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March 31

When did the Moon become tidally locked to the Earth ?

Can we put a date on it ? StuRat (talk) 02:14, 31 March 2016 (UTC)

This has some calculations and comes up with an answer for you. --Jayron32 02:29, 31 March 2016 (UTC)

That page in turn cites this one, which states that such estimates can be very far off. --69.159.61.172 (talk) 05:58, 31 March 2016 (UTC)

It's tidally locked everyday. :) --DHeyward (talk) 08:51, 31 March 2016 (UTC)

Is it easier for a tornado to make a cow fly or a car or a mobile home/trailer?

The car is several times heavier but it's several times less dense. The trailer might be less dense than the car no? At least when empty. Since it has no engine and more volume to area. It is less aerodynamic and more easily toppled than an SUV. It semi-traps the wind under it as long as the trailer doesn't leave the ground and then increases the area facing the wind (up to a point) as the trapped air effect decreases which should give it some rotational momentum on the long axis for the wind to accelerate. So maybe a mobile home is the easiest of all?

Could a tornado suck up a tank? (high weight, high density). Sagittarian Milky Way (talk) 18:00, 31 March 2016 (UTC)

Tanknado? Now that's scary. You think baseball-sized hail is bad, try HE shells! But why the shells you ask? Because A-Team is coming in for a landing!Wnt (talk) 18:18, 31 March 2016 (UTC)

I believe you have the right ideas, that density matters more than weight, and that the shape also matters quite a bit. The hardest thing to lift is something dense and flat to the ground and attached to it, like pavement. If the pavement is ripped from the ground, you have had a massive tornado. A trailer with no engine would be low density, so would be blown around easily, if not properly anchored to the ground. Cars and trucks would be next, and the cow would be after that, then the tank would be hardest of all to lift. StuRat (talk) 02:02, 1 April 2016 (UTC)

Certainly trailer parks here in Texas are commonly described as "Tornado magnets" - that's because they frequently figure in the news when there are tornadoes touching down. People assume that tornadoes more frequently hit trailer parks - but in truth it's just that they are vastly more likely to be damaged than more substantial (ie denser) structures. The goal when designing such structures is often to enclose the maximum amount of living space with the least amount of material - so they do tend to have low densities when compared to cars and cows. SteveBaker (talk) 13:17, 1 April 2016 (UTC)

Self-driving cars and drafting

I've read that theoretically, a person driving right behind a semi rig can cut gas consumption by as much as half - but he'd have to drive in a suicidal position, within a meter of the back of the rig, and of course it could brake at any time.

However, it strikes me that the self-driving cars being produced should have many advantages - they should have real-time machine-to-machine communication that allows them to coordinate drafting maneuvers from both ends. Do they have enough control via their interfaces to literally dock a specialized fitting to securely link them as a "train"? (Besides reducing the risk of jarring mismatches in position, this would allow the rear cars to deliver their fair share of the energy, so that the front car is not paying full freight when everyone else gets half off) Does the car in front have enough lead time when it sees a hazardous situation that the ones behind can either unhook and evade or else just brake in synchrony?

Has anyone tested anything like this? I'd imagine that you could link any number of self-driving cars together in a train, essentially without limit since each provides its own power. Ideally they should have some kind of specialized structure (maybe even just a set of streamers, or perhaps something more solid) that would make them "one" so that the wind would flow past as if they were a single vehicle. But they'd need some kind of common, manufacturer-independent protocol for communication, docking, and connecting to reduce air resistance, probably other things. Is there any such protocol? Wnt (talk) 18:30, 31 March 2016 (UTC)

I should note I've found some mentions, not very detailed, of "flocking" of self-driving cars [1] but this still implies considerable distance between them. Wnt (talk) 18:38, 31 March 2016 (UTC)

About a month ago, a self driving car found itself unable to avoid hitting a bus when both were traveling at very low speeds: [2]. Given that, I'm not sure the technology exists now to safely allow for self-driving cars to try the maneuvers you are describing. Of course, in the future, who knows? --Jayron32 18:45, 31 March 2016 (UTC)

This was a report about testing self-driving lorries, which would lock into a convoy with just a few feet between vehicles: http://www.bbc.co.uk/news/technology-28834774 217.44.50.87 (talk) 19:24, 31 March 2016 (UTC)

Lockheed Martin has worked on convoys of autonomous vehicles. For example, this 2014 press release, U.S. Army and Lockheed Martin Complete Advanced Autonomous Convoy Demonstration, describes technology for convoys of self-driving military vehicles. I'm pretty sure I've seen presentations from Lockheed Martin and others that describe fuel economies, although wind resistance is not the only advantage. Here's a decent starting place: a 2015 review presentation of interesting technologies investigated by the Army's TARDEC Tank and Automotive Research, Development, and Engineering Center. Fuel economies of modular autonomous vehicles are described. Nimur (talk) 19:28, 31 March 2016 (UTC)

That picture in the first (Lockheed Martin) link shows vehicles quite widely spaced - the convoy doesn't really look like it's worried about fuel, but rather is reducing the number of people exposed to (presumably) potential hostile action. Even the BBC link in the response above that presumes vehicles meant to work together in advance. But what would really seem to make sense is that the car comes onto the freeway, finds a line of other autonomous cars of all descriptions, and they all roll down as one big unit until the next exit where a subset might break off. For which a more universal standardization would be needed. Still -- these are pretty good finds, thanks. Wnt (talk) 21:07, 31 March 2016 (UTC)

They don't have to be self-driving cars. They only have to be self-braking and have some form of communication.Llaanngg (talk) 00:09, 1 April 2016 (UTC)

Well, to physically hitch them I'd expect you'd want robust computer-controlled steering, so that going over a pothole at the wrong time doesn't scratch your pretty chrome bumper (or worse) when you're trying to link up. Wnt (talk) 01:10, 1 April 2016 (UTC)

I'm rather surprised that nobody has mentioned the Mythbusters episode where they figured out that you have to be incredibly close to see any sort of real fuel savings. Dismas|^(talk) 01:23, 1 April 2016 (UTC)

@Dismas: In a way this question was inspired by that episode - I think I remember them getting close to 50% savings at that ridiculously close distance. That's why I was suggesting a physical connection (and even some kind of shell to keep air from getting into the remaining space between vehicles) rather than just a "flock". Wnt (talk) 17:20, 1 April 2016 (UTC)

See also slipstreaming wch cites "a 10% increase in efficiency of certain hybrid vehicles".--Shantavira|^{feed me} 06:52, 1 April 2016 (UTC)

It's worth mentioning that the lead vehicle gets benefits from slipstreaming as well as the one at the rear - but the largest gains are in the middle of the pack.

