UTC)

OK Im not going to make one. Im just interested in the theory of how they work.--92.28.80.252 (talk) 17:46, 18 April 2011 (UTC)

A question of which I do not now how to title

Relative motion of the Earth

I heard a demonstration that said during the time when it was first proposed that the earth circles the sun, which many did not believe; so they gave an example to try to disprove this new theory. (Considering the earth moves 30 km/s) "Then consider a tree with a bird, and under the bird is a worm. The bird swoops down and tries to snatch the wrom. So if the bird takes a second to get down, then it is assumed that would have moved 30 km away from the tree and the worm by time it gets to the ground." How would counter that? Bugboy52.4 ¦ =-= 18:09, 18 April 2011 (UTC)

The argument is the same as saying that if you're in a car driving down the road, and you reach for the steering wheel, you won't be able to touch it because it will have moved a hundred feet down the road while you are reaching. Looie496 (talk) 18:12, 18 April 2011 (UTC)

In fact, Galilean relativity was invented by none other than... Galileo (!) to express the mathematics of relative motion. Briefly, this formulation provides exactly the "counter-argument" that you're looking for. Galileo used the analogy of a ship - the sailors are co-moving with the ship. Nimur (talk) 18:17, 18 April 2011 (UTC)

But why does the bird move with the earth. If I was on top of a moving car, why don't I fall of? If I'm in the car, then thats because the seat stops me. Bugboy52.4 ¦ =-= 18:21, 18 April 2011 (UTC)

The seat isn't "pushing" you forward, except during the time while the car is accelerating forward. After it reaches its final speed, you and the car travel at the same velocity. You and the car both have inertia, and travel with the same velocity, so there is no net horizontal force from the car acting on you. If the car changes speed, you will either slam back in your seat, or slam forward into the seatbelt. If the car doesn't change speed, you don't change speed either, and the net effect of the contact force against the seat-back is zero. Nimur (talk) 18:29, 18 April 2011 (UTC)

Nowadays I'd refer you to Newton's Laws of Motion. An object in motion stays in motion. The bird would keep moving with the earth simply because nothing stops it from doing so. Galileo seems to have covered it in his analogy of the a ship by pointing out that if you jump on a ship you don't go flying backwards, you just go up and down relative to the deck just like on solid ground.

Once your car is moving at a constant speed you don't actually need the seat-back to keep pushing you forward. You could balance easily. (However, when the car CHANGES speed or direction the seat and the seatbelt will speed you up or slow you down with the car.) APL (talk) 18:34, 18 April 2011 (UTC)

So do you need to be in contact, what if I was floating? Or if I tried playing catchbetween two different cars? Bugboy52.4 ¦ =-= 18:39, 18 April 2011 (UTC)

You don't need to be in contact. Space shuttle astronauts are traveling at tremendous speed but they just sort of float around lazily. If you were to fire the shuttle's engines while they were floating, then the shuttle would speed up and the astronauts wouldn't. Result : Astronauts smashing against the back wall. That's why they have to sit in their seats and buckle-up before they maneuver the ship for any reason.

Playing catch between two cars would work fine on the moon where there's no air. However, here on Earth something as light-weight as a ball would probably get slowed down too much by the air. (The air outside your car is still standing still, even if you're driving through it at 55mph.) APL (talk) 18:58, 18 April 2011 (UTC)

Yup, you might be able to get away with it if you are going slower though since air resistance increases in an exponential fasion (at least in the subsonic realm). Googlemeister (talk) 19:00, 18 April 2011 (UTC)

One way I like to think about this — and if this is entirely wrongheaded, someone please correct me — is that you notice when you are in a reference frame when you come up against another reference frame. So when you're inside the car, you're surrounded only by things in your reference frame (including the air you are breathing), thus if you go at a constant speed, it doesn't feel like you're accelerating at all (and you go along with the car just the same as if it were sitting still). If you open a window, suddenly air from another reference frame is swooping in at you, and you know you're traveling through the air (or, alternatively, that the air is traveling towards you while you are sitting still). If you were on top of the car, you'd be in constant contact with the reference frame and know it quite acutely that you were traveling through a medium. If you're inside the space shuttle, you're not going to notice it moving very much at all if it is constant (and the air surrounding you is moving with it). Step outside the space shuttle and you'll be a lot more aware (especially if you let go!). Similarly, the bird is surrounded not by empty space, but by oxygen, all of which is rotating with the Earth and going around the Sun and throughout the universe etc. as well. You'd have to get to a vantage point where you were really outside of the Earth's reference frame (e.g. in space) before you'd start to worry about its speed of rotation, the fact that it is going around the Sun, etc. — you'd need contact with another inertial frame before it was acute to you that you were in an inertial frame. (You can tell if you are in an accelerating frame; not if you are an inertial one.) --Mr.98 (talk) 19:12, 18 April 2011 (UTC)

You are confusing reference frame with atmosphere.

If there was no air on Earth the bird would still continue to move with the Earth.

If you step outside the space shuttle you continue to move with the space shuttle exactly as if you were still inside.

If you are outside the space shuttle holding onto a hand-hold and then you let go, you'll stay with the ship and be able to grab the hand-hold an hour later with no problem. (Assuming you don't push-off. then you'll go flying of course.) APL (talk) 19:36, 18 April 2011 (UTC)

Yes, that's true. If we imagine a bird on the moon, the bird is moving at the same speed (more or less) as the moon itself as it glides around the Earth (and around the Sun, and the Milky Way, etc.). It's not the air itself that matters, it's the fact that it's not in contact with anything moving in another reference frame. In the Earth examples, the air is still in your reference frame. But it isn't the air that matters, per se. --Mr.98 (talk) 18:44, 19 April 2011 (UTC)

You're dead on 98. actually Galileo's ship analogy is a poor one since the air outside the ship is not moving with the ship, and a sailor who could stay in the air a little longer would be left behind.190.56.107.69 (talk) 19:21, 18 April 2011 (UTC)

The ship analogy specifies that the sailor is below decks. APL (talk) 19:37, 18 April 2011 (UTC)

And, though I assumed most readers would know it, for the record we should state that Galilean theory has been superseded by Newtonian mechanics, and later by Einstein's formulation of relativity. Both of those provide more complete and accurate explanations. Nimur (talk) 21:10, 18 April 2011 (UTC)

I added a more useful (sub)title. StuRat (talk) 19:33, 18 April 2011 (UTC)

If I was holding on to a rocket and I would let go, wouldn't I get left behind considering its moving? Bugboy52.4 ¦ =-= 19:57, 18 April 2011 (UTC)

First of all, If you're on Earth then gravity and air complicate the question, so lets assume this is happening in space.

