Wikipedia:Reference desk/Archives/Science/2012 March 19

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March 19

Why do energy saving lights (Compact fluorescent lamp) stop working?

Excluding external factors, which could break the lamp physically, what makes it stop working? Does any sort of chemical reaction take place in the gas? — Preceding unsigned comment added by 186.106.190.177 (talk) 00:10, 19 March 2012 (UTC)

The UV arc is sustained by gaseous mercury. This eventually gets absorbed by the phosphorous coating and even the glass tube. Eventually there is not enough. That’s when candles and reed lamps come back into their own – as God intended.. --Aspro (talk) 00:33, 19 March 2012 (UTC)

There are several different ways that a fluorescent bulb or tube may eventually fail. In addition to mercury depletion (which manifests as pink tubes) a number of of other failure modes are listed at fluorescent lamp#End of life. TenOfAllTrades(talk) 00:43, 19 March 2012 (UTC)

The most common failure "symptom" I see is black deposits on the glass at the electrode ends, what is the process there? Roger (talk) 10:34, 19 March 2012 (UTC)

It's in the article section that I linked...check the section about sputtering of the emission mix. TenOfAllTrades(talk) 15:16, 19 March 2012 (UTC)

For CFL's it's more likely the electronic ballast that fails: From reference 63 of the Compact fluorescent lamp article:

• Ontario’s Electrical Safety Authority will issue a warning later this week to notify users of the unexpected way compact flourescent light bulbs expire at the end of their long lifespan. Ted Olchena, a provincial code engineer with the authority said he plans to post the warning on its website. The bulbs come to an end by charring around the base, producing smoke and emitting a bad smell. That has scared some homeowners into calling fire departments, he said. But there have been no reports of fires resulting from flourescent bulbs in Ontario, Olchena said. The upcoming advisory will explain that this is a normal way for those energy­efficient bulbs, (which can last up to 10,000 hours) die.

84.197.178.75 (talk) 16:34, 19 March 2012 (UTC)

Arecibo message detection distance

How far away could the Arecibo message possibly be detected? Bubba73 ^{You talkin' to me?} 01:16, 19 March 2012 (UTC)

In the best case scenario, the Arecibo message is received at another planet while that planet has its own Arecibo-sized dish pointed directly at Earth. In this case, even a pessimistic estimate gives the Arecibo a detectable range of 10,000 light years [1]. In that same page, it is noted that Frank Drake claims the technology exists to boost this range ten-fold. It's hard to imaging, however, that we'd manage to land the message right on an equivalent detector. I recall seeing a calculation (but I can't recall where), that a modern radio receiver without a directional dish like Arecibo would detect the message from ~400 light years at most. A non-directional emitter and a non-directional detector would have a drastically reduced range, and with modern equipment (I'm told), you may have trouble communicating with Alpha Centauri. Someguy1221 (talk) 04:09, 19 March 2012 (UTC)

Polyethylene II

In the previous question, it was established that both cyclobutane and but-2-ene are possible dimers of polyethylene.

Which one has the lowest ground state?

What are the end groups of longer polyethylene polymers - what is the identity of the icosamer?

Plasmic Physics (talk) 02:09, 19 March 2012 (UTC)

Because all 3 substances (cyclobutane, (Z)-2-butene and (E)-2-butene) are gases under standard conditions, the standard enthalpies of formation should answer your question. According to the NIST Chemistry Webbook, it is -10.8±1.0 kJ/mol for (E)-2-butene and -7.7±1.3 kJ/mol for (Z)-2-butene. According to this, the standard enthalpy of formation of cyclobutane is +53 kJ/mol, so (E)-2-butene should have the lowest ground state. Icek (talk) 12:12, 19 March 2012 (UTC)

Seems useful to link your previous question. Here it is. Note carefully that I never said "2-butene" was necessarily the isomer. One of the neat tricks of catalysts is that they don't reduce the activation energy every reaction you can imagine, therefore the result is not strictly "most stable of all isomers you can imagine" (as if it were a free-for-all equilibrium ruled by LeChatelier). DMacks (talk) 04:40, 20 March 2012 (UTC)

OK. I'm asking these questions to find the habit of polyethylene polymers — what an individual molecule looks like. Plasmic Physics (talk) 00:44, 21 March 2012 (UTC)

LED lamp

I got an LED lamp today to replace a Compact fluorescent lamp. The quality of light is poor and it gets very hot. Why does it get hot? It is LED and every other LED I've ever seen never got hot. — Preceding unsigned comment added by 98.145.71.230 (talk) 06:14, 19 March 2012 (UTC)

It's unclear to me what other LEDs you've (ever) seen. All LEDs generate heat. It just depends on the amount. I don't know the number or type of LEDs on your light. But if it's intended to replace a compact fluorescent lamp then this is probably at least 8W in total. This isn't comparable to a single 5mm LED, not even say a 5050 LED. Even a 1W power LED is potentially an order of magnitude less (although if you've ever dealt with even a 1W LED at close to maximum rated current I'm sure you'll know LEDs do generate heat). If you have LED/s using a large amount of power in a small volume the heat becomes more obvious (a much more difficult to deal with). Also as mentioned in our LED article, LEDs exhibit droop meaning as the current through the LED increases the efficiency goes down, i.e. percentage of heat to useful light goes up. Dealing with heat output is in fact one of the big issues when designing bulb replacement LEDs, unlike with incandescents and to a lesser extent CFLs, LEDs really don't like heat, efficiency goes down (since current is generally fixed this means the light gets less bright) and LED lifespan will also decrease significantly if it's hot for a long time. It's easy to design for heatsinking requirements in purposely designed fixtures although with high power LEDs heat is always something you have to deal with in some way. (E.g. if you're dealing with the raw emitter you can easily kill the LED if you don't attach it to a heatsink.)

