Wikipedia:Reference desk/Archives/Science/2008 November 7

= November 7 =

Properties of Water
I was wondering how one may define wet in terms of water and more specifically, would water be classed as wet? For example, something soaked in water would be defined as wet, but would water(or any liquid) itself be classed as wet? 86.9.14.125 (talk) 00:17, 7 November 2008 (UTC)


 * Wet means covered, soaked with, infused with... a liquid. Liquids are, by definition, covered/soaked/infused with liquids - so they are wet. --  k a i n a w &trade; 00:52, 7 November 2008 (UTC)


 * It's to do with surface tension - and the affinity of water for whatever substance it's trying to wet. "Wet" basically means having a super-thin layer of water on whatever touches the water.  If you put your hand into a bucket of dry sand - most of the sand falls off when you take your hand out.  With water - you put your hand in the bucket and when it comes out, it's covered in a thin layer of water.   That's not true of all liquids - mercury (for example) is a liquid that doesn't wet things at all.  If you were to pour some mercury onto your hand, it would flow off and essentially none would remain there.  So that's an example of a non-wet liquid.   But it's really not that simple - there are surfaces that water doesn't wet - the reason we wax our cars is to alter the affinity of water for the surface and to cause the surface tension to cause water to make little droplets that roll off the car easily.  The waxy metal doesn't really get wet - when a water droplet rolls away - the metal underneath is dry.  Putting detergent into water lowers the surface tension and lets it wet things that it would ordinarily be unable to...so use soapy water to wash your car.  Use plain water for the final rinse to wash off the soapy water and replace it with high-surface tension water that'll bead up and roll off easily.


 * SteveBaker (talk) 05:15, 7 November 2008 (UTC)


 * Everyone probably knows this, but mercury is poisonous and can be absorbed through the skin, so don't pour it on your hand. --WikiSlasher (talk) 10:00, 7 November 2008 (UTC)


 * Yes - that's true. Sadly (or perhaps not), when I was learning chemistry in high-school, this unfortunate fact was not realised - so I've done the experiment many times...it's a great deal of fun - right up to the point where the brain damage kicks in.  I recall the year the rules changed...we went from asking the teacher to let us have a half teaspoonfull to play with (it's fun to whack a drop of it with a ruler and watch it "shatter" into tiny little perfect spheres with a mirror surface - that you can push back together again quite easily)...to the day someone broke a thermometer and the building had to be evacuated for an hour while the hazmat crew came in and cleaned it up.  Anyway - you may have to take my word for it - but mercury isn't "wet" despite being a liquid...and it has these weird upside-down meniscus's when you pour it into a glass container.  (ie when the surface of water meets glass, the water bends up to 'embrace' the glass - but mercury curves downwards as it attempts to avoid contact as much as possible.  Mercury is also insanely heavy...somehow your brain just can't adjust to the fact (which may have something to do with that brain damage thing!) - and it always seems very 'alien' stuff. SteveBaker (talk) 18:03, 7 November 2008 (UTC)

Photsensitivity and Effects of UVA Light on Copper Sulfate Solution
My research has shown that a photosensitive jelly-like resin of micromolecules that are exposed to UV light fuse into long cross-linked molecules (polymers). In other words went from mushy to hard.

The same thing with gel use in dentistry and artifical nails. A powder and liquid are mixed, brushed on and "harden" when exposed to UV. They no doubt crystalize.

Based on my earlier assumption, if these things are photsensitive to the effect of UV, wouldn't a solution of copper sulfate be effected by UV (black light/uva). I believe the effect would either accelerate growth and/or crystal size or structure. Two previous response weren't sure of any or little effect.

Any insight on some sources I could consult?

Thank you again,

JM —Preceding unsigned comment added by 206.66.221.83 (talk) 00:53, 7 November 2008 (UTC)


 * This is not necessarily crystalization. UV light causes increases in polymeric cross linking, which is what happens in the resin.  From our article on cross linking, "Cross-linking can also be induced in materials that are normally thermoplastic through exposure to a radiation source, such as electron beam exposure, gamma-radiation, or UV light.".  This process is not like crystalization.  This is comparing apples to oranges.  I am not saying that UV light will not effect crystalization of copper sulfate, indeed you should probably do a controlled experiment to find this our for yourself, though I speculate (see the answer to this same question above) that it will not.  The crosslinking process and the crystalization process are entirely unrelated to each other, and you cannot draw parallels between them.  --Jayron32. talk . contribs  01:13, 7 November 2008 (UTC)
 * BTW, if you are looking for some help on crystalization in general, perhaps Arnold L. Rheingold is someone you should attempt to contact. He is, perhaps, the nation's foremost crystalographer and may have some insight into this problem... --Jayron32. talk . contribs  01:15, 7 November 2008 (UTC)