If the rocket is firing, then it is speeding up.

If the rocket is speeding up, then you need to hold on or you'll be left behind.

When the rocket engine stops, the rocket will still be coasting at a very fast speed, but it won't be speeding up anymore.

If it's not speeding up, then you can safely let go. Both you and the rocket will continue to drift in the same direction at the same speed forever. Unless you push off from the rocket, you'd be right next to it forever.

Hope this helps. APL (talk) 20:11, 18 April 2011 (UTC)

And again, it's worth emphasizing emphatically that the question is whether you are accelerating (speeding up or changing direction) or not. If you are not accelerating (in an inertial reference frame), then you're fine — you and the rocket are by definition going at the same speed in the same direction. If you are accelerating (turning or changing speed), then you're not in an inertial reference frame at all, but an accelerating frame, and all bets are off! --Mr.98 (talk) 18:44, 19 April 2011 (UTC)

A classic thought-experiment to help unpick this is to consider a fly in a moving car. When you drive at 10 mph, the fly can fly from the back of the car to the front in a few seconds. When you drive at 70 mph, the fly can still fly from the back to the front in a few seconds. Why? Does the fly have the ability to fly faster than 70 mph? But then, the fly is no quicker flying front to back: when the car travels at a steady speed, it doesn't matter to the fly how fast the car is travelling relative to the outside world, because everything in the car, including the air and the fly, are moving with it. The fly only has problems when the car speeds up or slows down: when the car accelerates, the fly has to catch up. It can do so simply by landing on part of the car, because the acceleration is not typically great enough to overcome the simple ability of a fly to stand on something. 86.164.75.102 (talk) 23:20, 18 April 2011 (UTC)

Might be worth mentioning that objects of different mass, such as a rocket ship and a person "fall" at the same rate. The only thing that makes this seem like not the case is wind resistance. On the moon, a feather and an anvil would hit the ground at the same time if dropped from the same height. Secondly, falling due to gravity is not affected by velocity perpendicular to the "ground". For example, a bullet fired horizontally will hit the ground at exactly the same time as something simply dropped from the same height, it will just be a lot further away. (again ignoring wind resistance). Vespine (talk) 00:15, 19 April 2011 (UTC)

Another query: Storage of Information

The only two subjects that have rely left me confused are those on the brain and computer chips (I have studied even quantum mechanics, but that's not as hard as these, at least for me). How is information stored in these two substrates: How does the brain store such information as memory, and how does a computer do the same? Does it store this information in the form of matter or energy? Bugboy52.4 ¦ =-= 20:07, 18 April 2011 (UTC)

For computer ram see Dynamic_random_access_memory#Operation_principle.

For computer storage see Hard_disk_drive#Magnetic_recording.

The mechanisms of the brain are not entirely known yet. It looks like short term memory is stored electrically, but long-term memory seems to involve actually re-wiring neurons. But there's a summary here : Long-term_memory#Biological_underpinnings_at_the_cellular_level

Hope this helps. APL (talk) 20:16, 18 April 2011 (UTC)

There's also Neuroanatomy of memory for a higher level look at the nuts and bolts. Clarityfiend (talk) 00:57, 19 April 2011 (UTC)

You may find Magnetic_core_memory easier to grasp than its modern analogues. Information is quite literally stored by the physical configuration of little magnetic donuts. Note that the analogy between computer memory and human memory is quite weak. SemanticMantis (talk) 20:20, 18 April 2011 (UTC)

How is the magnetic core memory any more physical than a hard drive memory? Dauto (talk) 20:46, 18 April 2011 (UTC)

You can see the physical location of bits in iron-donut core-memory without a microscope; but you can't actually see the bits, which exist as the presence or non-presence of an invisible magnetic field. Most modern electronics rely on "invisible" principles of physics (like voltage and current), and therefore require a little abstract thought. Very modern electronics are ultra-miniature, adding another level of confounding abstraction. Nimur (talk) 21:06, 18 April 2011 (UTC)

In other words, by more physical you actually mean more visual? Interesting... Dauto (talk) 21:13, 18 April 2011 (UTC)

Nimur was not the one who made the "physical" comment. APL (talk) 21:24, 18 April 2011 (UTC)

I know. But he answered my question with a very puzzling explanation. Dauto (talk) 21:33, 18 April 2011 (UTC)

Nimur has answered basically how I would have. I was trying to help the OP by giving an example that I think is easier to understand than modern RAM, in part because of its macroscopic nature. I didn't mean to imply that the physical position of the cores changed when the bits they encode change. If the OP is interested in cases where information is stored by physical configurations, compact discs and vinyl records are some common examples, and Mercury_memory is a type of physical memory that has used by digital computers when RAM technologies were not readily available. SemanticMantis (talk) 21:49, 18 April 2011 (UTC)

Tachyons. —Preceding unsigned comment added by 165.212.189.187 (talk) 13:21, 19 April 2011 (UTC)

transmutation

Would bombarding the nucleus with alpha particles move the element up the table of elements and would high voltage electricity have a effect on this? —Preceding unsigned comment added by 82.38.96.241 (talk) 20:52, 18 April 2011 (UTC)

It can. Neither alpha particle nor nuclear transmutation goes into it much, but for example californium is made that way from curium, which is made that way from plutonium. The high voltage electricity might be used to deflect alpha particles in some desired direction, or to power the magnets of a cyclotron; indirectly it gives the particles the kinetic energy needed to react successfully. Wnt (talk) 22:06, 18 April 2011 (UTC)

The OP may want to do some research into the work of Glenn Seaborg. The Wikipedia article and section Glenn_Seaborg#Pioneering_work_in_nuclear_chemistry hints on some of his work, but it roughly amounts to smashing alpha particles into large nuclei and hoping they stick. --Jayron32 00:29, 19 April 2011 (UTC)

Bouncy bicycles

Pneumatic bicycle tyres have less rolling resistance than solid tyres, both according to what I saw on a pop science program on TV, and from personal experience when I once used un-puncturable solid tyres which required much more effort to pedal.

