Wikipedia:Reference desk/Archives/Science/2012 January 26

= January 26 =

liquid nitrogen in an electric kettle
I've seen experiments where people throw hot water into liquid nitrogen (I'm not sure how dangerous that is). I wondered what would happen to liquid nitrogen in an electric kettle (switched on)). Would it just break the kettle? Would the element respond differently at low temperature and cause the fuse to blow? --129.215.47.59 (talk) 02:19, 26 January 2012 (UTC)


 * I am not sure if it helps but you may want to click here and search for "Making better clouds is easy". Von Restorff (talk) 02:33, 26 January 2012 (UTC)


 * It might break the kettle. Perhaps softer metals, like copper, might hold up a bit better.  The electrical element might break, too.  If not, then it should work normally. StuRat (talk) 02:46, 26 January 2012 (UTC)

I've stored liquid nitrogen in metal containers. They never broke or became brittle; they're just dangerously cold and inconvenient if uninsulated. But I never did anything foolish, nor tried to heat the nitrogen on a stove. The most important thing to remember when storing liquid nitrogen is to provide a pressure relief - especially if you're going to heat it. Usually, this takes the form of a loose-fitting cap. Here's an OSHA MSDS for cryo nitrogen, and here's a more useful "general safety guide" from the government's Cancer Research Center. For good measure, here's the SLAC/Stanford Environmental Safety and Health manual for liquid nitrogen. Remember: explosions are caused by overpressure, not by combustion (despite what Hollywood has mis-taught everybody). Heating (or even just working with) a cryo liquid, like liquified nitrogen, without providing a safe pressure release is asking for very dangerous, deadly trouble. If you'll be producing large quantities of gas, watch out for asphyxiation hazard. Nitrogen is odorless and tasteless and colorless, and looks an awful lot like regular air, (and mixes well enough to displace room air). You might not have time to notice the lack of oxygen, if you do something stupid. Nimur (talk) 07:23, 26 January 2012 (UTC)
 * Sometime ago, I saw footage from an experiment with pigs, it was ammusing. There was a food trough inside a fume hood purged with nitrogen, the pig had to stick its head in the fume hood in order to reach the trough. Whenever it did, it fainted and fell over outside of the fume hood, before waking up and trying again. I don't actually remember the point of the experiment. Plasmic Physics (talk) 10:35, 26 January 2012 (UTC)
 * The Horizon series documentary How To Kill A Human Being featuring Michael Portillo (first broadcast 2008 on BBC Two) examined alternative methods of humane execution, including Nitrogen asphyxiation. The point of the experiment with the pig was to demonstrate that it was not painful, as the pig kept heading back for the food under the fume hood, repeatedly being rendered unconscious. The sequence with the pig appears in the last ten minutes of the programme. Note that the researcher who put on the pig demonstration was Dr. Mohan Raj (not the actor) of University of Bristol, who has researched humane methods of livestock slaughter. -- ToE 14:11, 26 January 2012 (UTC)
 * From a quick search, it looks like an electric kettle holds 1 to 2 liters; according to liquid nitrogen this volume is multiplied by 700 on evaporation to gas at 20 C. Now 1400 liters is a lot of water, but it's actually only 1400 x (1 decimeter)3 or 1.4 cubic meters of air.  A typical room (even a closet) probably averages at least two meters in each dimension, which means that the oxygen would be diluted down by only 17.5%.  Of course, that applies only when the gas is well mixed into room air - if you somehow knock yourself out and collapse at the foot of the kettle with the cold vapors down into your throat, or (in the classic oopsie) you want to make a cute visual effect of dry ice or liquid nitrogen in a hot tub you and your friends are bathing in, things can get tragic. Wnt (talk) 14:32, 26 January 2012 (UTC)
 * Apparently most electric kettles have a heating element made of nichrome; this has a temperature coefficient of resistivity of 0.4 x 10-3, which is only 1/10 that of silver, copper, aluminum, or tungsten. Liquid nitrogen, at -196 C = -216 K below room temperature, so by the formula on that page resistance is reduced by a ratio of 0.0004 * 216 = 0.0864 = 8.6%.  Unless the kettle is very near the capacity of the breaker indeed, it should not trip it due to the cold.  But the first kettle element I found on a Google search mentioned an aluminum element, which being ten times more susceptible might be more likely to trip it. Wnt (talk) 14:58, 26 January 2012 (UTC)