Well, I also conjecture that UV light will accelerate crystallization for a simpler reason. Perfecting the symmetry and shape of the crystals aside, if the evaporation rate of the solvent (pure water in this case) is increased, then the rate of growth will increase, correct? Based on this, and that shorter wavelengths have more energy, I would think that the crystallization would indeed be accelerated (against growth under, say, visible light) because more energy means more heat, and more heat means a quicker rate of evaporation. Am I correct to think of it in such a way?

Again, insight is much appreciated.

Thanks,

JM —Preceding unsigned comment added by 69.251.231.30 (talk) 04:31, 7 November 2008 (UTC)


 * Not necessarily. Take for example the fact that microwaves cause water to heat up very rapidly, and microwaves are LOWER energy than visible light.  There is not a simple "higher energy causes materials to heat up faster" correlation.  There is no reason to suspect that UV light at the same intensity as light in the visible spectrum is unlikely to cause any changes in the temperature or evaporation rate of the water.  I'm not denying that you may find a change in crystal growth under UV light; however given my understanding of the processes involved, I do not expect it, unless of course I can see data.  Data means either find someone who has done this experiment, or do it yourself.  --Jayron32. talk . contribs  04:45, 7 November 2008 (UTC)

Interesting, though could you elaborate (thoroughly, if possible, for I am oblivious in this topic) as to why increased energy would not accelerate the solvent's rate of evaporation? And yes I shall conduct the experiment sometime, and I'll report my findings.

Thanks very much, as usual I greatly appreciate your responses.

JM —Preceding unsigned comment added by 69.251.231.30 (talk) 03:20, 8 November 2008 (UTC)


 * It has to do with modes of energy transfer. Heat is merely molecular motion; faster molecules contain more heat than slower moving molecules.  Photons do not usually, in-and-of-themselves, have sufficient momentum to move an entire atom or molecule, and thus do not actually cause molecules to move faster.  Photons can cause electrons to move; but at this level of motion, quantum mechanics takes over, and we aren't dealing with a nice intuitive Newtonian world anymore, and quantum mechanics doesn't really cover molecular motion in what we conventionally understand as heat.


 * The reason microwaves are able to heat water is because of something called dielectric heating. Basically, polar molecules like water can be set "spinning" by the passing waves, and this spinning increases the molecule's kinetic energy, thus heating up the bulk substance.  The deal is, in order for this process to work, the wavelength of the light energy has to be "tuned" to the size and shape of the molecule it is working on.  Its sort of akin to resonance, in the sense that only specific wavelengths of light will set the molecule spinning.  For water, this wavelength is about 12 centimeters, in the microwave range.  Other wavelengths simply wont have the effect of heating up the water.  Think about it; you can sit a bowl of water in sunlight for hours, and it won't ever boil.  It will reach roughly air temperature, but it will never boil.  Put it in a microwave oven (and remember, microwaves are longer wavelength than visible light, and thus lower frequency, and thus lower energy), and it will boil in 3 minutes.


 * The deal with UV light is that it can effect all sorts of chemical processes, because UV light is of an energy necessary to begin to break covalent bonds inside of compounds. However, it has very little effect on other kinds of bonds, specifically the sorts of intermolecular forces that keep water in a liquid state aren't particularly effected by UV light.  Also, the sort of ionic bonding that occurs in copper sulfate crystalization wouldn't seem to be particularly susceptible to UV radiation.  If you doubt this reasoning, do the experiment.  You said you had some blacklights; put a bowl of water in a box with a standard fluorescent bulb and another in a box with your blacklight.  Monitor the temperature of the air and of the water in each (evaporation rates will be higher in warmer temperatures, and both lights will generate some heat just due to electrical resistance).  If you see greater evaporation in the box with the black light (accounting for any differences in air temperature between the boxes) than you have proved me wrong... --Jayron32. talk . contribs  03:47, 8 November 2008 (UTC)

Ok, thank you very much for that explanation. Two more questions: what are some examples of compounds that could be effected by UV light, and could you specify where I could find information about the breaking of covalent bonds by UV light? I have been searching for books and sources on the internet but have not been able to find much about that.