Bearing that in mind, would it require less effort to pedal my bicycle if I replaced the seat post with a springy suspension seat post? I imagine that less energy would be wasted moving my body up and down over rough surfaces. Thanks. 92.29.122.67 (talk) 20:54, 18 April 2011 (UTC)

It depends on the type of terrain you're on. Typically, road-bikes are rigid; mountain bikes have suspensions, and we have an entire article on bike suspensions discussing all sorts of detail. Nimur (talk) 21:00, 18 April 2011 (UTC)

The reason the terrain matters is that, depending on the magnitude and wavelength of the bumps, a given fixed suspension may either reduce or increase motion of the passenger. So, with anything less than an active suspension system, you take your chances. In the case of a bicycle, I suspect that the weight of the suspension system would be more of an energy drain than you would expect to ever get back. I'd also be skeptical on saving energy, since your torso will then move up and down relative to your feet on the pedals, and your hands on the handlebars. This will require muscle movements to adjust for the constantly changing position. So then, why do mountain bikes have suspensions ? Mainly to reduce injuries, I suspect. StuRat (talk) 22:12, 18 April 2011 (UTC)

The information here about bicycle suspension suggests that a suspension seatpost would probably be an improvement, particularly as I recall how much hard work and effort solid tyres required to pedal. 92.15.15.88 (talk) 08:33, 19 April 2011 (UTC)

Ethanol Efficiency.

An unsourced article stated that if the total energy imput was calculated to buy,plant,water,fertilize,harvest,process etc to produce a gallon of ethanol, it would exceed the energy available in that gallon produced.It did not take into account the associated loss of food production it replaced.Would someone care to comment? John Cowell.124.186.141.14 (talk) 22:33, 18 April 2011 (UTC)

Please don't use this forum as a discussion site. If you have a question, please ask it. Are you wondering whether the claims are accurate? Looie496 (talk) 22:52, 18 April 2011 (UTC)

YES! —Preceding unsigned comment added by 124.186.141.14 (talk) 23:00, 18 April 2011 (UTC)

It depends on how that ethanol is produced. Ethanol from corn kernels seems like it might fall into that category. Other sources, like sugar beets, seem better. Even better yet is if normal waste products or low value crops could be turned into ethanol, such as silage. The corn kernel, for example, is only a tiny portion of the total corn stalk, and is also useful as food, so using it to make ethanol in not wise. As far as whether it's good for the environment to use ethanol, other factors also matter, like whether fossil fuels are used to power the tractors and other equipment, or if they can be powered from more Earth-friendly technologies. StuRat (talk) 23:04, 18 April 2011 (UTC)

The answer you seek is here. Even if what you said was true, which was probably the case not that long ago, that's not an argument to "not pursue" a line of research. Lots of technology starts off as "un-viable" and requires a lot of R&D to become commercially useful. Just look at fusion power research, at the moment it's not creating ANY net energy at all, but there are still groups researching it in hope that one day it might become viable. Vespine (talk) 23:14, 18 April 2011 (UTC)

I certainly agree that research should continue, but the current issue, at least in the US, is that the government is subsidizing ethanol production from corn kernels, which seems unwise, since it's such an inefficient process. Perhaps those funds should instead be used to further research into better ways to make ethanol. To continue with your analogy, would it make sense for the government to subsidize large, production scale, fusion reactors, even if they used more energy than they produced ? StuRat (talk) 19:08, 19 April 2011 (UTC)

Ethanol fuel energy balance (linked from the ethanol fuel article cited above) looks like a decent stab at the topic. Wnt (talk) 03:17, 19 April 2011 (UTC)

The corn grown here is not used to make food, at least not directly. It is fed to cattle and hogs, or turned into ethonol. Losing out on food is a tiny problem in the USA. Googlemeister (talk) 15:10, 19 April 2011 (UTC)

Using corn for ethanol can affect food prices in many ways:

1) Corn prices go up.

2) Products containing corn products (like corn syrup) go up to.

3) Animals fed corn meal go up.

4) If land used for other crops is then switched to corn production, the lower supply of those other crops causes their prices to rise.

The result is that, if all our cars were 100% (corn kernel) ethanol powered, our food prices would skyrocket. So, we need to find a better way to make ethanol or give up on that goal. StuRat (talk) 19:00, 19 April 2011 (UTC)

We should avoid getting into a forum, but to ask a question: why does the U.S. have to put up with a 1.3-fold efficiency when Brazil gets an 8-fold efficiency? Do we really have no where to grow sugar cane? So much land in Louisiana and other Gulf states is being lost to rising seas and slumps over depleted oil wells and loss of sediment to various construction-related causes, seemingly without opposition. (Just look at the online maps of those states versus the actual satellite photo) Why isn't it cost effective to rescue some of the deluged land Holland-style and grow sugar cane on it for fuel? Wnt (talk) 19:45, 19 April 2011 (UTC)

Binding energy

In our article section Nuclear fission#Origin of the active energy and the curve of binding energy, it says the specific binding energy of elements like nickel and iron is greater than the specific binding energy of uranium. Doesn't it mean to say that it's lower, and that's where the potential energy difference lies? Comet Tuttle (talk) 23:03, 18 April 2011 (UTC)

The definitions provided in the binding energy article (referenced from the article you cite) suggest that the binding energy refers to the energy needed to dissociate the nuclear particles. Thus, if nickel and iron have higher binding energy, it will require more energy to separate nucleons. Doesn't that seem consistent? -- Scray (talk) 23:51, 18 April 2011 (UTC)

Hm. Yes, it makes sense, but I'm confused over where the energy comes from when a nuclear fission event occurs. If the specific binding energy (meaning "binding energy per particle") of a 238-particle nucleus is lower than the specific binding energies of the two fission products — suppose they're 38-particle and 200-particle nuclei, with no stray neutrons emitted — then it seems to me that the fission product nuclei have gained energy (per particle) as a result of the fission, which is the opposite of what occurs. Where's the defect in my understanding? Comet Tuttle (talk) 00:08, 19 April 2011 (UTC)

I am sure others have more updated nuclear physics knowledge than mine, but one thing I do recall that has always been true - always consider entropy, because it always wins. -- Scray (talk) 00:17, 19 April 2011 (UTC)