 * Caution: Placing something extremely cold into something extremely hot might cause the liquid to flash into vapor in an explosive fashion, with liquid getting splattered out of the kettle. If the opening were so small as to confine the vapor expansion, the overpressure might cause the metal to burst (like a small bomb). Some sort of metal fracture due to sudden contraction, or breakage of the heating element seems possible. Edison (talk) 20:11, 26 January 2012 (UTC)


 * Surely a teakettle isn't designed like a bomb, though! There's a place to pour in the water, which is not designed to resist pressure, and you'd be hard pressed to pour in the nitrogen without having it open. Wnt (talk) 01:52, 27 January 2012 (UTC)
 * I just think back to childish experiments with explosive substances such as nitrogen tri-iodide, or steam in a confined space, and what might go wrong in a worst-cast incident. With a wide-mouthed kettle and a loose-fitting lid, splatter would be the risk. If a way were devised to try the experiment with a very small opening or a tight lid, explosion potential would increase. Edison (talk) 06:24, 27 January 2012 (UTC)
 * Liquid nitrogen splatter isn't generally dangerous - it doesn't stick because it's surrounded by gas. You can coddle a drop in your hand if you want.  Yes, all kinds of crazy things can happen when kids play with steam and closed containers, but... it's a kettle.  It's not supposed to be closed.  The other thing to remember is that liquid nitrogen gives off the most gas when the first drops you pour strike the container.  You can't get it all in the container, then have it "detonate".  Now yes, you can create a pretty dramatic explosion with liquid nitrogen if you try, e.g. by putting a couple of inches at the bottom of a two-liter bottle weighted with a brick, rapidly seal the cap, rapidly dump it into a trash can of water, rapidly cover it with a piece of plywood you would like to send to treetop level, and rapidly retire to a safe distance.  But that's a different kettle of fish... Wnt (talk) 16:54, 27 January 2012 (UTC)
 * WP:BEANS. Von Restorff (talk) 09:48, 29 January 2012 (UTC)

Black umbrella in summer???
Dear frens, i hav a doubt....in summer we are asked not to wear black and dark clothes....but we use black umbrellas....why? -regards — Preceding unsigned comment added by Naviappu (talk • contribs) 15:34, 26 January 2012 (UTC)
 * At a guess I would say that a) it's "stylish", and b) if you've got your umbrella up then it's generally raining which means it's unlikely there will be much sun to heat up your umbrella up-- Jac 16888 Talk 15:38, 26 January 2012 (UTC)
 * An umbrella is also not in direct contact with your skin, which means it's not going to heat you up as much as black clothes will. Smurrayinchester 15:46, 26 January 2012 (UTC)


 * Black and white offer opposite extremes of blackbody radiation, or just the absorption of light, and each have their purposes. And culture varies also.  The poor Arabic ladies in chadors wear black in summer; then again, they are more or less supposed to be indoors, where they may be emitting more infrared than they absorb.  For an umbrella my guess is that it's more important for it to block the sunlight than for the fabric to stay cool; hopefully the breeze will do that anyway. Wnt (talk) 15:47, 26 January 2012 (UTC)


 * The rain has a cooling effect on the umbrella (when water evaporates it absorbs heat). --Colapeninsula (talk) 16:04, 26 January 2012 (UTC)