With intelligent vehicles (even if they aren't 100% self-driving), the communications lag should be a vanishingly small percentage of the time for braking - where mechanical parts have to move. I don't think it would make much difference to stopping distances for all of the vehicles to spend a millisecond be informed by the one in the lead of the intent to brake (or turn, or accelerate, etc) so that it happens at the exact same time. My concern would be about the degree of maintenance of these vehicles. If someone in the middle of the pack has worn brake pads and bald tires - then the possibilities for disaster become rather severe. SteveBaker (talk) 13:09, 1 April 2016 (UTC)

Well, the self-driving car ought to know if it has those mechanical problems, and know whether it can compensate to match the other cars with its current status or not. And the case of the abrupt failure is one of the reasons I suggested a linkage, so that the other cars would physically help the damaged one keep going at the same speed - while, of course, collaborating as a unit to brake it safely as it would with its own self-driving algorithm, even as those not adjacent peel off and reform, until they can leave it safely alongside the road and go on. Wnt (talk) 17:24, 1 April 2016 (UTC)

Perhaps some of this belongs in Platoon (automobile). Jim.henderson (talk) 17:11, 1 April 2016 (UTC)

Now that is a relevant keyword. (though I thought men joined platoons and vehicles joined convoys...) Wnt (talk) 17:27, 1 April 2016 (UTC)

Anti-wart vaccines

Is there a vaccine that prevents one from getting "ordinary" warts on the skin, e.g. plantar warts or the ones pictured in File:Wart ASA animated.gif? Wart#Prevention only really addresses genital warts, ignoring everything else except for a single unsourced statement saying that ordinary anti-genital-wart vaccines don't affect plantar warts. HPV vaccines appears to concern itself entirely with the genital-wart types of viri. Not surprisingly, I can't find anything else on Google. Nyttend (talk) 19:24, 31 March 2016 (UTC)

The Plantar wart article indicates there is no vaccine. ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:34, 31 March 2016 (UTC)

See Plural form of words ending in -us#Virus, incidentally. "Viri" is attested - that doesn't mean it's correct. Tevildo (talk) 19:49, 31 March 2016 (UTC)

Hm. I thought all warts were caused by HPV, and wart supports that. But there are hundreds of strains of HPV, so this becomes a very detail-oriented question about how and if vaccines against viral infections have any cross efficacy to closely related strains. There is also the matter that we would never expect a vaccine to do anything much to a current infection. So of course an HPV vaccine will not affect a plantar wart, but it conceptually might still be able to decrease chances of new infection. I'll do more searching of the academic literature to see if I can find any clarification. SemanticMantis (talk) 19:54, 31 March 2016 (UTC)

Here [3] is a research paper that discusses a bit about vaccine development strategies and a little about what works for what. SemanticMantis (talk) 20:20, 31 March 2016 (UTC)

Our entry on Gardasil says it is up to a 9-valent vaccine, i.e. it is a mixture of vaccines against HPV 6, 11, 16, 18, 31, 33, 45, 52, and 58. The bad news is that our wart article blames common warts on "2 and 4 (most common); also types 1, 3, 26, 29, and 57 and others", and plantar warts on "HPV type 1 (most common); also types 2, 3, 4, 27, 28, and 58" - which means that only one (58) is shared with the vaccine. The good news of course is that the vaccine does largely prevent genital warts and cervical cancer caused by them. Which means that it is reasonably likely that a similar 7-valent vaccine could target common warts, or a 10-valent vaccine could target common + plantar warts, and actually provide protection. Wnt (talk) 01:06, 1 April 2016 (UTC)

April 1

are we inside a black hole?

the schwarzschild radius of the observable universe is like 13 billion light years. there's a lot of dark matter too, could we be inside a huge black hole? if we were though, wouldn't one side of the universe (the side facing the center of mass) be completely black to us?— Preceding unsigned comment added by 203.213.116.10 (talk • contribs)

You appear to have answered your own question. Beyond that, all we could do is provide you with references (if they exist) that suggest we're in a black hole or attempt to explain why there wouldn't be a huge black spot. We can't really answer questions, just repeat answers that already exist. Ian.thomson (talk) 02:03, 1 April 2016 (UTC)

Sort of like a black hole does. Shock Brigade Harvester Boris (talk) 02:04, 1 April 2016 (UTC)

Well, I think the best evidence that we aren't inside a black hole is that the universe appears not to be contracting, but expanding, and not only expanding, but the rate of expansion is increasing. Secondly, I'm not sure you could explain the Cosmic microwave background from inside a black hole. Vespine (talk) 04:06, 1 April 2016 (UTC)

I think it is possible to have a standard cosmological spacetime with a finite total volume and mass, surrounded by an infinite Schwarzschild vacuum. Possibly if the matter recollapses, then it's inside the Schwarzschild radius, but I may be misremembering. This is sort of like a "universe inside a black hole", but I don't know what sort of process would produce it, and the evidence doesn't support a recollapsing universe anyway.

One side of the universe wouldn't be completely black. It would look almost the same as the actual world (including the CMB), and it would be almost the same, unless we were close to the edge of the matter region (and if the matter region is large enough, the chance of that is negligible).

In a recollapsing universe, you eventually hit a curvature singularity no matter what you do. The collapse is sometimes described as a "union of black holes." If you can't avoid ending up in a black hole, then by definition you're already in it. So maybe recollapsing universes are "inside a black hole" by definition, even without the surrounding Schwarzschild vacuum. -- BenRG (talk) 07:33, 1 April 2016 (UTC)

As said above we don't seem to be in one because the rate of expansion is increasing. I don't know what being inside a white hole would be like but perhaps this is it. Dmcq (talk) 11:51, 1 April 2016 (UTC)

Hearing loss from music performance

(Professional advice disclaimer noted.)

http://www.bbc.co.uk/news/entertainment-arts-35938704

"A renowned viola player is suing the Royal Opera House for ruining his hearing and his career during rehearsals of Wagner's Die Walkure."

Does anyone here know of any papers on what the type of injury is likely to be? ShakespeareFan00 (talk) 10:21, 1 April 2016 (UTC)

The news report you linked to says Acoustic shock. Our article doesn't include any papers but a search should find ones like [4] [5] [6] [7] [8] [9] [10]. These in turn link to other research. Stuff like [11] may also be relevant. Nil Einne (talk) 12:11, 1 April 2016 (UTC)