The article you linked indicates that electrostatic repulsion is the principal repulsive force in the nucleus. From a simplistic point of view, wouldn't that force provide substantial energy for fission? Here's a quote from farther up in the article you cited: "Once the nuclear lobes have been pushed to a critical distance, beyond which the short range strong force can no longer hold them together, the process of their separation proceeds from the energy of the (longer range) electromagnetic repulsion between the fragments. The result is two fission fragments moving away from each other, at high energy." -- Scray (talk) 00:27, 19 April 2011 (UTC)

About 80% of the total energy liberated in an individual fission reaction can be calculated just by taking into account the kinetic energy from the electromagnetic repulsion. The rest is from some of the spontaneous radioactive decay that occurs during the fissioning, instantaneous gamma rays produced in the fission, and the kinetic energy of the neutrons that pop out. --Mr.98 (talk) 15:35, 19 April 2011 (UTC)

(ec) I find it makes more sense when you don't think of binding energy as a "thing" but as the balance between the strong and electromagnetic forces. In uranium fission, you are taking a nuclei that is just on the line of being stable and wobbling it so that the electromagnetic forces suddenly outweigh the nuclear force (because the wobbling makes it act as if it were two distinct positively charged nuclei that happen to be right next to each other). You can calculate almost all of the fission energy by treating them as two very positively charged particles repelling from one another. That act of repulsion is the release of that binding energy, in a sense — in the same way that the springing of a spring is a release of stored mechanical energy. Looking at the "curve," you have a net energy release whenever you move up — whenever the binding energy per nucleon increases (on either side of the curve). --Mr.98 (talk) 00:22, 19 April 2011 (UTC)

The binding energy is always negative; elements in the middle of the periodic table have a "lower" per-nucleon binding energy inasmuch as −2 < −1, but a "higher" per-nucleon binding energy inasmuch as |−2| > |−1|. It's a linguistic inconsistency, not a physical one. -- BenRG (talk) 00:32, 19 April 2011 (UTC)

That's one of the problems I always have in this sort of discussion. Lower can mean both "more negative" or "smaller in magnitude". For example, a process which has a ΔH = -1000 KJ can be said to be more exothermic (release more energy) than one which has a ΔH = -500 KJ. However, we could also say that the first process has a lower enthalpy, being that -1000 is a lower number than -500. The trick is to be very precise in ones language; when we say that something has a higher binding energy, do we mean it has a greater magnitude of energy (say a larger absolute value), or do we mean that it has a more positive (i.e. farther to the right on the number line) value. It's a constant source of confusion, and you are correct that this is a linguistic problem that unfortunately has yet to be fully fixed. --Jayron32 00:37, 19 April 2011 (UTC)

You may find it helpful to think about the binding energy as being equivalent to (or at least precisely proportional to, depending on your preferred units) the mass defect of the given nucleus/nuclei. The mass defect is the difference in mass between the unbound, free, fully-separated nucleons (protons and neutrons) and the mass of the fully-assembled nucleus; the atomic nucleus will always have a slightly lower mass than the sum of the masses of its constituent particles. The mass defect is exactly equal to the total binding energy of the nucleus (by E=mc²), and it is the amount of energy that would be released if one were to assemble a nucleus 'from scratch'. As Jayron32 notes above, there can be some confusion about sign versus magnitude when comparing these values. It's important to realize that the binding energy isn't energy that a nucleus has or carries around with it. The binding energy is energy that was released when the nucleus was formed, and which you would have to put back in in order to break it up.

In the case of the fission of a heavy nucleus, we can make several equivalent statements:

• the total binding energy of the fission products will be greater than the binding energy of the initial nucleus;
• the binding energy per nucleon is higher in the products than in the initial nucleus (since we didn't change the number of nucleons);
• the combined mass defect of the products is greater than the mass defect of the original nucleus;
• the total mass of the products is less than the mass of the original nucleus; and
• the total energy (E=mc²) of the products is less than the total energy of the original nucleus.

Hope that helps. TenOfAllTrades(talk) 13:16, 19 April 2011 (UTC)

April 19

A few questions about the Japan earthquake...

1. How much radiation is in the sea water?
2. How did the radiation even get out of the nuclear power plant?
3. What types of radioactive material was in the water? (e.g. Was it radioactive metals, gases, or liquids? Did it have gamma rays, alpha rays, or that other kind of rays?)

PLEASE answer. — Preceding unsigned comment added by MickWithoutGlasses (talk • contribs) 00:45, 19 April 2011 (UTC)

Please read our article on the Fukushima I nuclear accidents.--Shantavira|^{feed me} 06:40, 19 April 2011 (UTC)

"System of Care" health care

Using Google I discovered that System of Care is a health care thing...www.systemofcare.org, which is exactly what I wanted to know...but I was hoping hat Wikipedia could have a page about System of Care. Trust me, Im not the guy to write the Wikipedia page. Thanks and I love Wikipedia! 99.188.102.172 (talk) 02:28, 19 April 2011 (UTC)

One definition given for system of care is "a framework within which health care is provided, comprising health care professionals; recipients, consumers, or patients; energy resources or dynamics; organizational and political contexts or frameworks; and processes or procedures. Current theory recognizes that an analysis of the provision of health care requires knowledge of the systems of care."

It looks like this is probably well covered by our article health care system --- Medical geneticist (talk) 16:05, 19 April 2011 (UTC)

Atomic number and qualities of the element

Perhaps I am misunderstanding this concept completely, but how can adding one proton to an atom completely change the qualities of the element? For example, Hydrogen has one proton and is extremely flammable and explosive while Helium has two protons and is completely inert. I do not understand how adding or subtracting a single proton can change so much about an element when it is seemingly such a small change. It seems very random that an atom with three protons would be a silvery metal while an atom with 29 protons would be a reddish-orange metal, and so on through all the elements. Is there a reason why the qualities of the elements change based on atomic number or is is just a mystery of nature? Any help understanding this would be greatly appreciated, thanks! Zipidiedoda (talk) 02:54, 19 April 2011 (UTC)