Thanks,

JM —Preceding unsigned comment added by 144.126.9.247 (talk) 23:02, 8 November 2008 (UTC)


 * See this google search. I didn't open many of them myself and look that hard, but in the first page of the search, you can see phrases like "The shorter wavelengths or ultraviolet portion of the sunlight spectrum are able to break most covalent bonds..." and "Ultraviolet radiation is powerful enough to break many covalent bonds. Alone it can degrade PCBs, dioxins, polyaromatic compounds, and BTEX (1)." and "Ultraviolet light with a wavelength of 266 nm is capable of breaking the C–C and C–H covalent bonds,...".  If you search these more thoroughly, you may be able to get more information about the mechanisms involved.  If you need any more help, just ask away!  --Jayron32. talk . contribs  01:09, 9 November 2008 (UTC)

Yes that helps me a lot, thank you. While most of the pages I searched through in the Google search you provided didn't go into much detail, it has given me the topic I can go research in a library; bond fission. That aside, could you suggest some sort of easy to obtain compound that has covalent bonds (and can be used as the solute in crystallization through precipitation) which could be broken by UV light, since as you said copper sulfate would not be vulnerable in this way.

JM —Preceding unsigned comment added by 69.251.231.30 (talk) 02:43, 9 November 2008 (UTC)


 * Table sugar, aka Sucrose, can be crytalized quite easily (sugar crystalized by evaporation is often called rock candy ) and it is a covalently bonded substance. UV light could reasonable be expected to degrade the sucrose molecule, quite likely into fructose and glucose.  In fact, the addition of high fructose corn syrup to fudge helps keep large crystals from forming and keeps the fudge a smooth texture; I would imagine that if UV light could be used to generate fructose in situ that would have a similar effect.  --Jayron32. talk . contribs  03:05, 9 November 2008 (UTC)

Well, that is all I need to know for now. If I have more questions later on I'll post an entirely new question. You've been a great help, and I thank you for it again.

JM —Preceding unsigned comment added by 69.251.231.30 (talk) 06:56, 9 November 2008 (UTC)

Extinct gull species
Does the current fossil record indicate the existence of extinct gull (or 'proto-gull') species larger than the Great Black-backed Gull? --Kurt Shaped Box (talk) 00:56, 7 November 2008 (UTC)


 * Only one comes to mind... ;-) --98.217.8.46 (talk) 14:06, 7 November 2008 (UTC)


 * MWAAAAAK! ;) --Kurt Shaped Box (talk) 20:15, 7 November 2008 (UTC)

Advancing human evolution
What can I do to help advance human evolution, or failing that, advance the evolution of other promising species? Neon Merlin  00:58, 7 November 2008 (UTC)
 * Genetic engineering for the short term. See technological singularity for the long run. 98.221.85.188 (talk) 01:06, 7 November 2008 (UTC)
 * Have lots of nookie; encourage all others to do the same? --Jayron32. talk . contribs  01:07, 7 November 2008 (UTC)
 * Won't that contribute to overpopulation? And isn't overpopulation something that a sufficiently evolved sentient species should be able to avoid? Neon  Merlin  03:59, 7 November 2008 (UTC)
 * Depending on your genetic contribution, the best thing may be to avoid the nookie.  Plasticup  T / C  06:29, 7 November 2008 (UTC)
 * Except that nookie increases the rate at which new, random genetic variants arise. You'd want to maximize the number of random events to increase the chance that beneficial events would occur.  Of course, we'd eventually have to figure out a practical way to develop self-sufficient colonies on other planets, but that's a mere practical concern.  If you want to make evolution work better, just make it happen faster!  Thus, MOAR SECKS FOR ALL! --Jayron32. talk . contribs  04:03, 8 November 2008 (UTC)
 * The die-off after over-population reaches critical levels might help to cull the weak. Having nookie is only half of it.
 * We enjoy the end result, but evolution itself isn't really pleasant. It mostly involves either dieing or just barely not dieing. APL (talk) 13:40, 7 November 2008 (UTC)