Using a large number of efficient power LEDs like the Cree XM-L at a low current would improve efficiency (although you also have to consider driver efficiency) but this significantly raises cost and may make design more difficult particularly if you need high surface brightness. And many manufacturers just use cheap generic Chinese LEDs with unclear efficiency and poor/inaccurate CCT binning and poor CCT rather then those from Nichia or Cree or other Japanese and American manufacturers who have high efficient LEDs generally with good binning etc. (Of course using cheaper Chinese LEDs may be better if you do get decent efficiency and you can afford to use more so don't need to drive them so hard.) Also LEDs are more efficient at high correlated colour temperatures whereas many people in the Western world prefer lower CCTs more comparable to incandescents, and higher colour rendering index LEDs are also less efficient. So ultimately it's a tradeoff between many factors.

Nil Einne (talk) 07:08, 19 March 2012 (UTC)

If by "very hot" you mean "too hot to touch", then it does sound like something is wrong. Is the lamp inside a fixture ? I believe LED lamps are normally designed to be used without a fixture, to allow cooling. I also agree with Nil that your household current may be too high. The driver is supposed to be able to compensate for that, but it sounds like yours isn't (either by design or defect). So, short answer is that the lamp doesn't seem to work for your application, so I suggest you return it and go with a compact fluorescent bulb. StuRat (talk) 07:18, 19 March 2012 (UTC)

Here's the relevant section from our article:

A single LED is a low-voltage solid state device and cannot be directly operated on standard high-voltage AC power without circuitry to control the voltage applied and the current flow through the lamp. In principle a series diode and resistor could be used to control the voltage polarity and to limit the current, but this would be very inefficient since most of the applied power would be dissipated by the resistor. A series string of LEDs would minimize dropped-voltage losses, but one LED failure would extinguish the whole string. Paralleled strings increase reliability by providing redundancy. In practice, three or more strings are usually used.

Another section talks about the use of a rectifier to convert A/C to DC. Either you have a low quality LED lamp that's using a resistor, or the heat is coming from the rectifier. In either case, it's just no good for your application. If you're dead set on going with LEDs, you might try hooking up a dimmer switch, so you can reduce the excess voltage and current that way. StuRat (talk) 07:31, 19 March 2012 (UTC)

There's a lot of confusing info above. The 'household current' has nothing to do with it being too high (the statement doesn't actually make sense) and I never suggested it did. If the light is very poorly designed, it may be designed for 220V and if the OP's voltage is 240V this won't work so well but there's no real reason to think this is the problem. It's fairly unlikely in a LED lamp replacement (for starters the only way you could use a resistor is if the lamp had a very large number of LEDs in series), most do use buck drivers. Far more likely if the lamp does get too hot it just means whoever designed the lamp didn't heatsink it properly. Also as I explained above, while some of the heat is coming from the driver, a big part of it is in fact coming from the LED. The idea LEDs don't generate heat is false. Nil Einne (talk) 08:40, 19 March 2012 (UTC)

Nobody said that. They should generate some heat, yes, but far less than an incandescent light, due to the much higher efficiency. Therefore, even an LED lamp without a heatsink shouldn't get as hot as an incandescent. From the OP's words "very hot", it sounds like it is getting that hot, so something is defective. Regarding your statement that "it may be designed for 220V and if the OP's voltage is 240V this won't work so well but there's no real reason to think this is the problem", why isn't the excess heat a reason to suspect this ? StuRat (talk) 08:50, 19 March 2012 (UTC)

I agree with Nil Einne. Competent designers of ANY electronic equipment design it to cope with reasonably expected input voltage ranges. For sale in countries variously noted as 220V/230V/240V, the recommended upper design limit is 254V. There's no reason for a LED light design engineer not to go for the same 254V limit. Leds dissipate power so naturally they get hot, not fogetting energy losses in the power convertor cicuit. High power LEDs dissipate a lot of power in a very small space, so, naturally, if not adequately heatsinked, they will get very hot. Keit121.215.133.147 (talk) 12:49, 19 March 2012 (UTC)

Any LED lamp designed to fit a standard incandescent fixture should be properly heatsinked and shouldn't be producing poor quality light. It sounds like something is not working as intended, or the bulb doesn't fit the lighting needs of the poster. The fact that a competently designed bulb should work properly doesn't really tell us anything about whether this bulb is working properly. Rckrone (talk) 14:59, 19 March 2012 (UTC)