The thread title is far different from the specific question. The Royal Opera House musician claimed that loud music during one set of rehearsals for one piece had caused this "acoustic shock" similar to a syndrome experienced by call center personnel in India whose headphones had squeals or were generally loud. This is despite his having been provided with hearing protection which reduced the sound pressure level by 28 dB, which is close to the best protection obtainable via earplugs or over-ear protectors. The claimed syndrome rather than rendering him unable to hear soft noises caused him to have psychological trauma when he heard a variety of sounds, such as his infant, or random household noises. The more common manifestation of actual "hearing LOSS from musical performance" is that the 130dB or whatever from combined percussion and brass in an orchestra or band causes lower hearing sensitivity at some frequencies, or causes tinnitus which interferes with hearing, as reported in a study from Michigan State University, which reported 71% of the Chicago Symphony musicians had experienced hearing loss. Some classical or popular musicians use special earplugs by companies such as Etymotic Research mentioned in a journal article which are supposed to let them hear the ensemble better than standard industrial hearing protection. Even in high school bands, having a number of trumpets playing a few feet behind one is painful and likely damaging to hearing. Studies with rats show that loud noise literally destroys permanently the "hair cells" in the cochlea which are the sensory receptors for sound. Edison (talk) 17:07, 1 April 2016 (UTC)

Comment: This is a bit of an aside as I don't know of any scientific studies -which is what the OP is seeking. I sometimes used to get asked to stand in for a sick band member. With a stack of Marshal amps behind me, the only way I could hear my contribution (which was why I was there) was to listen to a on-stage speaker such as this in front of me. Therefore, I think the ambient noise level can reach a point where ones hearing gets too overloaded. Rather than resort to wearing wax ear plugs (which attenuated too much, some of the frequency I wanted to hear) I stood on stage with ear defenders. To make it look like a stage act, I added curly wire antennas to them with little balls at the end. Whether this particular musician is suffering hearing lose due to his employment or whether its due to natural hearing loss that he would suffer regardless -I do not know. Yet, I can't help but notice when I bump into some of my old chums, many now need hearing aids. Some have further admitted, that they where still in their very early twenties when they first noticed hearing loss. But because they could afford a sound mixer at gigs and in the studio, it did not bother them too much at the time. But such is the isolation and frustration they now experience in their 60's and 70's, when trying to follow any of their grandchildren's conservations, they wish they knew then, what they know now. Food for thought. --Aspro (talk) 13:07, 2 April 2016 (UTC)

Nuclear renewable energy

What is the relation between the nuclear energy and renewable energy?--5.2.200.163 (talk) 15:06, 1 April 2016 (UTC)

They both contain the word "energy". --Jayron32 15:09, 1 April 2016 (UTC)

Thats a pretty fececious remark J. Below your station I suggest. Why not try to be helpful?--178.101.224.162 (talk) 23:57, 1 April 2016 (UTC)

Nuclear energy is in no way "renewable". The fuels that it uses (uranium, for example) are mined and will eventually run out just like coal and oil. Even fusion power requires materials like Deuterium that are not "renewable" (at least not rapidly).

The closest relationship I could imagine between nuclear power and true renewables (such as wind, wave and solar power) is that nuclear energy could provide a stop-gap measure as we phase out hydrocarbon fuels and phase in those true renewables.

Nuclear fission reactors cannot be a long-term solution to mankind's energy needs.

SteveBaker (talk) 15:44, 1 April 2016 (UTC)

I'm not deliberately trying to be facetious, but why are wind, wave, and solar power "renewable"? They depend on a unidirectional flow of solar energy. Over a sufficiently long time, the sun will consume all of its hydrogen, and that's a one-way trip. It will take billions and billions and billions of years before a new cluster of hydrogen coalesces under its own gravity, collapses into a core, and reaches critical mass-density to sustain hydrogen fusion inside a new sun. Eventually, over the timescales of tens or hundreds of billions of years, the universe will run out of free hydrogen from which we can form more stars.

Meanwhile, fossil fuels only take a few million years to collect a large quantity of biological kerogen, and then a few more million years to cook into useful petroleum, gas, and other fossil fuel. On geological timescales, fossil fuels form really fast. We can renew our fossil fuel resources much faster timescales than we can renew the hydrogen-resources that fuel our sun. The only downside is that nobody in present company will survive long enough to wait for the next batch to come out of the proverbial geological oven.

The terminology "renewable resource" refers to a renewal process that happens within a specific timeframe. Most of the time, that timescale is compared to the characteristic life-cycle of a human, or a group of humans.

Depending on how far out you're thinking, though, no energy source is renewable: our universe is on a steady march governed by the laws of thermodynamics, paraphrased thusly: (1) you can't win; (2) you can't break even; (3) you're guaranteed to lose.

If we're speaking purely in terms of physics, solar energy (and wind energy, and so on) are not renewable. Their actual advantage has nothing to do with the fact that they're renewable. They really have advantages because they are (potentially) low cost; (potentially) low-polluting; (potentially) low ecological impact; and because to first order approximation, they provide an "approximately" infinity-sized hot-bath reservoir for the enormous Carnot engine that is human society. None of those advantages have anything to do with "renewability." Don't get me wrong - I'm a huge fan of hydroelectric power generation, wind power generation, and so forth ... but I think we should be pitching for these excellent alternative sources of energy on their real merits, not their illusory merits.

Nimur (talk) 16:46, 1 April 2016 (UTC)

Not since Protagoras laid the touchstone for the Age of Enlightenment in his words "Man is the measure of all things" should anyone need apologise, or be mocked by a pedant, for qualifying renewable energy sources as those which are naturally replenished on a human timescale. The writers of ISO 13602-1:2002 are not pitching an illusion when they state that a renewable resource is "a natural resource for which the ratio of the creation of the natural resource to the output of that resource from nature to the technosphere is equal to or greater than one". AllBestFaith (talk) 20:38, 3 April 2016 (UTC)

1) Nuclear energy could last us for a very long time before we run out of uranium. That's not technically "renewable", but what we really care about is having an energy supply that lasts so long we don't need to worry about it running out, and it qualifies in that case. According to our peak uranium article "The identified resource at the start of 2013 was enough to supply reactors at current consumption rates for more than 120 years, even if no additional uranium deposits are discovered in the meantime." And, of course, there are many undiscovered reserves and even the possibility of extracting it from sea water at some point in the future. As with most natural resources, the higher the price goes, the more can be produced, since the less accessible reserves can then be accessed, at higher costs. However, unlike other natural resources, the price of uranium has little to do with the cost of the energy produced by it. That's because a huge amount of energy is produced by a very small amount or uranium, and most of the cost of the energy is due to the construction, operation, and maintenance cost of the reactor, transmission costs, etc.

2) Breeder reactors may possibly be used to make fuel for nuclear reactors, from other radioactive materials. Of course, those radioactive materials might themselves eventually be used up.

3) Nuclear fusion reactors should one day provide even more energy.