 * You can't "advance human evolution." It will go where it goes, advancing as it may will. If you mean, "try to consciously affect the statistical presence of certain traits in the gene pool over others," take a look at the page on eugenics. There are some ethical, philosophical, and practical issues involved, and whatever you do, you're unlikely to make any difference whatsoever as there are just too many bodies in the gene pool (and a gene pool that doesn't suffer massive shocks—LOTS of death or isolation or forced sterilization or what have you—will tend to revert to the mean over time). As for the other question, I have no idea what you mean by "promising species." --98.217.8.46 (talk) 01:22, 7 November 2008 (UTC)
 * Evolution no longer means only genetics. Memetics - and all manner of other things are driven by evolution.  Some of those work on amazingly short timescales. SteveBaker (talk) 05:07, 7 November 2008 (UTC)
 * ".... to help advance human evolution" you'd have to know what destination you were heading for. Remember the old joke "Yesterday we were close to the brink, today we are one step further."Lisa4edit (talk) 07:51, 7 November 2008 (UTC)


 * You could try to win a Darwin Award =P --WikiSlasher (talk) 10:03, 7 November 2008 (UTC)
 * Unfortunately, trying automatically disqualifies you. You need to make it look like an accident. --Tango (talk) 10:25, 7 November 2008 (UTC)


 * Persue the brightest & healthiest man/woman (depending on on your sex) you can find. If there are others, honest evaluate yourself and if they are better in every other way, then look for someone else. More seriously though, you can't really advance evolution. Evolution is a natural process. Once you start consiciously interfering to achieve a desired outcome (something like artificial selection or eugenics), it's questionable if it's evolution any more IMHO. And evolution doesn't really advance anyway. It just happens and things change. Nil Einne (talk) 11:51, 7 November 2008 (UTC)


 * Develop a really nasty disease. The people who are left will evolve to have better defences against it. And it would be good for he environment. Set up a dictatorship and do selective breeding. Then you can decide what constitutes more advanced. Set up yet another religion complete with aliens and abductions and have eugenics as a central tenet. I'm sure I could think of some other good ways of improving the genetic stock given a little while to think about it. More interesting I think though is what would you consider as improving people? Dmcq (talk) 18:28, 7 November 2008 (UTC)


 * Best bet is to try to make the race smarter. Make a really popular movie or book which makes the reader believe it's uncool and unsexy to be stupid, and that it's cool and sexy to be clever. Sort of an anti-forrest-gump movie. Vote for intelligent, pro-intelligence, pro-education politicians. Fight to change education from a system to prepare school-leaving children to obediently fill their predefined roles at the bottom rung in society, to a system that helps them to challenge those roles. DewiMorgan (talk) 22:04, 8 November 2008 (UTC)


 * That's Lamarckism. Just getting people better educated won't make them evolve. Getting bright people to marry each other won't change things in general (except if we diverge into two different species) Dmcq (talk) 17:41, 9 November 2008 (UTC)

Global warming and climate change.
what is the present reactions in the world due to Global warming and climate change?SAJITH 07:24, 7 November 2008 (UTC) —Preceding unsigned comment added by SAJITHANCHAL (talk • contribs)

See Global warming and Climate change and then look through the categories and links within. The reactions are varied across the world and many nations are taking different stances and positions on how to deal with the threat these cause. Have a look at the Kyoto protocol (or is agreement?) as well. 194.221.133.226 (talk) 09:45, 7 November 2008 (UTC)
 * Reactions to Global warming and Climate change is dependent on the political orientation of the organization or the individual who will react. Greens tend to be most caution on this issue and strictly favors measures which will reduce green house gas emission, pollution etc. Overall, the world is indifferent with this and the siltation is not hopeful.  Otolemur crassicaudatus  (talk) 08:48, 8 November 2008 (UTC)

Tangent Galvanometer
I want to wire up a tangent galvanometer, but the thing is, I don't know how to specify the number of loops that I want by connecting the wires in the proper places. For example, there are four knobs for the wires to be connected and three options as to the number of loops (2, 50 and 500).Each number is printed in the middle of two sets of knobs. How do I connect the wires to get 50 loops? Do I connect the wires to the knobs between which 50 is printed? Or do I connect the first wire to the one just before 50, and the last wire to the last knob? Please help. 117.194.225.101 (talk) 10:27, 7 November 2008 (UTC)