 * Wow. From the comparison with clothing I just assumed they had to be using a beach umbrella or something.  Yeah, in the rain this is just silly and it doesn't matter at all. Wnt (talk) 17:55, 26 January 2012 (UTC)
 * It rather depends on the desired effct. For these chaps, thermal efficiency is probably not an issue. Alansplodge (talk) 18:03, 26 January 2012 (UTC)

solar storm
dear frens, i went thru topic solar storm in wikipedia...i got information...but i need practical information if anybody can help me...how does solar storm affects us? actually,wen i went thru wikipedia..i was not clear in concept of how earth's magnetic field wud b affected..can anybody pls tell me.... -regards — Preceding unsigned comment added by Naviappu (talk • contribs) 15:36, 26 January 2012 (UTC)


 * A solar storm can refer to various related phenomena: solar flare, coronal mass ejection, geomagnetic storm; you can read about their effects Solar_flare, Coronal_mass_ejection, Geomagnetic_storm. For examples of the effects on earth see March 1989 geomagnetic storm and Solar storm of 1859.  Possible effects include radio interference, satellites being temporarily damaged (possibly disabling TV, radio, telephone, GPS satellites), power blackouts, damage to telephone lines (or formerly to telegraph lines), false read-outs on military early warning systems, and damage to pipelines. For people in airplanes or astronauts in space there can be significant radiation exposure.  The 1859 storm saw fires started by sparks from power surges, so that's probably a risk as well. --Colapeninsula (talk)


 * Note that the Earth's magnetic field protects us by redirecting charged particles towards the Earth's poles, where they create the Northern Lights near the north pole and Aurora Australis near the south. StuRat (talk) 18:04, 26 January 2012 (UTC)

Gamma ray lenses?
What material is used to design lenses for gamma ray photography? Germanium seems to be used for infrared. Electron9 (talk) 16:54, 26 January 2012 (UTC)


 * None. --Aspro (talk) 16:58, 26 January 2012 (UTC)


 * Compound parabolic refractive lenses have been used for hard x-rays up to ~60 keV, . Mikenorton (talk) 17:11, 26 January 2012 (UTC)


 * If one wants to gamma ray examine (say) a steel reinforced concrete bridge support, then a   gamma point source is placed behind it, and the imagining film the other side, the film just  records the shadows,  …. no lenses involved. Collimators can be employed for satellite borne instruments but they are not exactly lenses. It would  help if the OP indicates what the  application is the he is is asking about. Difficult question we can answer immediately; impossible questions and those requiring telepathy take just that little bit longer. --Aspro (talk) 17:46, 26 January 2012 (UTC)


 * I was thinking about an equivalent to a handheld infrared camera (flir). Translated to this it would be some kind of gamma source and for example an mobile phone that is imaged in detail. Electron9 (talk) 18:10, 26 January 2012 (UTC)