So, by combining these, we have centuries of nuclear energy available. It may, ironically, last longer than some actual renewable energy sources, like wood. This is because, if we were all to burn wood to provide energy, we would use it up faster than we could grow new wood. Then there would also be massive pollution created by burning all that wood. StuRat (talk) 16:32, 1 April 2016 (UTC)

How about radioisotope thermoelectric generators, how do they fit in in this context? And also isotope and elements regeneration by neutron capture and beta decay?--5.2.200.163 (talk) 16:39, 1 April 2016 (UTC)

Reply to first first question: Doesn't really fit in at all. Those portable devices are comparable to a can of Sterno used for camping. That is, the amount of energy produced by them is totally insignificant in the overall picture. (We could in theory each have a portable nuclear generator, but the risk or accidents and terrorism is just too high to do that.) StuRat (talk) 16:47, 1 April 2016 (UTC)

Nuclear energy is off the charts clean per unit energy created..."renewable energy"20:06, 2 April 2016 (UTC)68.48.241.158 (talk) is not particularly clean per unit of energy created if look at all the infrastructure and maintenance costs etc etc etc...nuclear power plants are basically capable of powering entire metropolitan areas by themselves....the downside is the small chance of catastrophic disaster, and dangerous waste products etc..there's no efficient manner of storing large quantities of energy either so when the sun doesn't shine and the wind doesn't blow etc, renewables are useless...68.48.241.158 (talk) 19:24, 2 April 2016 (UTC)

They just need to take my advice and build nuclear reactors deep in abandoned mines, in stable geologic areas, far from population centers, so they can just push the nuclear waste they generate into an already constructed concrete bunker at the end of the mine, rather than hoping to ship it across the country some day to a nuclear waste storage facility that has yet to be built. The cooling towers would still be on the surface, and they would need high tension wires to send the electricity to cities. If they did have a meltdown, they would just evacuate the mine and bury it in concrete. If the reactors survived to their projected lifespan, they would again bury the mine in concrete, and would only need to post a few guards, since drilling through all that concrete to get to the nuclear waste would take terrorists way too long to go unnoticed. StuRat (talk) 19:34, 2 April 2016 (UTC)

I'm no expert but I'd imagine there's all kinds of technical difficulties in what you propose, might hugely increase the cost of nuclear power, defeating the purpose..just dealing with the hydrology needed underground... 68.48.241.158 (talk) 20:06, 2 April 2016 (UTC)

The problem with the mine idea is, mines tend to flood, so any reactor placed in an abandoned mine would be under water in short order. Generation 4 and later reactors are being designed as "walk away safe", where one could quite literally walk away from the reactor and it wouldn't melt down. Add thorium into the equation of nuclear reactors, nuclear waste (depleted uranium) is "burned" in the reactor, leaving little waste at the end of the fuel cycle and that waste would only be radioactive for a few hundred to a few thousand years. That's a whole lot less than what waste we have now, being dangerous for hundreds of thousands of years. But, for more renewable (nuclear isn't renewable at all, once you run out of fissionables, you're out), fusion power would be the way to go. Alas, we've yet to get gain in fusion, all fusion processes currently operate at a loss - more energy going in than being generated. Solar is nice, however heavily polluting in making the solar cells and storage batteries and highly inefficient (top of the line solar cells are only around 10% efficient). Wind is nice, when there's wind. Tidal power is workable, if disruptive to shorelines. Every technology has its strengths and weaknesses, as TANSTAAFL.Wzrd1 (talk) 20:51, 2 April 2016 (UTC)

Try mines in deserts to avoid flooding. The opening can be sealed while in operation, too. I am aware that nuclear fission can be done safely, but the problem is that we must rely on large corporations and government agencies to ensure that they are safe, and nobody much trusts them to keep us safe anymore. So, to get public support, we need to move them to some place where we don't have to trust people who have proven they can't be trusted. StuRat (talk) 22:04, 2 April 2016 (UTC)

Nuclear power proposed as renewable energy might be the answer to the OP's question.--Heron (talk) 09:18, 3 April 2016 (UTC)

Radium mixed with uranium

If radium is mixed with uranium in a container, is the halflife of the uranium decreased? By a lot, or hardly measurably?Eva M. Kahn (talk) 20:01, 1 April 2016 (UTC)

Outside of certain unusual environments such as the interior of stars, nothing changes the halflife of any element. --Guy Macon (talk) 20:05, 1 April 2016 (UTC)

(ec)No, radioactive decay, to an excellent approximation, is completely unaffected by external chemical or physical processes. There are some possible quibbles (especially for unusual decay modes like K-capture, which doesn't apply to uranium), but at any level you're likely to care about, the decay of the uranium is unaffected. --Trovatore (talk)

I was just about to correct myself. See Radioactive decay#Changing decay rates. --Guy Macon (talk) 20:09, 1 April 2016 (UTC)

I suspect you are confusing two different things. The half-life refers to it's tendency to decay all on it's own. You can, however, cause it to undergo nuclear reactions quicker than that, by exposing it to various forms of radiation. A nuclear explosion is the ultimate example of this. StuRat (talk) 20:16, 1 April 2016 (UTC)

I'm wondering if the OP has read about reactors that can burn nuclear waste. In this case, many of the radioactive isotopes – (combined with other fuel which maintains a high neutron flux) are fissioned well before there normal half-life expectancy.--Aspro (talk) 14:36, 2 April 2016 (UTC)

That's not changing the half-life of the nuclides — it's transmuting them into different nuclides that have different half-lives. But I agree it's possible that that's what Eva M. Kahn had in mind.

If that's the idea, then adding radium is not likely to work well. Radium is an alpha emitter, and what you want is neutrons. But in any case you're generally not trying to get rid of uranium, as at least U-238 is so long-lived that it doesn't have much radioactivity. It used to be used to make a bright red-orange glaze for pottery, and it was safe to eat off of. --Trovatore (talk) 19:22, 2 April 2016 (UTC)

That's not strictly true. Uranium painted dishes were safe to eat off of if the glaze remained intact, as the food would never touch the uranium. As for disposing of nuclear waste, one of the selling points for thorium reactors is that one can place depleted uranium into the thorium reactor and it'd "burn" it, leaving short half-life waste. Of course, the shorter the half-life, the more radioactive it is, but it has to be stored for a much shorter amount of time.Wzrd1 (talk) 20:55, 2 April 2016 (UTC)

I don't know why you would want to get rid of depleted uranium. It's useful for all sorts of things, and so slightly radioactive that for most purposes it's not really a problem. Breeder reactors (and perhaps thorium reactors; I haven't heard that but could be true) can use DU to get energy, and that's different of course, but getting rid of the DU is not the purpose. --Trovatore (talk) 21:17, 2 April 2016 (UTC)

True or false? "Winter ice maximums in the arctic are relatively unchanged. Summer minimums are lower."