 * Sounds like you would connect one wire to the 50 and the other wire to the common knob. But I have never seen this kind of equipment.  If you used 2 and 50 you would get 48 loops. Graeme Bartlett (talk) 21:32, 7 November 2008 (UTC)

That is what I want to know.Is there a common knob at all? And, if I used 2 and 50, wouldn't I get 52 loops? 117.194.226.154 (talk) 03:57, 8 November 2008 (UTC)

I asked this question almost two days back. Am I to take it that no one in the Science Desk has ever seen a tangent galvanometer?? I need the answer by tomorrow, so after that, even if someone does come up with an answer, it'll be of no use to me, because the last practical class of the term will have ended by then. Please hurry! Thanks in advance. 117.194.226.234 (talk) 05:21, 9 November 2008 (UTC)


 * I think the scarcity of answers may be because the question is specific to a particular instrument that no-one on the help desk has seen.


 * I'm guessing here but it seems most likely to me
 * that the posts on either side of the printed "50" have 50 turns of wire between them,
 * that the two posts that have 50 and 2 between them have 52 turns of wire between them,
 * that the "end posts" which have 2, 50, and 500 between them have 552 turns of wire between them, and so on.


 * Depending how resistive the wire is, an ohmmeter might allow you to check how the apparatus is wired. CBHA (talk) 06:18, 9 November 2008 (UTC)

current electricity and magnetism
do the electrons really trace a parabolic path while carrying current?

what is meant by magnetic meidean? —Preceding unsigned comment added by Kunal pdj (talk • contribs) 11:33, 7 November 2008 (UTC)


 * I wouldn't think that electrons carry current, rather they carry charge and the flow of charge is the current. (Mere semantics, I know...) Electrons in a current typically go through a wire so I can't see how they would trace a parabolic path unless the wire is parabolic. They can however trace a parabolic path as they move through a uniform electric field. --WikiSlasher (talk) 12:05, 7 November 2008 (UTC)


 * If you're asking about whether or not electrons really trace the classic 'orbital' path like they do in the old Atom illustrations, (Like this one : Image:Flag_of_IAEA.svg) then the answer is "no". See Atom for more discussion on this, but basically they exist in probabilistic clouds around the nucleus and don't really trace any coherent path within that cloud.
 * (As an aside, I can't believe that illustration doesn't appear anywhere in the 'atom' article.)APL (talk) 13:36, 7 November 2008 (UTC)

impossibility of stable equilibrium in electrostatics
3 point positive electric charges are fixed at the corners of a cube. I have to design an experiment to show that a ninth positive charge placed at the centre of the cube will not be in stable equilibrium. I thought I will place small charged sphere in place of 'ppoint charges'. But how to charge a sphere? I know I can use induction or conduction or friction method to charge them but for that I will need another charged object. How to get a charged object..will they be naturally occuring? Please help..name any experiment to charge objects.. —Preceding unsigned comment added by Sruthi Narasimha (talk • contribs) 12:20, 7 November 2008 (UTC)
 * By 3, do you mean 8? Algebraist 12:22, 7 November 2008 (UTC)
 * No, It doesn't make sense. Eight occupied corners give rise to complete equilibrium, since the net field at the center is always 0, as long as the point charges are identical. Nevertheless, the second part of the question isn't clear. Why do you need charging, if you say the ninth is charged ? In whatever positions you choose for the first three, you find two of them symetrical, thus counterbalance each other, leaving the third acting alone. BentzyCo (talk) 13:30, 7 November 2008 (UTC)
 * Equilibrium is easy, but the question asks for stable equilibrium. That's far harder. --Tango (talk) 14:59, 7 November 2008 (UTC)
 * I see. I hope that's what the asking user meant, indeed. Anyway, the type of equilibrium is defined by the tendency of the system once it's shifted out of its equilibrium. In the case of a stable one, the ninth charge tends to return to the center of the cube, in order to minimize the potential energy of the system. This is the case assuming 8 identical point charges at the corners, namely the ninth charge would oscillate about the center periodically. In the case of shifting the charge along one of the main diagonals it would oscillate linearly, while in the general case, its oscillations would be spatial. BentzyCo (talk) 16:24, 7 November 2008 (UTC)
 * No, a charge won't oscillate around the center, whether it's positive or negative. Probably the easiest way to see this is from Gauss's law. There's a stable equilibrium at a point x iff the net force at that point is zero (F(x) = 0) and a small perturbation in any direction leads to a restoring force (for ε sufficiently small, for all |r| = ε, F(x+r) · r < 0). But that implies that the normal force integrated over the sphere of radius ε is nonzero, while by Gauss's law the same integral must be zero, since there's no charge inside the sphere.
 * In any case, as I said last time, the original poster understands the theory. The question is about actually doing the experiment. I'd answer it except that I don't know anything about doing experiments. -- BenRG (talk) 17:35, 7 November 2008 (UTC)
 * The "experiment" cannot be done as described. It can be calculated as a "paper experiment", i.e. you can do the calculations to show what the expected results are, however it is literally impossible to do it as described.  Here are the problems you will run into:
 * It is impossible to suspend 8 objects, unsupported, in space at the corners of a cube under standard conditions, i.e. in your kitchen. Gravity will just take over. You could POSSIBLY do it in a controlled free fall, such as in orbit aboard a space shuttle, but for practical purposes, your not going to book passage on one just to do this experiment.
 * A point charge is a theoretical construct. You can certainly put an electric charge on object; rub a balloon against your head; viola.  However, this has several problems.  a) Its not a point charge, but a charged area spread across 3 dimensions b) its not of uniform charge density, that is different parts of the object will have different charges and c) its of an indeterminate magnitude, that is if you repeat the balloon rubbing experiment 8 times, there is no guarantee that each balloon will receive the same amount of electric charge and d) the charge will dissipate over time, as it moves around the object, or just dissapates into the air.  The stability of the 8-point equilibrium requires point charges of uniform magnitude; anything other than this exact set up will be unstable from the start, making it impossible to ad your ninth charge in such a way as to do the experiment.
 * Maybe you should find a different idea to prove experimentally; because I can't fathom a way you could do this to any reliability in real life... --Jayron32. talk . contribs 19:14, 7 November 2008 (UTC)