 * It's "deep" into research territory when you start talking about imaging using gamma rays. When we image with light and infrared and ultraviolet, we use things that generally look "sort of" like glass lenses and mirrors, forming refractive and reflective surfaces.  As we get farther out from the visible spectrum, in either direction, "optical" materials start to look stranger.  Toward the low frequencies, we get radio telescopes, which still look a lot like a Newtonian reflector, but aren't so "shiny and silvery and reflective" (when viewed by the naked human eye in visible light).  On the other side of the spectrum, the optics get really weird really fast, as the wave energy becomes higher (and the wavelengths become shorter).  You may find the Chandra X-Ray Observatory enlightening: X-ray optics are pretty weird (they look nothing like optical optics).  The key fact to keep in mind is that it's easier to "bounce" than to "bend" a very high energy ray - so the "optical elements" in such imaging systems tend to use oblique incidence (very very shallow angles) on very precisely designed material surfaces to steer the beam toward the imaging element.  Consider, Compton Gamma Ray Observatory, too.  The angular resolution is very poor - basically, by steering the aperture, you can get some angular resolution; but the images are pretty blurry.  There's a schematic at NASA Goddard's Compton Gamma Ray Observatory webpage.  I'm sure if you're more interested, we can dig up more technical papers.
 * The key concepts to keep in mind: if you want an image, you don't need a "lens" or a "mirror." All you really need is coherent scattering.  If you generalize this concept, you see that it's possible to image using any type of wave: SONAR and ultrasound uses sound waves; optical light photography uses electromagnetic waves in the visible spectrum; radio astronomy uses radio signals (and sometimes, as in the Very Large Array, usees synthetic aperture imaging; and so on.  Nimur (talk) 20:30, 26 January 2012 (UTC)
 * Thanks!, I thought of gamma photon reflection against heavy metals some minutes after the last post. But considered that they most likely would be absorbed and that the idea was DOA. Seems it I was not completely wrong ;) The links were interesting, but many of the articles lack good illustrations of the actual instruments. Electron9 (talk) 00:03, 27 January 2012 (UTC)
 * Here's a webpage with a fairly good diagram of the EGRET, the Energetic Gamma-Ray Experiment Telescope on CGRO. As you can see, the instrument doesn't look much as you might expect a conventional visible-light telescope to be designed.  And here's a sample image, VP9185, a high-level data product synthesized around 2002 from data collected in early year 2000.  As you can see, a gamma ray image isn't really a great "photograph" - but, that's what the object looks like in the gamma spectrum.  (In this case, the target was a wide field view of the Milky Way - and it's a loooong exposure photo, around two weeks, with arguably the world universe's worst case of rolling shutter distortion).  The target's effectively "at infinity," though, so there's not much visible smearing.  Nimur (talk) 02:08, 27 January 2012 (UTC)