My basic understanding: if you change the atomic mass, you're actually going to also change the electron configuration. The things you're talking about (reactivity, etc.) are chemical functions that have to do with the electrons, not the nucleus. Things like color, the way in which the atoms fit together in a lattice, how they bond with other atoms (and release energy, etc.) are chemical reactions that depend on the electrons, not the nucleus. --Mr.98 (talk) 02:59, 19 April 2011 (UTC)

It's true that adding protons requires adding electrons, because the positive charge pulls them in. But what is mysterious is how orbitals work, which is tied to the strange science of quantum mechanics. It turns out that if you have one electron "circling close around" a nucleus, it can readily accept another that is spinning "in the opposite direction". So hydrogen will strive to pair up. Yet throw in a third electron, and there's no way for it to spin around in the same way - it gets stuck in the upper bunk, so to speak, and you have lithium, which is eager to get rid of it wherever it can. Now why can't three or four electrons circle just like two - it's complicated, and hard for me to understand in an intuitive way. It has to do with eigenvalues and the Schroedinger equation and the Pauli exclusion principle and all sorts of nasty math, which is derived not because it seemed like the logical way things ought to happen but because it fit the experimental data. Wnt (talk) 03:08, 19 April 2011 (UTC)

The problem is that the model of electrons existing as little balls orbiting a nucleus in various ways, and physically "spinning" actually doesn't work. The actual model is based a lot more on the wave-like properties of the electrons. There are two perspectives on the actual electron, both of which work perfectly well with the quantum mechanical model, and so we tend to think of them both working simultaneously. First, you can think of an electron as a complex standing wave (like the wave formed on a plucked guitar string as it vibrates). The orbitals around an atom represent harmonics of the fundemental wave. Secondly, you can think of electrons as existing as three-dimensional Probability distribution around the nucleus, indentifying the relative probability of finding a particular electron in a particular location after an abritrarily long time. After you make a scatter graph of all of the possible locations one could find that electron in the space around the nucleus, you basically get the shape of that 3D standing wave I just described above. This probability/wave definition of an electron is what the wavefunction and the Schroedinger equation is all about. The deal with the Pauli exclusion principle is that, if we define an electron in these terms, then we cannot have two electrons that have identical wave functions; two wave functions can only co-exist in the same space if they are orthogonal to each other; that is if the two waves cannot interfere in any way; much as two sine waves at right angles cannot interfere. This is where "electron spin" comes it; this is the property that introduces orthogonality to the wave function. It's not a real "spin" in that there is no little ball rotating on its axis; nor is there two "balls" orbiting in different directions. Its just a (somewhat arbitrary) name given to the part of the wavefunction that allows two electrons to coexist in the same physical space. --Jayron32 04:15, 19 April 2011 (UTC)

Hmmm. You say electron spin isn't really spin, but... when electron spin flips (relative to one spatial dimension), it is transmitted away by means of a photon with an angular momentum of hbar. The spin of this photon is a real thing, isn't it? A circular polarization that can be measured by passing it through filters. Am I wrong? Wnt (talk) 05:38, 19 April 2011 (UTC)

Sort of. Particle spin in physics is a real physical property of particles; it's just not actually spin. When you tease the property out of the particles, it behaves like angular momentum, and it has directionality (the terms "spin up" and "spin down" are real aspects of particle spin, relative to the z-axis). However, with point particles, its not clear how such a particle can physically spin. The name is an artifact of treating electrons like little balls; this is explained well at Spin_(physics)#Elementary_particles. So, it's called spin because mathematically it behaves like classical angular momentum, but there's nothing to spin in fundemental particles, since they have no internal structure to spin about. Spin is likely more related to some sort of fundemental directionality; the other aspects of a particle's fundemental properties (like charge and mass) make no directional distinction, spin uniquely does. It is through spin that we can establish a non-arbitrary set of axis to define the regions around an atom, for example. Still, though it is tempting, one must resist using this property of spin to treat electrons like little balls; they clearly don't behave that way. After all, quarks aren't really colored, and fundemental particles don't really taste like anything. --Jayron32 05:58, 19 April 2011 (UTC)

No—color has nothing to do with color, and flavor has nothing to do with flavor, but spin really is spin. Just as you can develop a theory of instantaneous velocity by taking the limit of the velocity over successively smaller time intervals, so you can develop a theory of "angular momentum at a point" by taking a limit of successively smaller spinning objects. The theory you get when you do that is the theory of particle spin. That doesn't mean that particles are spinning points. They might be very small spinning objects, or they might be Kerr–Newman black holes, or something else entirely. It's a mistake to take any aspect of the standard model of particle physics too literally. All we really know is that particles behave like they're spinning. -- BenRG (talk) 07:01, 19 April 2011 (UTC)

When I look at the figures in circularly polarized light, it certainly looks like something is really spinning. A photon should look like a brief segment of such a wave, trailing off at either end, moving forward like a corkscrew. Now I am still slightly hazy on the spin number of light - apparently it is hbar in two dimensions, adding up to sqrt(2) hbar - but my impression is that the spin of the light in the dimension of its travel should match this classically perceptible rotation. Question: can you actually look at the E and B fields of a circularly polarized photon and derive hbar as the angular momentum of the energy it carries?? Wnt (talk) 07:40, 19 April 2011 (UTC)

The deal with particle spin, which I guess by BenRGs reasoning is that it is OK to call it a spin (by a certain perspective) is that it doesn't interact with other, macroscopic angular momenta, outside of the "particle spin" mode. Think about a molecule like triplet oxygen. Triplet oxygen has "particle spin". This is because, as a diradical, it has unpaired electrons. This particle spin generates a magnetic field (as all spinning charges, point particle or macroscopic) should. This creates the property of paramagnetism in groundstate dioxygen. However, the oxygen molecule itself also has a rotational axis (two in fact) perpendicular to the bonding axis of the molecule. Being a non-spherical molecule, that means it has an axis about which it can rotate. However, I am not aware of any way that the particle spin has any effect on the rotation of the molecule; I am willing to accept that I am wrong on this, but it is my understanding that the "particle spin" which accounts for dioxygen's paramagnetic nature does not increase the molecule's rotational energy, which is pretty much exactly what spin is. If we take the situation to a classical analogy; we could look at dioxygen like a helicopter. The existance of angular momentum of the helicopter blades tends to introduce a torque into the helicopter, that's why there needs to be some counteracting force in the form of some stabilizing counterrotation of some sort. Likewise, if the "spin" of oxygen's electrons were the same sort of thing, it would only stand to reason that this would cause the molecule itself to rotate. The same should happen in ANY radical molecule, such as Nitrogen dioxide. I am not aware of such an effect, though I am open to an explanation of either a) why my reasoning is wrong or b) where such an effect actually occurs. --Jayron32 13:24, 19 April 2011 (UTC)