Is it true that "Winter ice maximums in the arctic are relatively unchanged. Summer minimums are lower." (according to User:DHeyward above). If there is some source of this, it could be interesting to update some articles. --Scicurious (talk) 22:28, 1 April 2016 (UTC)

https://nsidc.org/data/bist/bist.pl?annot=1&legend=1&scale=75&tab_cols=2&tab_rows=2&config=seaice_extent_trends&submit=Refresh&hemis0=N&img0=trnd&hemis1=N&img1=plot&mo0=03&year0=2016&mo1=09&year1=2016 Maximum extent is in March. Min extent is in September. Rate of loss is roughly an order of magnitude difference between max and min. Not sure what articles you think need updating. --DHeyward (talk) 23:16, 1 April 2016 (UTC)

I suppose Polar_ice_cap#North_Pole and Arctic sea ice decline could benefit from this information.--Scicurious (talk) 23:24, 1 April 2016 (UTC)

Take it to WP:RDL, guys. Evan (talk|contribs) 19:07, 3 April 2016 (UTC)
The following discussion has been closed. Please do not modify it.
Just to be pedantic: its minima and maxima. Still, not everyone had the experience of Latin in school.--178.101.224.162 (talk) 00:00, 2 April 2016 (UTC) Just to be pedantic: (1) it's it's; and (2) both plurals are correct for those nouns. This is English, not Latin. --69.159.61.172 (talk) 08:00, 2 April 2016 (UTC) Thats an American dictionary. Not English.--178.101.224.162 (talk) 00:09, 3 April 2016 (UTC) The speak English in the United States. Perhaps you noticed that... --Jayron32 03:28, 3 April 2016 (UTC) Also it's my error: it's it's not its. Glad we cleared that up. The American Heritage dictionary confirms: Its is the possessive form of the pronoun it and is correctly written without an apostrophe: The cat licked its paws. The contraction it's (for it is or it has) should always have an apostrophe: It's the funniest show I've seen in years. TearsOfaClone (talk) 12:00, 2 April 2016 (UTC)

The time series graph on this NSIDC page is probably useful in answering the question. --Tagishsimon (talk) 00:11, 2 April 2016 (UTC)

April 2

Book recommendations on the history of automotive engines

Hello, Science Reference Desk. I am looking for recommendations on full-length, non-fiction books (or equivalently-detailed internet resources) that review the modern history of automobile engines. I'm looking for a book that goes into considerable depth beyond our articles. For context, this query is prompted by some extensive reading I have already conducted; I've absorbed about everything I can from our articles... This morning on BBC World Service radio, I learned that after switching to 1.6 liter, six cylinder turbocharged hybrid engines, Formula 1 cars are now completing laps faster than when they used ten cylinder, 3.0 liter BMW engines just a few years ago (interview paraphrased in the web news story). This progress seems incredible, and it's no surprise that it's flared some brand rivalry! I'm trying to determine whether these technology results are representative of progress across the entire automotive industry, or if F-1 engines have evolved so far out on their own branch of the technology tree that these improvements cannot translate to ordinary cars - (needless to say, turbo-compound engines used in these racecars look an awful lot like turbocharged aircraft engines!); or if the new race results are consequences of other changes independent of engine performance. Nimur (talk) 16:34, 2 April 2016 (UTC)

doubt you'll find a full-length general interest book specifically about what you're looking for....vaguely related though: this is among the greatest (and best-selling) pop autobiographies ever written and about American auto industry...has all kinds of cool stuff about the creation of the Ford Mustang.. https://en.wikipedia.org/wiki/Iacocca:_An_Autobiography 68.48.241.158 (talk) 19:14, 2 April 2016 (UTC)

I'd second that. It generally takes a while to get good technical books about motor sport, so I don't believe there is detailed information about the latest engines in book form. If you were to buy Autocourse from the last few years, you could start to read some of it. The other issue you'll face is that F1 isn't just about engine power: aerodynamics have a huge part to play, and the influence of changing regulations on the amount of aerodynamic assistance makes a huge effect on lap times. For example, the Coandă effect had a huge influence on F1 design a few years ago, but is pretty much outlawed now. Also, note that unrestricted 1.6 turbo engines would blow 3 litre NA engines into the weeds: in the 1980s both were allowed: the 3L engines produced 5-600 BHP; the 1.5L turbos knocked out well over 1000 BHP in qualifying form. And a final comment: the F1 fraternity does not refer to the current generation as the "engine": they are power plants, comprising an internal combustion engine, a turbo with a heat recovery system (MGU-H) and a kinetic recovery system (MGU-K) with a battery storage system and an electric power unit to boost output. If you read in places like https://www.formula1.com/ some of this will be covered.--Phil Holmes (talk) 19:29, 2 April 2016 (UTC)

Indeed, thank you for the terminology correction! Nimur (talk) 15:25, 3 April 2016 (UTC)

You might want to try a magazine like Popular Mechanics. While not exclusively devoted to car engines, I bet they go into more detail than car magazines, which tend to be very superficial on mechanical details. If you go to a library with those in stock, you can probably find quite a bit of detail on car engines found in issues over the decades. StuRat (talk) 19:25, 2 April 2016 (UTC)

Cough. I'd be a little surprised if Popular Mechanics covered the design parameters of Formula One systems pushing out around 900 BHP from 1.6L power plants. They tend not to be home maintenance projects. The estimate from Bernie Ecclestone today was that Mercedes-Benz spent around $700M developing theirs, so it's a bit more than a garage project.--Phil Holmes (talk) 20:55, 2 April 2016 (UTC)

For Formula One, yes, it's uber advanced, everything custom. Other racing classes can be done in one's garage, one of my neighbors was a race car driver and was always working on his race car. Engines have improved tremendously over the course of my life, with increasing compression, greater efficiency and even transmissions being more efficient in using the energy the engine delivers to them. I wonder at times, just what will the next half century bring?Wzrd1 (talk) 20:59, 2 April 2016 (UTC)

Thanks guys. Nimur (talk) 13:56, 3 April 2016 (UTC)

Do bacterium have feelings/aspirations?

Lets take paramecium for example. Does paramecium have feelings, such as happiness and sadness? Moreover, how do they know what they are doing? Surely they must have aspirations, such as "I must eat 6 pieces of algae today and then reproduce". --FallinggoApartio (talk) 19:51, 2 April 2016 (UTC)

Did you mean to post this yesterday? DrChrissy ^(talk) 19:54, 2 April 2016 (UTC)

No. That would require nerve cells and a brain. Think of a computer program, does it have feelings and aspirations, or does it just do what it is programmed to do, without any thought at all ? StuRat (talk) 19:54, 2 April 2016 (UTC)

StuRat, be careful what you assume to require nerve cells and a brain. Some organisms can surprise you.