Note that 'poor quality light' was undefined. As I stated in my first reply, a lot of people in the Western world appear to prefer warm white lamps similar to incandescents but it's not a universal preference so there's no reason why someone shouldn't produce a cool white LED lamp. If the problem is colour temperature then the fact the OP doesn't like it doesn't tell us the bulb isn't properly designed. If the problem is poor CRI, again since it gives higher efficiency and since some people may not feel the need for high CRI, it seems to me there's no reason why a designer needs to only use higher CRI LEDs. And again if the OP made a poor choice in CRI it's not the fault of the designer. This is particularly true if these factors were part of the product packaging, documentation or advertising. If the LED has a fairly unusual tint, as can be particular problem with cheap Chinese LEDs, then you can perhaps argue it was a poor design choice. Nil Einne (talk) 15:42, 20 March 2012 (UTC)

I once bought a batch of "cheap" 12 v led lamps for halogen lamp fixtures. The 20 or so leds gave less light than a single 20mA led. There's money to make, and led lamps are easy to produce, in contrast to for example fluorescent lamps. Makes it ideal for businesses specializing in fake merchandise 84.197.178.75 (talk) 15:02, 19 March 2012 (UTC)

Agreed, talking about how it should work is irrelevant, as clearly it's not working as it should. A lot of these are probably made in China, by people who either don't know how to do it right or don't care, with either no regulations or no enforcement of regulations requiring them to do it properly. Considering all the cases of poison in items coming out of China, it should be no surprise that they also make some defective products. As for the retailer, they probably never tested the item, and, even if a substantial number of returns occur, they still might well make a profit if this LED lamp is considerably cheaper for them to buy, from those who don't return it. StuRat (talk) 19:20, 19 March 2012 (UTC)

I question whether we have conclusive evidence the lamp isn't working properly since we have no real idea what the OP means 'gets very hot' and 'quality of light is poor'. It's possible the OP simply has unrealistic expectations and made a poor choice given their personal preference, the details of the light being fully disclosed in the packaging, advertising or website info. It actually wouldn't surprise me if the LED lamp is poorly designed since as I said in my first reply this is a very common problem in the LED replacement lamp world but without more evidence it's just an educated guess. Nil Einne (talk) 15:42, 20 March 2012 (UTC)

Actually an LED lamp can get fairly hot. It probably shouldn't be as hot as an incandecent, but it could easily be too hot too hold for any length of time. This isn't ideal for efficiency or lifespan reasons but it also depends on the LED as some are rated to handle higher temperatures then others. (E.g. the Cree XM-L is rated for a junction temperature of up to 150 degrees C.) Note that one of the reasons for the increased efficiency of LEDs is because they don't produce much light in the infrared unlike with incandescents, so just looking at the increased efficiency and saying it shouldn't get so hot is misleading. And how hot the light feels also depends a lot on various factors too, like where the heat is concentrated and whether your exposed to it. E.g. halogens tend to be more efficient then normal incandescents but try touch an exposed halogen and you will likely burn yourself (and break the lamp). While you can burn yourself on incandescents, it's far less likely if you're careful. (Halogens of course are designed to get hot.)

And an LED lamp intended for a compact fluourescent lamp replacement without any heatsinking will either be so dim to be useless or heat up very quickly and kill itself, so in that respect you're correct it won't get hot if you leave it for long enough. (Note I never said 'no heatsinking' I just said insufficient.) To be blunt, it sounds like you have no real experience with high output LEDs and don't really appreciate how much heat they can produce (again let's ignore the driver circuitry). I'm not saying this is unusual, until I myself had experience, I didn't really appreciate this either but it does mean you should be careful when providing advice.

P.S. I realised some of my earlier statements may have been misleading. Although high CRI LEDs are less efficient, it doesn't mean they generate a substanially greater amount of heat. One of the reasons for the lower efficiency is undoutedly because of the different wavelength profile needed for higher CRI which results in a light with a lower apparent brightness.

P.P.S. There's no reason to think it's the problem because the chance someone would design an LED lamp in a way that it will do that is slim. As I've said, if it's not designed to accept 240V, the most likely thing is it will die when you try to use 240V with it. And there's no reason to think it wasn't simply insufficient heatsinking or poor design in general, which is a far more common problem in the LED replacement lamp world.

P.P.S. Just to be clear, I don't disagree with APL that one of the common flaws in LED replacement lamps is poor driver circuitry, which may get very hot. However one of the flaws in design is the electronics themselves aren't heatsinked nor connected to the LED heatsinking (and thermal transfer through the circuit board is usually limited). So usually the driver circuitry may get very hot but it won't affect the LED or heatsink temperature that much. Note that even though electronic design may be poor and get hot, it doesn't mean it's using a linear regulator or resistor. Vvery likely it is using a buck driver, just a poorly designed one.

Nil Einne (talk) 15:25, 20 March 2012 (UTC)

I've noticed that some of the cheap no-name brand of LED bulbs have cheap ancillary electronics (Primarily a rectifier, I assume?) that overheat like crazy and eventually burn themselves out. Which is too bad because the LEDs themselves run cool and will last for a very long time, it seems like a shame to connect them to a circuit board that's perpetually on the verge of setting itself on fire. APL (talk) 11:45, 20 March 2012 (UTC)

Questions for the OP (so we never buy this product):

1) Where was this product manufactured ?