I'm really on the wrong side of the bar here, but to quote [15], "The energy of a molecule can be approximated as the sum of the contributions from its different modes of motion (translation, rotation, and vibration), the distribution of electrons, and the electronic and nuclear spin. Given that the energy is the sum of independent contributions, ... the partition function is a product of contributions: q = q^T q^R q^V q^E q^S where T denotes translation, R rotation, V vibration, E the electronic contribution, and S the spin contribution. The contribution from electronic spin is important in atoms or molecules containing unpaired electrons. For example... the Cs atom, which has one unpaired electron ... contributes a factor of two to the molecular partition function."

Now to what degree I understand it, this means that the amount of entropy, at least, is increased by the spin (it doesn't really increase the molecule's energy in the sense that the spin can't drop to zero, though pairing spins is welcomed in chemical bonding). The molecules with electron spins are free to move energy back and forth between those and rotational spins, and since angular momentum is always conserved I assume that means that the direction of the electron vs. molecular spin matters. So I really would think of electron spin as a "real" spin, contributing to the overall motions going on within the molecule. Wnt (talk) 19:38, 19 April 2011 (UTC)

That quantum spin can couple arbitrarily to other quantum numbers is unsurprising. This sort of thing happens between all of those various contributions. More telling in your description is that the rotation of the molecule and the spin of the molecule are treated differently; that is the quantum rotation (that of the entire molecule spinning) is an independent value from the quantum spin value (that calculating by summing all of the individual electron spins). To me this tells me that there's no special physical relationship between those energies, over that one would expect from any coupled modes of energy, such as vibration and electronic. In other words, spin and rotation are unique properties, and though each is classified in terms of "angular momentum" they actually measure different things about a molecules. --Jayron32 19:54, 19 April 2011 (UTC)

Basic answer (as oultined above) is that chemical properties are determined by electrons, not by nuclei and nuclear processes. So it is not adding or subtracting protons (or neutrons) that changes the chemical behaviour of an element - it is the associated addition or subtraction of electrons. Example - sodium chloride has chemical properties very different from either sodium or chlorine, yet its atomic nuclei are standard sodium and chlorine nuclei; its chemical properties are due to the transfer of electrons and the resulting creation of ionic bonds. Gandalf61 (talk) 11:03, 19 April 2011 (UTC)

Chemical properties are primarily determined by electrons (especially in covalent compounds), but those electrons are certainly affected by the protons (especially in ionic compounds). For example, they are what actually causes the ionic charge once the electrons have been added or subtracted to get a closed shell. "Why is it +2?" Because it has 2 more protons than electrons. Why two? Because that's how many electrons it lost to get to a stable electronic configuration. But why did it have "two more than a stable electronic configuration" when it was neutral? Because that's how many protons it had. And isotope effects are real chemical-reactivity differences due solely to mass not atomic number or charge. DMacks (talk) 12:50, 19 April 2011 (UTC)

Why only 92

Since we know of only 92 naturally occuring elements, the other 17 transuranics (maybe more now) being man made, and since we know about the enormous pressures, temperatures and densities that exist in many types of stars, it seems inconceivable that mother cosmos has not been able to make any elements with a greater atomic mass than U238. questions. (1) Has there ever been any observed indications of natural elements heavier than U238

(2) If the answer to (1)is no, then does this indicate some naturally limiting factor. If yes, then what is it.

(3) If "no" again then what might that limiting factor be.Phalcor (talk) 03:02, 19 April 2011 (UTC)

Neptunium (239, I assume) is actually produced in trace amounts in natural uranium ores. (On the other hand, technetium was synthesized). See nuclear shell model - nay, rather Magic number (physics) - for a diagram of how the largest known nuclear shell marks the end of the stable sequence at bismuth. Some believe that an island of stability of super heavy elements exists, but the number of neutrons for the best isotopes keeps going up, making it very difficult even for supernovae to make them out of smaller nuclei with fewer neutrons. Wnt (talk) 03:13, 19 April 2011 (UTC)

(edit conflict) (1): Yes, there have been observed natural elements heavier than U238: Plutonium is the heaviest Primordial nuclide, and is found in extremely small quantities in nature left over from the Earth's formation. It has also been formed more recently in the Natural nuclear fission reactor of Gabon, though again, has almost completely decayed by now. Other, heavier elements were (and are) created in stars, but are not found on Earth only because they've decayed away in the 4.5 billion years since Earth's formation. Buddy431 (talk) 03:16, 19 April 2011 (UTC)

But neptunium has amu of237.0482 less than U238 with amu 238.03 and plutonium exists in nature only as an isotope of U238. the Element plutonium is greater amu 244.0 but is man made as you said at gabon and other places and was used in (fat boy) over Nagasaki. If other heavier elements have existed and decayed then there would be a non-radioactive element left in it's place, such as uranium to lead. If I understand it correctly they do not simply disappesr. correct my if I'm wrong but all the elements 93 to 109 are all man made I'm asking about natural elements above 92 —Preceding unsigned comment added by 190.56.115.96 (talk) 04:32, 19 April 2011 (UTC)

Plutonium-244 is created by natural processes, and has an atomic number greater than 92. --Jayron32 04:40, 19 April 2011 (UTC)

(edit conflict with Jayron) Stars produce many elements above 92, but uranium is the heaviest element that decays slowly enough that it's still here on Earth after 4.5 billion years. The reason heavier elements decay faster is well answered by Wnt. In short, there's nothing special about U-238 is terms of being produced by nature, it's the decay times that determine whether it still occurs naturally on Earth. U-238 decays slowly enough that there's still a lot of it after 4 billion years. Pu-244 decays slowly enough that there's a tiny amount of it after 4 billion years. Buddy431 (talk) 04:44, 19 April 2011 (UTC)

Thanks guys I just checked out "island of stability" Guess I should have done that earlier Huh. Boy do I feel updated, or dated or something. Yeh I'm gonna have to throw some books away and graduate to the computor.190.56.14.159 (talk) 05:32, 19 April 2011 (UTC)

is there a formula from wavelength in nanometres to RGB?