"Studies on Physarum have even shown an ability to learn and predict periodic unfavorable conditions in laboratory experiments. Professor John Tyler Bonner, who has spent a lifetime studying slime molds argues that they are "no more than a bag of amoebae encased in a thin slime sheath, yet they manage to have various behaviours that are equal to those of animals who possess muscles and nerves with ganglia – that is, simple brains." --Slime mold#Behavior

--Guy Macon (talk) 06:13, 3 April 2016 (UTC)

Ultimately there is no way to know whether a paramecium has subjective experience, aka qualia. That's what makes it subjective. But it does seem unlikely. --Trovatore (talk) 19:58, 2 April 2016 (UTC)

If you're a materialist I think it's pretty safe to say that single-celled organisms don't have "feelings" or mental "experiences", any more than your skin cells do. Thought requires a brain. --71.110.8.102 (talk) 22:14, 2 April 2016 (UTC)

Yes, materialists would say that, that's true. However materialists really cannot explain the hard problem of consciousness. The extremists among them, the "eliminative materialists", simply deny that it's a problem. --Trovatore (talk) 22:44, 2 April 2016 (UTC)

Given what we know about qualia (nothing), I see absolutely no reason to think that single-celled organisms don't (or do) have qualia, whether or not you're a "materialist". -- BenRG (talk) 04:00, 3 April 2016 (UTC)

• Paramecium lack the portions of the nervous system which are responsible for producing feelings and emotions. --Jayron32 03:27, 3 April 2016 (UTC)

They respond to stimuli. Maybe between stimulus and response there are intermediate states that, once we've figured out what qualia are, we'll recognize as qualia. Maybe not. -- BenRG (talk) 04:00, 3 April 2016 (UTC)

"Paramecium lack"??? I'm almost used to seeing "bacteria" used as a singular noun, but "paramecium" as a plural is new to me. --Trovatore (talk) 04:06, 3 April 2016 (UTC)

Sedan vs. Hatchback

What advantages do sedans have over hatchbacks? Is it just a question of aesthetics? --Llaanngg (talk) 20:41, 2 April 2016 (UTC)

Hatchbacks potentially allow for more storage space than a sedan, given the large hatch access to the storage area and the two-box design that merges passenger and cargo compartments. clpo13_(talk) 20:52, 2 April 2016 (UTC)

I think you read the Q backwards, he was asking for advantages of the sedan. StuRat (talk) 21:00, 2 April 2016 (UTC)

Whoops, you're right. Dunno what I was thinking. clpo13_(talk) 21:03, 2 April 2016 (UTC)

Disadvantages of hatchbacks:

1) They get hot, due to all that extra window surface area, and it being more horizontal than in a sedan. (Could be an advantage in winter, if not covered in snow.) I had a game melt back there (King Oil). Louvers can help here, but they impair visibility somewhat.

2) The contents of the cargo area can be seen, both looking messier and tempting thieves. There can be some type of cover used to prevent this.

3) The opening height can be higher, and the hatch heavier, making it harder for short people to close the hatch.

4) The pneumatic(?) cylinders that hold them open can fail, and if the hatch falls on your head, it could cause serious injury.

5) Smells in the hatch area intrude on the passengers more. Imagine a bag of manure.

6) More window area to clear of ice in winter.

7) The more horizontal hatch is more likely to shatter in a hail storm. StuRat (talk) 21:00, 2 April 2016 (UTC)

That's the point, StuRat. I wanted to know why would someone prefer a sedan. I could only think about advantages of hatchbacks over sedans, like the example above your answer. They also have a bigger trunk lid for bulky objects. In a sedan, even if you had space for a fridge, you won't get into the trunk. Llaanngg (talk) 21:02, 2 April 2016 (UTC)

Safety could also be an issue. Sedans might have the center of gravity lower than a hatchback. Scicurious (talk) 21:10, 2 April 2016 (UTC)

And if you get rear-ended, the trunk can collapse and provide more protection. This paper only mentions that the car is more likely to remain driveable, but presumably there is some safety benefit for occupants as well. --69.159.61.172 (talk) 21:15, 2 April 2016 (UTC)

1 Aesthetics maybe

2 Weight, that side glass weighs, as does the ring bulkhead for the tailgate

3 CGZ

4 Body torsional stiffness

5 squeaks and rattles

6 temperature control

7 tailpipe emission ingress

8 tailpipe noise ingress

That list is definitely the sound of straws being grasped, the differences are slight and the practicalities favor the hatch for most people I'd have thunk. In India hatchbacks are lower status than sedans, hence the wide availability of models that would be hatchbacks anywhere else with a funny little bustle trunk. Greglocock (talk) 22:43, 2 April 2016 (UTC)

CGZ?Scicurious (talk) 22:47, 2 April 2016 (UTC)

Higher Center of Gravity in the Z (vertical) direction. StuRat (talk) 22:49, 2 April 2016 (UTC)

Yeah, that accident at CGZ? Caused by a hatchback. The more you know ====* clpo13_(talk) 22:51, 2 April 2016 (UTC)

• Since StuRat just makes up things he thinks he read some time, I might as well provide some references to make this section useful for the OP. See this article. --Jayron32 03:26, 3 April 2016 (UTC)

• Other than style, the only disadvantage they list is that potential thieves can view what's in the back. That's my point #2. I also notice that Greg's unreferenced list and Scur's unrefed response got no criticism from you, showing you are on a one-man vendetta against me. Should you really bring your personal grudges here ? Or can you just list your refs without the personal attack next time ? StuRat (talk) 07:10, 3 April 2016 (UTC)

It's difficult to find objective information on this subject, but there's this post on Reddit where someone has at least gone to the trouble of collating a number of responses to find the most popular features of sedans. To summarise, the top advantages were (i) looks, (ii) price, (iii) availability, (iv) trunk security and (v) acoustics. --Heron (talk) 09:07, 3 April 2016 (UTC)

Following up on the security issue, which surprises me ... in the UK every hatchback that I have used has had a rigid horizontal parcel shelf which conceals the contents of the boot/trunk/cargo area when the rear hatch is closed. The parcel shelf can be removed to maximise carrying space, but this would be a temporary measure for a specific purpose. Are parcel shelves not standard equipment on US hatchbacks ? Gandalf61 (talk) 12:28, 3 April 2016 (UTC)

Parcel shelves in hatchbacks are often inadequate or unsafe for heavy parcels. The Toyota Prius has no shelf, only a thin retractable plastic roller blind. AllBestFaith (talk) 14:42, 3 April 2016 (UTC)

Yep, that's the same down here (Poland), and as far as I remember, in Western Europe that's also the case. Unless the owner removed it for whatever reason, they all seem to have. It seems to be not only a case of security against thieves, but it's also about safety, to maintain the cargo in place in case of a rollover accident. Llaanngg (talk) 14:47, 3 April 2016 (UTC)

Do we use the calories in alcohol as energy?