2) What is the brand name ? StuRat (talk) 19:23, 19 March 2012 (UTC)

OT but if you want an LED replacement lamp which really doesn't get hot, you could try something made using [2] ;-) Of course given that we're talking about LEDs using picowatts, you'll have a very, very, very, very dim light even if you use 1000 of them. Nil Einne (talk) 16:00, 20 March 2012 (UTC)

We should probably compare the heat generated by LED lamps with that from CFL's, of identical wattage. Since they are both supposed to be about the same efficiency (with LED perhaps a bit better), they should both produce a similar total amount of waste heat. And CFLs I've used (up to 23 watts) only get warm, not too hot to touch, as incandescents of the same luminosity (100 watts) do. StuRat (talk) 21:57, 20 March 2012 (UTC)

LED lighting is an important technology which is likely changing in efficiency, cost, and construction. If someone familiar with the technology ( as several posters here seem to be) would update the article LED lamp, it would be a great benefit to Wikipedia. The article is full of "as of 2010" statements, so reading the article is like, well, reading about something in an old Encyclopedia Britannica. As for the heat buildup causing degradation of light output, why do new LED light fixtures have to imitate olf incandescent light bulbs? Incandescents have by nature a tiny filament, and commonly are behind a much larger diffuser such as a piece of opal glass or ground glass. If an LED light is going to last for years or decades, why not make have an array of LEDs that are spread over an area comparable to the diffuser placed over an incandescent bulb? That way the heat dissipation is less of a problem. Going forward, why would all LED lamps have to screw into an old incandescent lamp socket. rather than being a new fixture, for new construction? It should at least be an option. In the beginning of incandescent bulb use, some were installed in old gas lights, with the wires run through the gas pipe, but we did not insist on maintaining that style of fixture going forward. Edison (talk) 14:56, 21 March 2012 (UTC)

Good point. The ideal LED placement might be to have them evenly spaced as a grid along the ceiling. Hopefully this would eliminate any need for heat-sinks. I seriously considered buying strands of LED XMAS lights as a way to illuminate the room. StuRat (talk) 23:22, 21 March 2012 (UTC)

Geomagnetic calculator

Hey!Can anyone please tell me that this calculator,http://wdc.kugi.kyoto-u.ac.jp/igrf/point/index.html , provided north latitude and east longitude , altitude and year , gives geomagnetic field values in ECI or ECEF or in any other frame of reference? Any genuine info? Thanks --111.68.97.146 (talk) 07:09, 19 March 2012 (UTC)

Have your read through their PDF file ? Maybe there's more info in there: [3]. StuRat (talk) 05:03, 20 March 2012 (UTC)

Theoretically, under controlled circumstances, can plants live indefinitely?

Plants, especially trees, can live a very long time (especially when they are not cut). But is it possible, that under certain controlled circumstances (such as in a well-maintained greenery or something, or under a glass structure like I sometimes see in movies), they can live indefinitely? If not, exactly how can they die without natural disasters or animal intervention? Narutolovehinata5 ^tccsdnew 07:24, 19 March 2012 (UTC)

I believe, in theory, yes, some plants can live forever (others have a fixed lifespan). However, in practice, changing conditions in any given area will eventually kill them, especially since they can't evolve to better fit those changing conditions. Since the ice age cycle is on the order of tens of thousands of years, that may set the practical limit. StuRat (talk) 07:39, 19 March 2012 (UTC)

With technological intervention anything possible under the laws of physics is not off limit. You could have surgeons replace parts of the tree as they wear out using cloned parts. You can strengthen part of it with bionic components. Genetic errors can be undone if you keep a record of the original genome. You could mount the whole tree in side a artificial environment that will control environmental conditions to keep them constantly favorable, and mount the whole thing on rocket engines so that the tree can escape the Earth in the event of an impending asteroid impact. You may even wish to move the tree out side of the galaxy to escape gamma ray bursts etc. But left to its own without this help it will die. SkyMachine ⁽⁺⁺⁾ 08:04, 19 March 2012 (UTC)

See also Lomatia tasmanica (which might have been cloning itself for up to 135,000 years).--Shantavira|^{feed me} 08:22, 19 March 2012 (UTC)

What a sad cycle and waste of potential. At first, it just didn't have any plants to produce with sexually, it ends up cloning itself, and before you know you spend 135,000 years having no one but yourself. Sad but true. --80.99.254.208 (talk) 09:23, 19 March 2012 (UTC)

A relevant (though short) article is indeterminate growth, which is what allows some plants to have no theoretical upper bounds to their life spans. Given clonal growth, we also have to be careful about the notion of 'individual'. For instance, the L. tasmanica link above states that the genetic material is very old, but no one plant lives for that long. You may also be interested in Pando, who has been living in the same place for ~80,000 years. Lastly, even something as common and 'simple' as a dandelion has no fixed limits on longevity. SemanticMantis (talk) 12:52, 19 March 2012 (UTC)

There's a jellyfish that has the potential for immortality, Turritopsis nutricula. See also the tree Pinus longaeva and the article Immortality 84.197.178.75 (talk) 15:13, 19 March 2012 (UTC)

non-metal, non-nitrogenous acidic cations

What would be a counterion that would buffer carbonate's basicity that doesn't contain nitrogen or heavy metals? Are there any water-stable stabilised organic cations? — Preceding unsigned comment added by 74.65.209.218 (talk) 08:27, 19 March 2012 (UTC)