 * The experiment ought to work in two dimensions - that's much more do-able. Four charges at the corners of a square with a fifth in the center.  By keeping the apparatus horizontal - you can eliminate gravity.  By doing it on an air-hockey table you can eliminate friction - and that only leaves air resistance.  Alternatively - you could suspend the charges on long strings so they are all at the same level - the restoring force due to the string would have to be negligable - so very long, thin threads would be required. SteveBaker (talk) 12:59, 8 November 2008 (UTC)

Chihuahua and Great Dane
Is there any documentation or photos of a dog that is one half Chihuahua and one half Great Dane? Is it possible?--Emyn ned (talk) 14:10, 7 November 2008 (UTC)


 * With artificial insemination, it should be possible. Can't say I know what the result would look like, though. --98.217.8.46 (talk) 14:29, 7 November 2008 (UTC)
 * (EC) It is entirely possible geneticly. One could do it by simple artificial insemination techniques. As far as natural methods, the equipment may not be compatable... --Jayron32. talk . contribs 14:31, 7 November 2008 (UTC)


 * Male Dachshund x female Afghan Hound works and has happened naturally, FWIW. --Kurt Shaped Box (talk) 20:14, 7 November 2008 (UTC)

I guess, if done naturally, it would have to be a male Chihuahua and a female Great Dane....--Emyn ned (talk) 14:35, 7 November 2008 (UTC)
 * I think it would need to be even if done by artificial insemination. The puppies might grow too big to fit inside a female Chihuahua resulting in a miscarriage. What size are Great Dane pups? --Tango (talk) 14:55, 7 November 2008 (UTC)
 * Why do I picture a Chihuahua coming to the door with a dozen roses, a bag of dog-treats, and a step-ladder, and later a very undersatisfied Great Dane? DMacks (talk) 20:36, 7 November 2008 (UTC)
 * I don't care to speculate about the causes of your warped imagination... ;) --Tango (talk) 00:57, 8 November 2008 (UTC)
 * Beverly Hills Chihuahua has a fuckload to answer for, s'all I can say... --Kurt Shaped Box (talk) 01:01, 8 November 2008 (UTC)

heating with a gas stove
If someone had electrical heating, which was much more expensive in their area than natural gas (but unfortunately no gas heater was installed in their apartment), couldn't they heat by turning on all four of their stovetop ranges and opening their oven door, putting the oven on full, until their apartment quickly got quite toasty? Or is there some downside I'm not seeing?