Liquid Thorium Reactor
The Liquid Flouride Thorium Reactor sounds like it should be able to solve satisfactorily almost all of the problems associated with current nuclear power, as explained by this presentation at the House of Lords (Youtube). Now anything that solves all of our biggest issues without making new even bigger issues immediately raises my suspicions; is this really stacking up the way that they say it is? Falconus p t   c 18:33, 26 January 2012 (UTC)
 * I know India is very interested in making Thorium breeder reactors since they have abundant thorium reserves. I don't see any major problems. My guess is, shit takes time and money to make. ScienceApe (talk) 18:59, 26 January 2012 (UTC)


 * The problem is that reactor designs on paper look very straightforward and easy. Reactor designs in reality are full of many different types of engineering difficulties. When designing something as large and complex as a power reactor — and power reactors are quite large, and I heavily recommend touring one if you ever get the chance, to see exactly how big and complicated their whole facilities are — in the end it's often the case that the newer, "next generation" designs don't quite pay off to the degree as they are hyped to. Now, that's not to say that PWRs and BWRs are actually the best options on the table. But in designing a power reactor, one has to go for a very tough-to-reach sweep spot at the intersection of profitability and safety.
 * Looking at the article itself, I'm not convinced that the design there is really the best out of the possibilities for next-gen reactors. For one thing, reading the "Two versus Single fluid" section seems to imply that the engineering will be quite complicated, and the entire thing uses rather noxious chemicals at many stages (within the reactor, and for reprocessing). It sounds like there's a lot of room for error in the plumbing. It's also riddled with fallacies, e.g. "Burial in rock or clay is reasonable and safe by that time, because we've always lived with uranium in rock." This is true only if one neglects the fact that the natural concentrations of uranium in rock are very low — usually less than 1%, even in mineable ores. (There are a few very rare exceptions where you get concentrations up to 60% or so.) So it's not quite the same thing as uranium in rock, because presumably these wastes would be quite concentrated by comparison. I'm also unclear about the "annual fuel requirements". You can't power a reactor on natural thorium alone — it is fertile, not fissile. So there's some uranium missing from that graphic.
 * Frankly I find the idea that any nuclear reactor is going to be cheaper than coal a pretty suspicious statement. Coal is pretty damn cheap.
 * But anyway, it's hard to know. As the article points out, there haven't been many of these types of things built, and the ones that were built were built a long time ago. PWRs and BWRs are the "devil you know." That isn't to say that they're ideal — they're really not. But I would be suspicious of anyone touting the one reactor that's the easy solution to all of nuclear power's difficulties. Most experts who aren't in some way already committed to promoting nuclear power generally don't think there is one easy solution. --Mr.98 (talk) 19:51, 26 January 2012 (UTC)
 * "You can't power a reactor on natural thorium alone — it is fertile, not fissile. So there's some uranium missing from that graphic." I'm not sure what you are talking about. It's fertile, which means it breeds U233, and apparently according to some designs you can extract energy from the U233 produced in situ so what's the problem? ScienceApe (talk) 21:03, 26 January 2012 (UTC)
 * You need neutrons before the thorium transmutes into U233, to kick off the process. Where are those initial neutrons coming from? You've got to have some kind of uranium fuel in there to start up each load of thorium, at the very least. --Mr.98 (talk) 22:16, 26 January 2012 (UTC)
 * Is it possible to use some sort of tabletop (or larger) neutron generator, such as the laser+deuterium setup? Or do you need a seriously large amount of neutrons from the beginning to get the ball rolling? Wnt (talk) 01:29, 27 January 2012 (UTC)
 * You need a seriously large amount of neutrons. Remember that to get your chain reaction working, you have to have enough neutrons to keep generating more neutrons. And some number of those neutrons, then, are to breed the U233 (and thus are lost to chain). Now once you have the U233, you can keep the thing going again. But you are going to need a lot of neutrons to begin with (and the transmutation is not instantaneous — it will take a few weeks in most cases). It strikes me that you'd need to be able to keep a chain reaction going from the very beginning. I'm not saying it wouldn't work. It just means that some large number of your rods at any given time probably have to already have U235 or Pu239 or U233 in them. It also means you probably have to swap out the fuel in clever ways to make sure there are always enough there to sustain the reactions you need to breed more U233. But it complicates the very simple idea that you're just plugging in thorium and nothing but. The chemistry of the thing sounds fantastically complicated to me. My main criticism of the idea that this is a panacea is that no panacea actually turn out to exist once you start building them. But there are better and worse designs, to be sure. --Mr.98 (talk) 02:27, 27 January 2012 (UTC)
 * I don't think the article was claiming that it runs on Thorium alone, obviously it uses U235 as well to kick start the reactor. After that, U233 takes over. ScienceApe (talk) 03:53, 27 January 2012 (UTC)
 * Okay, thanks. I really hope that it works out the way that they are saying.  We shall see though. Falconus p  t   c 11:55, 27 January 2012 (UTC)

Voltage characteristics of bike dynamo
I want to use my bicycle hub dynamo to drive an LED (using a simple circuit with a rectifier and a capacitor to reduce flicker). The hub is a 2.4W, 6V hub but I'm not sure what this means in practical terms - can it never produce a voltage higher than 6V? What if the circuit is open?