I've used various phosphoranes (analogous to protonated nitrogen) as acids. All sorts of cryptands and crown-ethers can complex your choice of cation (and perhaps protect it from whatever concerns you about metals or nitrogen). Lots of borates exist with various pKa ranges. But what do you really mean by "buffer carbonate's basicity"? Carbonate is a base and carbonate/bicarbonate is already a buffer. If carbonate is too basic, bicarbonate gets you closer to neutral. Seems like you've got some context in mind...don't know what it is so it's hard to recommend something useful. DMacks (talk) 17:22, 20 March 2012 (UTC)

Ramjet

Why don't the expanding gases at the exhaust of a ramjet prevent the flow of air going into the inlet? Surely there is more pressure at the exhaust end than the inlet end; so what stops the gases going back toward the inlet? --92.28.88.124 (talk) 09:47, 19 March 2012 (UTC)

I would think that the forward motion of the engine combined with the rearward motion of the exhaust would keep the exhaust gasses from looping back around to the inlet. Or am I mis-reading your question? Dismas|^(talk) 10:00, 19 March 2012 (UTC)

Im talking about the internal situation.--92.28.88.124 (talk) 10:02, 19 March 2012 (UTC)

In the internal situation, the fuel-air mixture is moving toward the exhaust at a good pace. There is also a lot more open space behind the combustion chamber than in front of it. Both of these combine to produce a net forward thrust. I'm not a rocket scientist, so I cant gild that explanation with fancy words or math. Someguy1221 (talk) 10:06, 19 March 2012 (UTC)

It may be the OP meant by "flow of air going back the inlet" mean internal backflow and not an external loop around. I'm not a ramjet expert either, but I think you'll find that the answer lies in the physics of flame propagation. More space behind the combustion chamber compared to that in front is not the crucial factor in ramjet operation.

First, fuel can be arranged to squirt into the combustion chamber in the direction facing the exhaust. As the fuel moves toward the rear it get progressively burnt, resulting in ever increasing gas temperature and ever increasing gas velocity as seen moving toward the rear orrifice. Any tendency for flame to propagate forawards should be inhibited by the incomming relatively cold air. Picturing a ramjet side-on, if the aircraft is travelling at a certain speed, that will be the airspeed just in fron of the intake. As the air moves towards the combustion area, the air velocity drops due mainly to compression, but also due to friction. At the point where the fuel is injected the velocity is minimum. Progressively combining chemically with the fuel, air velocity progressively increases again reaching a value at the exhaust nozzle significantly greater that the initial intake velocity and producing a large forward reaction force driving the whole thing forward. This is not realy much different to a bunsen burner. A bunsen burner burning will push down a bit harder than just its weight, but of course not enough to be significant.

Ratbone120.145.147.138 (talk) 12:36, 19 March 2012 (UTC)

Think of it this way: if there were "reversed" flow in the engine, the combustion could not continue indefinitely. So, the flow reversal would self-limit; the combustion would cease; the overpressure would dissipate; and the forward flow would resume. Combustion could then restart. If the engine is poorly designed, this could result in a combustion oscillation; the absolute worst-case scenario would be a complete flow-reversal with hot exhaust-gases impinging on forward components of the engine; but in less severe cases, you'll just see high-frequency variations in the chamber pressure, turbine speed, and so on (in a ramjet, there's not a lot to "see," but you'll have poor below-spec I_sp and pressure oscillation). You may also see another symptom of this: "chuffing," or incomplete burning, resulting in gasps of sooty smoke blowing out of the back of the engine. Properly-operated, properly designed engines do not do this. You can see chuffing in a solid-rocket booster in this amazing composite video of the Space Shuttle Solid Rocket Boosters. (It should be noted, solid rockets are not ramjets, but the point is that a rocket engine "out-of-spec" will have reversed flow). What is occuring at ~5m40s into the video is that after the engine has already extinguished, and is falling back toward Earth's surface, atmospheric pressure is rising, so oxygen re-enters the combustion chamber, re-igniting the last remnants of the fuel. When this combusts, it overpressures, and blows the atmospheric oxygen back out, extinguishing the SRB again, decreasing the pressure in the combustion bore; ... allowing atmospheric oxygen back in, reigniting... hence "chuffing" or combustion oscillation. Again, let me reiterate: this is "out-of-spec" operation, in this case occurring long after the SRB has separated and completed its mission. Well-designed motors (ramjet, SRB, or otherwise) do not exhibit reversed flow during normal operation. Nimur (talk) 17:27, 19 March 2012 (UTC)

Also, on the specific subject of ramjets, don't forget the immense stagnation pressure on the forward air. The entire principle of a ramjet is based on balancing static and combustion pressures, which is why they work in sparse air at high mach numbers. Nimur (talk) 17:34, 19 March 2012 (UTC)

There is an exception, A valveless pulse jet is based on reversal of flow, and oscillation. 84.197.178.75 (talk) 17:51, 19 March 2012 (UTC)

Valid point... pulse jets are a sort of exception to almost all of my earlier explanation. But, at the risk of angering the pulse-jet fans, as far as I'm concerned, pulse-jets are "hypothetical." Lemme know next time you see one flying! Nimur (talk) 18:00, 19 March 2012 (UTC)

So it seems that Nimur is saying it is the stagnation pressure that prevents the combustion pressure pushing too far back (ie creating a balance). The ramjet article glosses over this point and i think the actual operation needs expanding much much further. Im not qualified to do it.--92.25.96.193 (talk) 19:04, 19 March 2012 (UTC)