Do we transform the energy contained in alcohol as energy? Could we keep alive longer drinking alcoholic beverages like vodka instead of just drinking water? --Scicurious (talk) 21:07, 2 April 2016 (UTC)

Yes we use it as energy, but no, you can't stay alive on ethanol alone, as we need many other nutrients. StuRat (talk) 21:09, 2 April 2016 (UTC)

I don't mean staying alive on ethanol alone. But given the choice to drink just water, or, water and vodka, what would let's use survive for a longer period? --Scicurious (talk) 21:12, 2 April 2016 (UTC)

That is going to depend on the balance between the water and the vodka. Admittedly the alcohol will have a small nutritional value - but it also has a fairly high toxicity which could cause organ damage if you took too much of it. 217.44.50.87 (talk) 21:33, 2 April 2016 (UTC)

Also, ethanol causes you to lose more water, as it increases urine production by interfering with vasopressin. So if running out of drinkable water is a problem, best to avoid the liquor. --71.110.8.102 (talk) 22:11, 2 April 2016 (UTC)

I seem to recall a case of a shipwreck, where they had their cargo of sugar and rum, and fresh water on the island. Some consumed the sugar and rum, while others just drank water, and the latter group lived longer. Not sure if this really happened, and even if it did, they may have died more quickly from overdoing it. A small enough dose of sugar and rum (with water) probably would have been better than water alone. StuRat (talk) 21:53, 2 April 2016 (UTC)

Do you mean sugar mixed with the rum? Otherwise, I don't see any reason for not drinking water with sugar. --Scicurious (talk) 22:11, 2 April 2016 (UTC)

No, not mixed. I'm not sure which is worse, sugar-water or rum. StuRat (talk) 23:04, 2 April 2016 (UTC)

Any citation for this nonsense, Stu?--TMCk (talk) 23:16, 2 April 2016 (UTC)

I've made no claim that to have stated a fact here, so there's nothing to reference. "I seem to recall..." means I'm not positive. I was hoping somebody else might recognize the story and have the facts. But, since Jayron apparently can't live without endlessly ranting at me, let's let him get his fill. (I was hoping that eventually he would have the sense to keep such rants off the Ref Desk, but alas he doesn't seem to.) StuRat (talk) 07:19, 3 April 2016 (UTC)

You said "No, do not mix." followed by a false rational and there is no "I seem to recall..." (not that it would matter much). You more than strongly implied the false fact that a sugary soft drink is at least almost as bad as wodka in a survival situation. One can only hope nobody, including youself, remembers and applies your ideas in a lifethreatening situation: "Throw the pop of the raft. The guy from the ref-desk said it could be even worse than wodka."--TMCk (talk) 14:54, 3 April 2016 (UTC)

Stu doesn't believe in providing citations for anything he says. He has said so repeatedly in the past. He believes everything he says is so blatantly obviously true, it doesn't need references. Which questions why it needs saying at all, if that were true. However, getting Stu to provide even a single reference to any question he answers is a futile endeavor. He will refuse to do so, and will instead now pick through your personal editing history for the one time you answered a question 3 years ago without a reference, and hold that aloft as why he doesn't have to provide references ever for any of his ridiculous speculations. Then he'll attack you relentlessly for that one answer you gave three years ago, trying to deflect the legitimate criticism against himself for never providing a single reference ever. --Jayron32 03:24, 3 April 2016 (UTC)

Re: "Stu doesn't believe in providing citations for anything he says", I've provided many refs for questions on this very page, so you are quite obviously lying. StuRat (talk) 07:23, 3 April 2016 (UTC)

A useful cut-and-paste: [ Citation Needed ]. (Note that it has three links in it, not one.) --Guy Macon (talk) 06:24, 3 April 2016 (UTC)

Indeed, if everything that you have is water, sugar and rum, I don't see why someone wouldn't take at least some sugar besides water. Maybe he should go lightly with the rum, though. Scicurious (talk) 23:42, 2 April 2016 (UTC)

• The basic answer is yes -- a person who has a bottle of vodka and some water will be able to stay alive longer than a person who only has the water. Looie496 (talk) 22:22, 2 April 2016 (UTC)

And how does the alcohol get digested/converted to energy by our bodies? Alcohol is not present (or only scarcely) in nature, so, why would we be able to process it into energy? Scicurious (talk) 14:40, 3 April 2016 (UTC)

Good question. One wishes there was an easily available online encyclopedia with an article about the topic. Shock Brigade Harvester Boris (talk) 15:06, 3 April 2016 (UTC)

Stupid cold fusion questions

These are all vaguely related to muon-catalyzed fusion, a process that would be the end of the energy problem if only muons were more stable...

1. Muon-catalyzed fusion is said to work because the nuclei are brought much closer together. Nuclei also seem to come closer together in a Bose-Einstein condensate. Apparently though the notion that this means fusion is considered a vulgar error. [12] As I understand it, the nuclei are smeared together in their probability distribution, but if the position of one is known the position of all the others is known to be somewhere else. Despite the talk of a "superatom", the distribution must be fairly broad then. Nonetheless... how dense can a BEC actually get? Does it get anywhere in the ballpark of fusion plasmas? There may be an answer here that I failed to understand - they give densities at a transition point, but I don't understand that to be the BEC itself?

2. It is possible to make a coherent electron source [13]. Does that mean that it is possible to bounce coherent electrons back and forth in a specially shaped chamber (or in a cyclotron-style ring), creating standing waves of electron density into which multiple nuclei might be drawn? And if so, can these waves be made smaller than the ordinary wavelength of electrons (and size of ordinary atoms/covalent bonds) by some clever wave mechanics or by making the electrons go really fast so they pick up relativistic mass comparable to that of a muon? Would that then mean they can undergo the equivalent of muon-catalyzed fusion?

3. If the electron wavelength can't be reduced, is it conceivable to do the same thing with muons, creating a coherent standing wave pattern, and pump the muons to such tremendous speed that their lifespan is extended for relativistic reasons? Wnt (talk) 21:53, 2 April 2016 (UTC)

If we could answer that Wnt, I don't think any of us would be editing WP. Instead, we would be busy trying to submit our papers to peered review journals in the quite confidence that there is a Nobel Prize waiting for us at the end of it.--Aspro (talk) 23:04, 2 April 2016 (UTC)

The Wikipedia articles Cold fusion and Muon-catalyzed fusion are not fora for general discussion about cold fusion. AllBestFaith (talk) 14:04, 3 April 2016 (UTC)

Has someone used them that way? I don't see your point. Wnt (talk) 22:08, 3 April 2016 (UTC)

April 3

Feeding a 5W IR LED

If one has a 5W infrared LED, how much volts of DC does one need to get maximum IR output without blowing it. And please also tell much CURRENT will be sufficient and how does one measure it ?  And will using a resistor (how much ohms will make us succeed, BTW has it always to be put on + wiret ? ) And will the resistor lower voltage or current, please. I hope somebody will be kind enough to let me know these things without suggesting me to filter rather confusing waters of Google....124.253.244.183 (talk) 00:37, 3 April 2016 (UTC) 

Our article on Ohm's law covers at least the last part of your question which seems like a homework question. --DHeyward (talk) 02:16, 3 April 2016 (UTC)