I'm NOT asking for legal, medical or any other regulated professional advice! —Preceding unsigned comment added by 82.124.214.224 (talk) 17:19, 7 November 2008 (UTC)


 * It wouldn't be very efficient. Real gas heaters have all sorts of fans and ducts to move the warm air throughout the house. Just opening up a stove would be a very poor way to heat your house. You'll wind up with a whole bunch of hot air around the stove, and by the time that diffuses throughout your whole home (if it does!) you will have burned a tremendous amount of fuel.
 * Think of old homes built before central heating. They had a fireplace in every room, and even so they often tended to be rather cool if you weren't standing right next to a fire. APL (talk) 17:33, 7 November 2008 (UTC)
 * Open fireplaces have a different problem. Hot air rises - so the heat from the fire goes straight up the chimney - however that air has to be replenished from somewhere - so it has to leak in around outside doors and windows.  Hence the fire can actually cause the house to be generally colder than it would have been without the fire!  The solution (which we have in our open fireplaces is to have a simple heat exchanger built into the grate - through which air from the room is blown - and which exits to the side or above the fireplace.  This warms the air in the room using the heat from the fire - which greatly reduces the lost heat - and cuts down on the amount of outside air that gets pulled in.   However, it's not perfect.


 * The problem with the oven is definitely one of circulation of the heat. Putting a small office fan next to the stove might help that.  I guess I'd also have nagging worries about combustion by-products from the stove - normally, you'd only operate the stove for an hour or so - but if you're going to have it on for a protracted amount of time - there could be issues here.  You should maybe consider installing some carbon monoxide detectors in the kitchen area.  SteveBaker (talk) 17:44, 7 November 2008 (UTC)


 * Granted that fireplaces have more than one problem, house-heating wise. But even a Franklin stove tends to only heat the area around the stove.
 * I've seen fans on hinges that mount in door-ways. Strategically placed these fans can warm up whole floors of large houses with a small Franklin stove. I'll bet a similar arrangement could work with a gas stove. APL (talk)


 * Actually, it worked quite quickly (less than 10 minutes). The idea is that I realized that the place got really hot very quickly whenever I was baking something!  It's a small place, we're not talking a whole house.  —Preceding unsigned comment added by 82.124.214.224 (talk) 18:03, 7 November 2008 (UTC)


 * There's also a safety issue if you have kids or pets, what with leaving all the burners on the top of the stove on... -- MacAddct1984 (talk &#149; contribs) 18:10, 7 November 2008 (UTC)
 * You didn't find that some parts of your apartment were far too hot while others still hadn't warmed up? APL (talk) 19:23, 7 November 2008 (UTC)


 * I did that and it worked fine for me. That's very probably because of a few special factors in my apartment. Firstly, I was aware of the exhaust fumes and used it only for initial heating, not to keep the temperature permanently. Then there were only a few very small rooms with no doors. And than, the proper heating was itself so poorly designed that nearly everything would be an improvement. 93.132.156.148 (talk) 19:12, 7 November 2008 (UTC)


 * You could quite easily kill yourself (or the person upstairs) by doing this: either incomplete combustion of the natural gas causes carbon monoxide poisoning, or combustion without enough fresh air coming in causes carbon dioxide poisoning. --Carnildo (talk) 22:06, 7 November 2008 (UTC)


 * DO NOT DO THIS. Stoves only efficiently vent when closed.  If left open to your apartment for long periods of time, toxic gases, as noted above, can build up and harm or kill you.  People die this way all the time:, , .  What good is the extra $20.00 per month you will save in heating bills going to do you when you're dead.  Don't fuck around with this stuff... --Jayron32. talk . contribs  22:14, 7 November 2008 (UTC)


 * Personally, I can heat two rooms quite well just by grilling some bacon. Grills aren't intended to work closed, neither are top burners. And I get food cooked at the same time. Yay. Do get any gas devices checked out regularly by qualified technicians. DewiMorgan (talk) 22:11, 8 November 2008 (UTC)
 * Read above safety notices. A friend of mine used to do this in an apartment that didn't have separate gas metering (in Australia you can only be charged for services that are seperately metered). She put the oven and burners on all day in winter. The place warmed up but it was very humid and there was a lot of condensation on the windows and ceiling. Polypipe Wrangler (talk) 04:36, 9 November 2008 (UTC)