The British saw lots of them in 1944 ;-). Nice videos on youtube of model airplanes with pulsejet engines. Very fast, very loud... 84.197.178.75 (talk) 18:22, 19 March 2012 (UTC)

Just in case you didn't get 84.197's point, see V-1 flying bomb which was powered by an Argus As 014 pulse jet. Over 8,000 were sent towards London in 1944. One threw my grandmother's Victorian dining table through a partition wall, then some windows and out into the street, so I doubt that it's really "hypothetical". We still have the table. Alansplodge (talk) 20:40, 19 March 2012 (UTC)

By all means, I'm familiar with the V-1, and I'm familiar with its characterization as a pulse-jet. I would prefer to call it a "poorly calibrated, primitive hybrid liquid-fuel/atmospheric-oxygen rocket." At this point we're mincing terms, though. It's regrettable about the table (is it still intact?), though I'm sure it's a fantastic antique/discussion-piece. Nimur (talk) 20:49, 19 March 2012 (UTC)

Yes, still intact - it's a very solid lump of oak. And you're quite right, you don't see many about these days, thank goodness. Alansplodge (talk) 21:05, 19 March 2012 (UTC)

Ah! make do and mend, that's the spirit. None of this new-fangled fabby-dabby Reduce, Reuse, Recycle. Thincat (talk) 22:09, 19 March 2012 (UTC)

Was this dining table laid at the time? I could be the source (HP obviously) for the rumors that the Nazi's had created flying saucers but then again...--Aspro (talk) 22:31, 19 March 2012 (UTC)

Reinforcing what Ratbone posted. The thrust is the result of accelerating the 'mass' of air passing through. Its reaction and not pressure that is important here because a jet doesn’t 'push' against anything. To put it in context: in the case of a jetboat, it is more efficient when the water is ejected out above the waterline rather than below it. The air coming into a ram is forced to slow down before it enters the combustion chamber and that increases the pressure well above that of the exhaust pressure -which is lower because it accelerates.--Aspro (talk) 22:14, 19 March 2012 (UTC)

Blimey! An apparently "straight" question from someone in LC's neighborhood. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:58, 20 March 2012 (UTC)

Just to sum up and clarify what I and Aspro have said:-

The air at the intake port is at the natural air pressure and temperature and the velocity relative to the engine is the aircraft forward velocity. As the air goes down the intake throat it is progressively compressed - this progressively reduces its velocity and also increases its temperature somewhat. The air velocity is minimum in the throat where the fuel is injected. At the point of injection, the fraction of fuel burnt is zero. As the air moves further towards the rear, the percentage fuel burnt gradually increases. The pressure, meanwhile, must pregressively decrease from its maximum at the point of fuel injection, back to natural atmosphere pressure at the exhaust port. As the fuel is progressivel burnt, towards 100% burnt at the exhaust port, the combustion progressively raises temperature and velocity, such that the the exhaust velocity considerably exceeds the intake velocity. As the pressure is maximum where the fuel is injected, and the flame is directed rearwards, any tendency to burn toward the front is inhibited. The excess air velocity of the exhaust/combustion product preduces a net reaction force driving the whole thing forward. Ratbone58.167.242.221 (talk) 08:47, 20 March 2012 (UTC)

So which part of the engine does the exhaust gases push against to give the vehicle forward momentum?--92.25.96.193 (talk) 17:42, 20 March 2012 (UTC)

That's a good question to ask but difficult to answer simply because it depends on the direction of the 'force vectors' which change throughout the length of the engine and in particular after the compression stage (before the compression stage they are negative) but I imagine that statement doesn't leave you any the wiser. So lets construction a Pons asinorum. If a hot gas is allowed to expand -it cools (bad). Therefore, the gas must be constrained radially so that it accelerates rearward only. All those surfaces that lay towards the perpendicular plane to the rearward axial flow have a higher force vector that points forward (or the push that your looking for). Yet, remember that that that is due to the initial compression. As the exhaust gas accelerates rearwards, its pressure starts to drop leaving behind it high pressure regions (the push – in all ways -it both accelerates and helps to maintain the pressure brought about due to the reduction of velocity of the incoming air and exerts pressure on the walls of the engine). Yet the main sum of the vector forces propels the aircraft forwards. That's also why a ram jet can't produce static thrust. In other words its only by stint of its forward momentum that the intake air can get compressed. The fuel's heat energy then ensure that the compressed air mass wants to expand and the design of the engine ensures that it only has one direction in which to do that. Though, I'm sure there must be an easier way to explain all this. You may have noticed also, that the upper stages engines of heavy transport rockets (Arian etc.) have an extended nozzles. This is so that at the low ambient pressure of the upper atmosphere, some of the radial velocity potential of the expanding exhaust gas a it leaves the throat can be captured as forward thrust. Don't tell me, your next question will be “why then don't we see more ram jet powered aircraft.” --Aspro (talk) 20:38, 20 March 2012 (UTC)