This is the information for the LED I will use, but a simple voltage/current curve is provided here. I can't figure out what will happen... the voltage will be forced to 6V? But then the 2.4W rule would be broken (as well as my LED)? --129.215.47.59 (talk) 19:12, 26 January 2012 (UTC)
 * Bike generators I own, which are pretty old, produce alternating current, by rotating against the tire rather than being in the hub. The voltage (and frequency) vary with the speed of rotation (low speed, low voltage & low frequency). At a given speed, an AC generator should produce a higher voltage with no load that with a load, since it has impedance. Unless it has some electronic regulation circuit (a real possibility), the voltage should increase above a normal 6 volts as it spins faster than "normal." Powering a resistive load such as an incandescent light bulb, Ohm's law would imply 400 milliamperes at the 6 volts "normal" RMS into a 15 ohm resistive load. A solid state load (nonsinusoidal waveforms) or reactive load will complicate the equations a bit, since there would be vars as well as watts. I would not expect the output to be at all a pure sinusoid in any event. If it has an AC output, you should consider a rectifier circuit such as a full-wave bridge rectifier (available as an integrated circuit, or easily constructed from discrete rectifiers) rather than a single rectifier. Edison (talk) 20:05, 26 January 2012 (UTC)
 * might be of use to you: it's a chap who's upgrading his old Sturmey Archer dynohub lights to use LEDs. SA hubs kick out about 2W rather than the 2.4W/3W that most dynamos do, but the principles will be the same. FWIW, voltage characteristics are dependent on the hub dynamo: the more expensive ones often have voltage- and current-limiting circuitry in them, but the cheaper ones don't. Some dynamos can hit 20V+ with no load, but most saturate at about 400mA (2.4W dynamos) or 600mA (3W dynamos). See also which has lots of graphs and figures. You can also run a real bodge-job LED lightbulb by getting a 12V MR16 fitting LED lightbulb and a voltage doubler circuit. Very bright, but no standlight and might increase drag slightly - at least for mine. The plus side is that LED lightbulbs tend to have circuitry inside them that cleans up the input so you don't have to. Brammers (talk/c) 21:22, 26 January 2012 (UTC)
 * You probably should check out forums and other sites where plenty of people have done this, and more, before. E.g.  . It sounds like there are a lot of things you haven't considered. For example, unless your dynamo already produces a clean output close to 3.5V, you would want a driver of some sort (probably a constant current buck) which you don't seem to have considered. (Trying to put 6V through the LED is likely to kill it and using resistors or a linear regulator like an AMC7135 if your normal output is 6V is a bit dumb with a dynamo, your efficiency is likely to be something under 65%.)  Also have you considered heatsinking? If you're planning to power the LED at 1A for example (perhaps not possible considering your dynamo), you'll easily kill the LED with insufficient heatsinking and at the very least would lose a great amount of efficiency. Air movement on a bike helps considerably but you still need to get the heat away from the LED in the first place. Also why did you choose the XP-G? A XM-L T6 will beat the XP-G S2 or even I expect S3 in efficiency under basically most loads. In your case, this is likely to mean more light output and easier heatsinking. While the XM-L will cost more, the price difference isn't that much (you can buy an XM-L for under US$10 shipped) and considering you're likely to be spending a fair bit more on all the other hardware, shouldn't be an issue. The primary reason I expect to choose an XP-G would be even with good optics, the XP-G would probably give more throw then the XM-L. (Of course a triple XP-G will probably beat a single XM-L in terms of efficiency.) And speaking of optics, have you considered them? P.S.  on drivers may be useful although it's not specifically directed at the bicycle market. Nil Einne (talk) 05:40, 27 January 2012 (UTC)
 * Isn't there something to stop a 2.4W dynamo from producing 1A at 6V? I figured that the voltage would drop because the dynamo couldn't produce enough current to maintain that voltage. If the dynamo produces only 400 mA, then the voltage shouldn't get that high. I bought the LEDs two years ago, and don't recall the reasons for those specifically. I'll worry about optics if I don't like the way the light looks on the road. 129.215.47.59 (talk) 11:15, 27 January 2012 (UTC)
 * Actually on further consideration you're right, you're not likely to kill the LED with your dynamo (well unless it's not heatsinked properly). However presuming you plan to use a single LED, it's still not going to work very well presuming you want a bright light. From what I read, dynamos are often fairly constant current at speed, so you're liable to be feeding 400mA to the LED. However this IMO isn't that bright even with an XM-L, let alone an XP-G. If your dynamo is capable of producing 6V at 400mA at resonable speed it will make more sense to use appropriate circuitry to either increase the current or alternatively use 2 XP-G or other LEDs in series. Both of these options seem fairly common. P.S. I didn't check the Pilom link earlier but it has a lot of good suggestions, including ways to get good light output at speed while still lighting up at resonably low speed. P.P.S. The XM-L didn't exist 2 years ago. Nil Einne (talk) 15:02, 27 January 2012 (UTC)

Can you make plastic from biofuel?
Topic says it all. ScienceApe (talk) 20:59, 26 January 2012 (UTC)
 * Read this. Turning plastic into fuel is also possible. Von Restorff (talk) 21:27, 26 January 2012 (UTC)
 * Wikipedia also has an article on this topic. See Bioplastic.  -- Jayron  32  22:21, 26 January 2012 (UTC)


 * Another angle is that cellulose can be used to make plastics (celluloid) and biofuels (cellulosic ethanol). SemanticMantis (talk) 00:51, 27 January 2012 (UTC)