Suppose I have a sharp band of 632.4 nm light. What RGB value should I use to represent it? John Riemann Soong (talk) 06:03, 19 April 2011 (UTC)

Depends how accurate you need it, if it's just rough, my guess is about 255,50,0. I'm seeing a few conflicting sources, some saying it's roughly red, some saying it's more orange. Vespine (talk) 06:38, 19 April 2011 (UTC)

Actually just found a site that does it, it says 255, 70, 0.. Hey! My guess was pretty good:) lol. Vespine (talk) 06:41, 19 April 2011 (UTC)

Because the RGB receptors in the retina don't match the RGB mapping of your screen (they actually use a different system, and vary from person to person), you will never get a "perfect match" for everyone. Dbfirs 07:14, 19 April 2011 (UTC)

I think that applet is somewhat inaccurate, as it claims to use this code, which links to this page, which displays this chromaticity diagram generated using the algorithm, which is rather different from this chromaticity diagram, which I trust more. You could use an eyedropper tool on the latter diagram to get a better answer. There is actually no correct sRGB value for any monochromatic color; you have to desaturate the color (mix it with white) to get something in the sRGB range. And color perception does vary somewhat from person to person. -- BenRG (talk) 07:34, 19 April 2011 (UTC)

I agree with previous replies; there is no single "correct" relationship. A lot of thought has been put into finding good approximations, see e.g. this article. (I had once seen a nicer one with the pictures directly in the article, but I can't find that one just now.) — Sebastian 17:56, 19 April 2011 (UTC)

electrodynamics

what is the physical significance of displacement current? — Preceding unsigned comment added by Karan khajuria (talk • contribs) 11:03, 19 April 2011 (UTC)

It produces a magnetic field just like an actual alternating current would produce. Dauto (talk) 15:17, 19 April 2011 (UTC)

Chickens in a container

I recently heard something on TV which has me puzzled. It was talking about a sealed shipping container containing chickens on a weighbridge. The show's presenter said that if you could get all the chickens to fly into the air at the same monent, it would not affect the measured weight of the container on the weighbridge. I sometimes fleetingly think I understand it, but can anyone explain, please? Si1965 (talk) 12:19, 19 April 2011 (UTC)

Mythbusters tackled this one, but I can't remember what they found. HiLo48 (talk) 12:27, 19 April 2011 (UTC)

See here. --Mr.98 (talk) 17:27, 19 April 2011 (UTC)

Flight works by increasing air pressure beneath the bird (and decreasing it above), which spreads out as a roughly cone-shaped region above and below it. This applies forces to the floor and roof of the container which add up to the weight of the birds. So, the overall weight of the container doesn't change. Now, if it was open at the top and bottom (say with gratings), then the air pressure would go down below the container, and apply to whatever was below it. If the bridge also had a grating form, then the air pressure change would continue to spread down and out, eventually being supported by the ground. The same is true, in reverse, for the decrease in air pressure above the birds. So, there could be a tiny difference when the birds were all in flight. However, note that not very many flying chicken could fit in the container, as collisions and the air pressure from those birds flying near the roof would both force lower chickens to fall back down. StuRat (talk) 12:44, 19 April 2011 (UTC)

Interesting. I could find no article dealing with chickens flying in shipping containers. Your answer may well be definitive Sturat, but it occurs to me that the kinetic energy transfered to the air by the wings would be completely or mostly used up by the work of compressing the air locally beneath the wing which would then completely decompressed in all directions, thereby the weight of the chicken would not be transfered to the bottom of the container.190.148.134.75 (talk) 14:33, 19 April 2011 (UTC)

Mythbusters used pigeons. They fly better than chickens. HiLo48 (talk) 17:34, 19 April 2011 (UTC)

The force does dissipate in all directions, but the downward component of the force eventually works on the ground.

Perhaps it would help to imagine something bigger and more powerful. Imagine a helicopter in a box. Clearly, the helicopter is pushing down on the bottom of the box.

The higher the helicopter is, the harder it is to notice that downward force because it's spread out over a wider area. APL (talk) 14:53, 19 April 2011 (UTC)

Unless said helicopter is in a box where the air disturbances can not dissipate. Googlemeister (talk) 15:04, 19 April 2011 (UTC)

Right. APL (talk) 15:10, 19 April 2011 (UTC)

In any case, chickens don't really fly all that well. The longest I ever saw one stay in the air was less then 10 seconds. Googlemeister (talk) 16:02, 19 April 2011 (UTC)

Also discussed in episode 8 of QI (H series).--Shantavira|^{feed me} 16:39, 19 April 2011 (UTC)

How sealed? If it is air tight, there should be literally zero mass difference. You are not weighing the bottom of the container, you're weighing the whole container unit, air, chickens, and all. --Mr.98 (talk) 17:27, 19 April 2011 (UTC)

Best of all, if it's air tight the chickens eventually stop moving so you can get an accurate measurement on your scale. APL (talk) 18:46, 19 April 2011 (UTC)

This is true - until the chickens stop moving up and down, the weight on the scale will keep changing. It will only average the same as when they're not flying. Wnt (talk) 19:14, 19 April 2011 (UTC)

Having sex after working out

Since body testosterone plays an important part in muscle growth, does having sex shortly after exercising intensely improve the results of the work-out? I am assuming the person in question assumes a more passive role in sex, in order to favour recuperation. Leptictidium (mt) 13:20, 19 April 2011 (UTC)

Assuming there is an effect, it is not clear to me why you would need to do this "shortly after exercising". What is known is that exercising an hour or so after the main intense exercise does help recuperation. Count Iblis (talk) 16:47, 19 April 2011 (UTC)

Excessive caffeine and hair loss

Is there any correlation between the two? I read that caffeine can cause numerous nutritional deficiencies (iron, vitamin b, etc) which can lead to hair loss; or that it could lead to an increase of DHT, which in turn causes hair to fall off at a faster rate. However, I didn't find any reliable sources concerning this.