Where does the reaction push back on? I'll see if I can make it simpler. The ramjet article, http://en.wikipedia.org/wiki/Ramjet has diagrams that are more complex than they need to be in order to understand the principle. The simplest ramjet comprises a cylinder, open at both ends, with an internal narrowing in the midle, like a venturi, but it is important to note that the mode of operation is not as a venturi (ie it is not about increasing velocity, it is about increasing pressure) It is the internal narrowing that causes the compression. This compression does of course mean that there is pressure against surfaces forward of, and just at, the point where it is narrowest. This pressure is acting acting against inward sloping surfaces as the narrowest point is approached. - the vector sum of all this distributed force impedes the forward motion of the engine/aircraft. The fuel is injected at the narrowest point, directed rearwards. Combustion causes temperature rise - this means that for the same pressure a larger volume must be provided. So, as the combusting air & fuel procedes toward the exhaust orrifice, an expanding of the tube is provided, so that pressure is back to atmospheric at the exhasut nozzle. Since the temperature after the narrowest point is greater than the temperature before the narrowest point, there is a greater surface area to push on, to accomodate the greater volume required. So, even though the expansion side pressure gradient is a almost a mirror of the compression pressure gradient, the greater engine surface subject to expansion side pressure means a greater total force than the net force on the compression side. Prctical ramjets don't have to have a narrowing of the tube - they can have an co-axial tapered plug, as shown in many drawings. This does not change the principle. Ratbone121.221.30.213 (talk) 02:40, 21 March 2012 (UTC)

additions and corrections update

I recently came across a possible addition to your "Neutrinos" definition. I hope I am doing this correctly. Consider the following artical from March 19, 2012, Lab New Daily: (link) — Preceding unsigned comment added by Drummer1088 (talk • contribs) 18:06, 19 March 2012 (UTC)

Thanks for the link. It is not necessary to copy/paste the entire article here; please see our Wikipedia:Copy-paste guidelines. I have removed the verbatim text of the article you pasted above, per our copyrights policy. I'll read through the article to see if there's anything new that should be added to our article; was there a specific item you felt was important from this article? Nimur (talk) 18:22, 19 March 2012 (UTC)

After review, our main neutrino article, and the more recent Faster-than-light neutrino anomaly, both already include reference to the official OPERA statement from February, 2012, which reflects that the experimental was most probably beset by instrumentation errors. Both articles seem to correctly indicate that the earlier reports of a faster-than-light speed have been debated, and are widely considered to have been in error, though the results are not "completely" discredited. Both articles also correctly indicate that further research by independent labs, including the ICARUS group, are being undertaken. Nimur (talk) 19:06, 19 March 2012 (UTC)

Liposuction to produce biofuel

I asked previously if it was possible to use animal fat to produce biofuel, and the answer was affirmative. So it got me thinking, is it possible for the government to offer free liposuction to fat people in exchange for their fat which could then be turned into biofuel? Is this economically feasible? ScienceApe (talk) 21:59, 19 March 2012 (UTC)

Whilst I'm sure it is feasible, it could never be economically viable - think about how much liposuction costs, compared to how much fat you get. Even with the price of petrol as high as it is here in the UK, it would probably be at least 100 times as expensive to remove that from a person. SmartSE (talk) 22:02, 19 March 2012 (UTC)

(WP:EC, I basically agree with Smartse above) In short, no, it is not economically feasible. The small amount of energy gained from the fat of even a large human is of negligible value compared to large institutional infrastructure and expert labor costs. A more reasonable idea is salvaging used deep fryer oil from restaurants. One such application of this is biodiesel. SemanticMantis (talk) 22:07, 19 March 2012 (UTC)

At a 20% fat to electricity conversion efficiency, 1 pound of fat is approximately equal to 1 kilowatt-hour of electricity, which is worth USD$ 0.10 to 0.15, depending on the exact location. --Itinerant1 (talk) 23:31, 19 March 2012 (UTC)

The medical cost of extracting the fat would indeed make the value of the human fat negligible. The fat, however, is sometimes used on other parts of the person's body (e.g. breast augmentation). Human faeces, on the other hand, might well be a somewhat more viable resource (and have long been used for this purpose in rural areas, I believe) to produce Biogas. 58.111.224.202 (talk) 03:24, 20 March 2012 (UTC)

Note that the Nazis did use human fat as fuel in extermination camps, but it was only economically viable because of how cheaply they got it, without the need for surgery. StuRat (talk) 03:38, 20 March 2012 (UTC)

Do you have a citation for this? It seems like it would have been more trouble than it's worth, besides perhaps a few candles or lamps for novelty's sake. This is especially true when you consider that most people who'd spent any time in those camps did not really have a lot of body fat. APL (talk) 11:36, 20 March 2012 (UTC)

This compilation of graphic testimonies discusses the fuel requirements for cremation in Auschwitz. It notes that some groups of bodies were relatively healthier vs others that were emaciated, and that the healthier ones required less external fuel supply in the crematoria--even though there would be more material needing to be incinerated--due to their own ability to fuel the fire. DMacks (talk) 17:10, 20 March 2012 (UTC)

Yes, one of the defenses the Nazis used at the Nuremberg Trials was to claim that they couldn't have murdered and incinerated the number of people claimed, because the camps didn't requisition enough coal for that. And, indeed, the figures for the amount of coal they used only seemed to be enough to heat the barracks, not to run crematoria. However, once the fuel value of the fat was figured in, in all made sense. Also note that a good portion of the people were sent to the crematoria upon arrival in the camps. Our view is biased, because the only ones we have on film are the few skeleton-like survivors or skeleton-like detainees who died in the last few weeks before liberation. The plump detainees who were killed years earlier were long gone. StuRat (talk) 21:38, 20 March 2012 (UTC)