A 5W infrared LED typically (there are 5W versions with other characteristics) has a forward voltage of 2V DC and current rating of 1000 mA (1 Amp). The calculation of the series resistor is by Ohms Law: Supply voltage - 2V / 1 Amp. For example, if the supply voltage is 12 VDC, the voltage to be dropped in the resistor is 10V at 1 Amp, so the resistor should be 10/1 = 10 ohms. The power dissipated in the resistor will be 10V x 1 = 10 Watts. You would normally choose a 10 ohm resistor of at least that rating, example 20W, as a safety margin. The above calculations presume that the power supply is capable of supplying 12V at 1 Amp. If you do the sums correctly, there's probably no need to measure the current, but a 1 Amp (or more) DC ammeter or multimeter set on an appropriate range will allow you to do so. The resistor limits the current that would otherwise flow if the LED were connected directly to the power supply; that current would almost certainly destroy the LED or power supply (in that order of likelihood). Akld guy (talk) 04:15, 3 April 2016 (UTC)

I don't mean to be snarky but "A 5W infrared LED typically (there are 5W versions with other characteristics) has a forward voltage of 2V DC and current rating of 1000 mA (1 Amp)." seems a little unlikely. I think you mean 5V. In which case rinse and repeat. Guy's advice below seems more robust.Greglocock (talk) 07:34, 3 April 2016 (UTC)

Yes, the obvious error is 2V @ 1A is only 2W. The question regarding what a resistor will do makes me believe that this is a homework problem and reluctant to provide a complete solution even if they specified an LED. I will say, that I doubt I would ever choose a single 5W IR LED to consume 5W of total power. The spec below has many solutions. --DHeyward (talk) 07:51, 3 April 2016 (UTC)

@Greglocock: and @DHeyward: You both need to take a look here. The voltage is 2V, the current is 1 Amp for several 5W models. Akld guy (talk) 09:26, 3 April 2016 (UTC)

@Akld guy: 5W is the power rating, not power consumed. Operating at 2V and 1A will consume 2W with luminous power below that. It's simply not possible to consume 5W with 2V and 1A. It's the same with the resistor you use above. The package/material might be a 5W rated package but it has nothing to do with how much power is actually consumed. You recommended a 20W resistor for margin but you also calculated it only consumed 10W using the same law that says the LED will only consume 2W. The package has margin which usually means the junction temperature is more stable over a variety of operating powers. This margin affects emitter frequency stability over power consumed (i.e. 2W CW is the same peak frequency as the 2W with 50% duty cycle - the same device in the 2W package has more variation). There are other issues that you can see using the curves in the datasheet that make the 5W package more attractive than the 2W package but it's clear that the rating is separate from the consumed power --DHeyward (talk) 10:03, 3 April 2016 (UTC).

Well, what's your solution to the OP's question. You haven't provided one. Akld guy (talk) 10:25, 3 April 2016 (UTC)

I think it's a homework problem and we don't do homework problems. He also doesn't provide power supply available or part. This is likely a combination of resistor with parallel and series diodes and part of a beginning course on current, power and the various laws that govern them. --DHeyward (talk) 15:59, 3 April 2016 (UTC)

The specific LED you are using will have a datasheet with that information. Here is a typical one:[14] --Guy Macon (talk) 06:33, 3 April 2016 (UTC)

Simple LED circuit with resistor for current limiting

Responsible answers can only be given when the component data sheet is found. A LED has a non-linear current-voltage characteristic, see Light-emitting diode so in the circuit illustrated we can calculate Ohm's law

$R={\frac {V}{I}},$ only at one (V,I) point for that particular diode. See the article LED circuit. The OP also needs to know that a series resistor reduces both the voltage across the LED and thereby the current (which is the same in both LED and resistor), that one can measure the current either by breaking the circuit to introduce an Ammeter or by measuring the voltage across the resistor R and applying Ohm's law

$I={\frac {V}{R}},$ and that it makes no difference to the LED whether the resistor is on its + or - wire. Operating a LED near its maximum output makes the manufacturer's data sheet essential and in some cases a different circuit, such as a constant-current driver, may be necessary.

The responses above have linked to two different LED data sheets. From Akid guy, Type LZ1-00R400 quotes typical V_F 1.9V @ 1A, so we may conclude that a 5V supply and 3.1 ohm resistor will suit. The power dissipations are 1.9W in the LED and 3.1W in the resistor; that implies a heatsink for the LED and preferably at least a 5W rated resistor. But look further in the data sheet: individual LEDs can have V_F @ 1A that vary from 1.7 to 2.7 V and this can have the consequence that resistors must be selected to match the brightnesses of an array of LEDs. From Guy Macon, Type LZ4-00R408 quotes V_F @ 700mA that vary from 6.8 to 10.8 V, and in this case I_F = 1A is its absolute maximum. One should realize that this device is actually specified as 4 LEDs connected in series and the series resistor circuit is less suitable (and power wasteful). A constant current supply circuit and a proper heat sink are needed for safe operation of this device. AllBestFaith (talk) 13:47, 3 April 2016 (UTC)

Actually Guy Macon's are not connected at all. The datasheet assumes that configuration for publishing tables and it would popular if you have two lithium ion 18650 batteries (popular torch rechargeable). if you only had 1 battery, it can be configured as 2 parallel and 2 series LEDs and the voltage would 3.6V instead of the nominal 7.2V with the same power delivery. --DHeyward (talk) 16:33, 3 April 2016 (UTC)

Well noticed! The LZ4-00R408 is a 4-diode package which may have external series connections, but doesn't have to. The data sheet mentions that the diode forward voltages match within ± 0.16V which suggests the diodes also work well when connected in parallel; then implicitly V_F @ 2.8 A is 1.7 to 2.7 V. AllBestFaith (talk) 19:54, 3 April 2016 (UTC)

Downloading the Science Wiki

Hello, is there any way to download the Science (Chemistry, Biology, Physics) Wiki so I can use for my studies even when I'm offline?

Could you clarify what you're asking for? If you're referring to Wikipedia articles, you might be interested in our Book creator page, though I'm afraid that it explains that the program is mostly on hiatus. The existing science books are listed here. You can also download individual articles as PDFs (there will be a link on the left side of the screen, just below the option to create a book). Matt Deres (talk) 12:41, 3 April 2016 (UTC)

The answer depends on what sort of device you are using and the capacity it has. See : Wikipedia:Database download. Haven't used it myself but one should (I think) be able to download articles by just category. --Aspro (talk) 16:51, 3 April 2016 (UTC)

Are you talking about pages on Wikipedia, or some other wiki? "Wiki" and "Wikipedia" do not have the same meaning. --71.110.8.102 (talk) 21:48, 3 April 2016 (UTC)

April 4

Is it possible that few sperm let us be near to female?

(no answer expected) 49.135.2.215 (talk) 01:03, 4 April 2016 (UTC)LIke sushi