24.202.44.252 (talk) 13:46, 19 April 2011 (UTC)

You are probably stretching things too far. A -> B and B -> C both being true in different studies does not always equate to A --> C in real life. There are so many different variables involved that you can't always put 2 and 2 together. This article suggests that caffeine might actually stimulate hair follicle growth at least in vitro. However, a little bit of personal WP:OR suggests that this is not working in vivo. Perhaps I should be using a caffeine shampoo for better effect. --- Medical geneticist (talk) 15:57, 19 April 2011 (UTC)

vitamins

I heard somewhere that a lot of the benefits of vitamin pills are counteracted by the body not absorbing the vitamins well. Is this because all the vitamins show up in a large bolus? In that case, wouldn't it make sense to divide the vitamins into say 4 parts and take one part every 6 hours or so?

Googlemeister (talk) 16:01, 19 April 2011 (UTC)

The bolus isn't really the issue, normally. There are many reasons why they may not be absorbed properly:

1) Some are fat soluble, so need to be taken when eating fats.

2) Some can't be absorbed in the presence of certain other foods.

3) Sometimes they give you a different form of the vitamin than you get from food.

4) And, of course, you may not have a deficiency in the first place.

One case where the bolus does seem to be a problem is when they give dialysis patients a week's worth of iron in a single ferritin injection. This causes local problems in the area of the injection. StuRat (talk) 16:25, 19 April 2011 (UTC)

materials treatment

Could waste materials be heated up to a high enough temperature leaving all the elements of the waste by themselves then could these be filtered by their density? —Preceding unsigned comment added by 82.38.96.241 (talk) 16:12, 19 April 2011 (UTC)

Possibly, but that would take a huge amount of energy. StuRat (talk) 16:31, 19 April 2011 (UTC)

At microscopic/atomic levels, this is (partly) how Mass spectrometry works, but it is difficult to envisage this being scaled up to macroscopic quantities economically. {The poster formerly known as 87.81.230.195} 90.197.66.111 (talk) 16:47, 19 April 2011 (UTC)

Methods based on this principle might be used extra terrestrially at some point in the future (possible by the Chinese at the speed they are advancing). http://www.permanent.com/i-distil.htm --Aspro (talk) 18:54, 19 April 2011 (UTC)

Absolute

I've been fixing the links to the disambiguation page absolute. I can't seem to find a Wikipedia article about absolute as the opposite concept of relative in a scientific context. Does anyone know of such an article? Thanks, P. D. Cook ^{Talk to me!} 17:39, 19 April 2011 (UTC)

There's Absolute time and space. Does that help? --Tango (talk) 18:11, 19 April 2011 (UTC)

There's also Absolute temperature (redirecting to Thermodynamic temperature), but I suspect that these are too specific for the OP. I'd say that is because "absolute" is rather a case for Wiktionary; it's hard to imagine a whole Wikipedia article about so general a term. — Sebastian 18:36, 19 April 2011 (UTC)

Well the concept of an absolute measurement (versus a relative one) comes up many times in math and science (temperature scales, coordinate systems, scoring systems, etc.), so I could see there being an article about it or maybe a section in a broader one. I had been considering just linking the articles in question to Wiktionary, and now that you mention it as well, I might do that. Thanks for your input. P. D. Cook ^{Talk to me!} 19:15, 19 April 2011 (UTC)

Testosterone Levels

In a recent article in the New York Observer, the author writes:

"Indeed, it's no longer so easy to be male. If you're a frog, this is literally true—modern environmental toxins can actually turn you into a female. If you're a human, they've merely halved your sperm count since the 1940s and zapped 15 percent of your testosterone since the 1980s."

We'll let the frogs be for now – are the claims he makes about men really scientifically documented? Wouldn't a 15% change in a testosterone levels have a profound effect on anyone? And, what types of environmental toxins, if any, could alter testosterone levels like this?

Alfonse Stompanato (talk) 18:37, 19 April 2011 (UTC)

Sounds very dubious. As usual, the impetus is on the originator of the claim to provide evidence; it's very hard to publish a scholarly article that provides counter-evidence to every inane claim ever made. Do you need help finding epidemiological studies of hormone levels? Nimur (talk) 18:49, 19 April 2011 (UTC)

The lower sperm count may have more to do with modern fashions, where tight pants overheat the testicles. Scotsmen had the right idea with kilts, apparently. StuRat (talk) 18:52, 19 April 2011 (UTC)

The article on Endocrine disruptors might interest you.--Aspro (talk) 19:08, 19 April 2011 (UTC)

FPGAs in space applications

Where can I find extensive information about that topic? Thanks. --Belchman (talk) 19:18, 19 April 2011 (UTC)

ChemDraw diagrams in articles

how can I place into body article chemical structures, like polysaccharides, written by Chemdraw programBermanel (talk) 19:53, 19 April 2011 (UTC)

You have to save the ChemDraw drawing as a graphics file format (png, gif, jpeg, etc.) rather than as a ChemDraw document (cdx, etc.). Use the ACS template when making your diagram, and save the image as a PNG format with a resolution-option of 720dpi. Then, upload that image file. An image is its own item on wikipedia, not "a component of another page". Finally, edit the page where you want the image to appear and enter the wiki-code for it. Wikipedia:Creation and usage of media files has lots of specific details about how to upload files and insert them in pages. WT:CHEMISTRY is a place to discuss chemistry issues related to wikipedia articles (formatting and style, etc). DMacks (talk) 20:20, 19 April 2011 (UTC)

Windward Islands and trade winds

Trade winds (yellow arrows) come from the northeast in most places, including at the Antilles.

There is a great comparison table at Windward Islands#Terminology, which shows that (in English) the "Windward Islands" lie to the south and west of the "Leeward Islands". How can this be, given that the trade winds come from the northeast? — Sebastian 21:23, 19 April 2011 (UTC)

IBM Watson's size in 10 years.

According to Moore's Law, how small would Watson get by his 10th anniversary? Would his entire hardware finally fit behind a game show podium? What would be his estimated dimensions? --70.179.169.115 (talk) 21:30, 19 April 2011 (UTC)

If cars followed Moore's Law from the 1925 Model T

At 6.9 HP per liter for a 1925 Model T, if cars started following Moore's Law from that year, how many HP would an economy car's 1.0 liter engine contain? What devices have that much power today, and what would that kind of power do?

Or if we only needed 1,000 HP, how big would the engine need to be? What other object(s) would it then be the size of? --70.179.169.115 (talk) 21:30, 19 April 2011 (UTC)