Wouldn't it have been protein and other components as well (excluding some such as bone and water perhaps)? According to Body fat percentage the average is around 25% (depending on gender, age and other factors) but it seems likely the average there would be significantly lower. [4] suggests the protein content is something like 16%. [5] gives 16.6% for protein and 14.9% for fat. I don't actually know what sort of figures we can expect for holocaust victims, given the atrocious conditions including starvation many likely only had muscle left for fuel. Those who were better off would have had more fat, but I wonder whether the protein was often just as big a contributor to body weight differences. Remembering that even if they had just arrived, very often they would still have been in poor conditions beforehand so I suspect even many of those who had just arrived weren't exactly 'plump' (nor very fit/muscular) even if they weren't the horrificly emancipated victims common in picture. The over double energy density of fat does mean fat may have been a bigger contributor even if body fat percentage was only 10% but I don't know if it's accurate to suggest it was primarily fat that was used as fuel. Nil Einne (talk) 00:15, 21 March 2012 (UTC)

Fat contains 9 kilocalories per gram, while protein only contains 4, so fat provides the majority of the fuel value. I believe almost all of that 4 is used in incinerating the meat itself (specifically, in vaporizing the water contained therein), so there is little or no excess energy provided by protein (unless it was dessicated first). Try lighting a slab of lean meat on fire. It doesn't work. Fat, on the other hand, burns well, once you get it started (anything which acts as a wick helps a lot). StuRat (talk) 02:07, 21 March 2012 (UTC)

But consider that Nazis weren't just rounding up people in the streets and sending them straight to extermination camps. Many (probably most?) victims came there from ghettos like Warsaw ghetto or internment camps like Westerbork transit camp, where they lived for years on extremely restricted food rations.

Even if there were some "plump detainees" on trains to extermination camps, that does not seem adequate to change significantly the amount of needed coal. Modern gas-fired crematoria consume 2000 cubic feet of natural gas per body, which is around 37 kg. Various online sources likewise quote fuel demands of Nazi crematoria at around 30 kg/body.--Itinerant1 (talk) 00:37, 21 March 2012 (UTC)

A bigger problem is that fuel requirement estimates would be normally based on bodies of well-nourished victims in the first place. Cremating bodies of emaciated victims would have required more fuel than average. Saying that not all of them were emaciated does not really help to address the discrepancy. (Which is, by the way, quite large: according to the article linked by DMacks, in order to harmonize data on coal shipments to Auschwitz and conventional Holocaust estimates of the number of prisoners killed there, camp authorities had to be able to manage with 4 kg of coal per body.) --Itinerant1 (talk) 01:46, 21 March 2012 (UTC)

The other major factor is running the crematoria continuously, versus letting them cool off between bodies. The amount of fuel required in the latter case is far greater. The reason is that most of the coal is required up front, to drive off all the water, and then then, later on, the excess energy is released. The difference is therefore whether this heat energy just goes up the chimney, or used to dessicate the next body. StuRat (talk) 02:02, 21 March 2012 (UTC)

But that raises another discrepancy: the article states that it would take 16 tons of coal per day to run all crematoria at Auschwitz if they were operated around-the-clock, on a continuous basis. At that rate, total coal shipments recorded for the period from April to October 1943 (six months) were sufficient to operate crematoria only for about 30 days, even assuming that they didn't use the coal for anything else.--Itinerant1 (talk) 05:42, 21 March 2012 (UTC)

I would guess they only kept one running full-time, and fired up others as the need arose. This would be almost as efficient as keeping them all going continuously. StuRat (talk) 06:37, 21 March 2012 (UTC)

Cecil Adams recently did an article on this : The Straight Dope : Is Excess American Body Fat a Potential Energy Resource?

Hope this helps. APL (talk) 11:33, 20 March 2012 (UTC)

Actually, you can harvest that energy economically, and indeed, with a greater-than-unity gain, if, instead of using liposuction, you simply put the affected people (mumbleincludingmemumble) onto a rational diet. Food calories are very expensive (in energy terms) to produce, so if you let people partially live on their build-in reserves, you do get a large saving in primary energy. And a calorie saves is a calorie gained. Now, about how to make people follow this diet I have no plausible idea... --Stephan Schulz (talk) 18:22, 20 March 2012 (UTC)

One additional factor is that the surgeon performing liposuction will typically extract fat from a limited number of sites in a single procedure, and it is generally considered unsafe to extract more than 5 liters (~10 lbs) of fat at once. A borderline obese person of average height has 30 lbs of excess subcutaneous fat spread under his skin and around 3 lbs of visceral fat inside the abdominal cavity. There are some visible excesses near the abdomen and the hips, but that's only part of total fat. It would be extremely unsafe to remove all excess fat in a single procedure.

One alternative method that could be close to break-even economically is to install stationary bike trainers with power generators in gyms and to hook them up to the grid. It would take a long time to recoup the cost of building and installing the bike trainer. A moderately fit person such as myself might be able to put out 200 watt of useful energy for extended periods of time, producing electricity at the rate of USD$0.03/hour. However, a sufficiently cheap bike trainer that is manned 12 hours a day everyday could pay for itself in several years and begin generating money after that. --Itinerant1 (talk) 20:45, 20 March 2012 (UTC)