Intensifying Artifical Light, ie from a Flashlight
My son is doing a scientific methodolgy project for school (he is in the 7th grade). His project is determining whether ordinary household light (ie lightbulb, flashlight) could be focused and intesifed by using a series of magnifying lenses. He was hoping he could ultimately focus it to the point where he could burn paper. Is this possisble? Why or why not? He has researched it on the internet but has only found articles on how to convert a Mag Light into a laser using diodes from a DVD burner. Very cool, but not the info he needs. Thanks in advance 63.118.244.130 (talk) 18:42, 7 November 2008 (UTC)
 * Whether you could burn paper this way would really depend on the strength of the source light and the strength of the lenses used. (It is possible using a strong magnifying glass and the sun). - 87.211.75.45 (talk) 19:07, 7 November 2008 (UTC)


 * Artificial or natural isn't a big deal, it's just the intensity (i.e. the amount of energy) of the light. You can't start fires with a magnifying glass and sunlight on a cloudy day, too much has been absorbed already.  Your average household flashlight probably doesn't have the power output necessary to ignite things, but I could be wrong.  You'd have to look at the total power output of the flashlight and compare it to the point at which paper ignites (451 degrees F, or Fahrenheit 451 if you like).  Determining the heat output might be done with a styrofoam container filled with a known amount of water and a thermometer (look at the definition of a calorie), and then compare that to the amount that the lens focuses the rays.  A "perfect" lens would focus all energy passing through the glass onto one exact point, but no such perfection exists, and you'd have to know what the best focusing point is (the lensmaker's equation may be of help, but just guessing).  Not sure of the exact multipliers involved, since paper will take less energy to warm than water (especially given that water doesn't absorb a lot of light, though reflected sunlight can be used to heat water).  You would also have to look at how fast the paper is cooled by the various ways heat transfers.  In short, you might be able figure it out, but it would be well beyond 7th grade science.  My guess, though, is that a flashlight and an magnifying glass can't make fire from paper, even with black paper, a really strong flashlight, and high power magnifying glass.  SDY (talk) 19:10, 7 November 2008 (UTC)
 * I don't think it's the strength of the magnifying glass that's important, that should just affect how far away the focal point is. The size of the lens determines how much light it actually focuses and thus how likely it is to be able to burn paper. A series of lenses is unneccesary, it won't focus more light than one lens and will actually cause less light to focus because the lenses aren't truly transparent. (It just occured to me you may mean leses next to each other instead of in line, that could help if you could get them to focus on the same spot, which will be difficult). In the end I just don't think you'll be able to get enough light from a flashlight. -- Mad031683  (talk) 20:27, 7 November 2008 (UTC)


 * Just to clarify, magnifying glasses merely focus or "concentrate" energy; they don't create energy out of nothing. The sun generates sufficient energy to light paper on fire; however there is likely not enough total energy output from a flashlight bulb to generate enough energy to light paper on fire.  A typical flashlight bulb draws 10 watts of power.  Even if the lightbulb was 100% efficient in turning electricity into light, that would mean it would be maximally capable of 10 joules of energy per second; the paper would be perfectly capable of disipating more than 10 joules of heat per second.  This calculation assumes that 100% of the electrical energy was turned into light energy, and that 100% of that light energy could be turned into heat energy.  These transitions are going to be far less efficient than that, so it is quite obvious that you couldn't do it with this equipment.  --Jayron32. talk . contribs  21:11, 7 November 2008 (UTC)


 * What the magnifying glass does, though, is that it makes it so that all that energy goes to heating a smaller piece of paper. If you have 10 j/s (very roughly 2.5 calories/second) applied to a microgram of water (10^-6 liters), that microgram of water will evaporate quite rapidly (far less than a second, at any rate).  That same amount of energy applied to a microgram of paper will cause it to hit 233 C (451 F), which I assume is the point of spontaneous combustion for paper.  SDY (talk) 21:27, 7 November 2008 (UTC)
 * Except that that microgram of paper is also in contact with other paper, and the air, etc. etc. If you had 1 microgram of paper in a vacuum in isolation, yes, I suppose you could make it burst into flame with an appropriately focused magnifying glass and a flashlight bulb.  However, under standard science fair project conditions (i.e. you could do this stuff in your kitchen) its likely impossible to make happen.  The question is not whether you could get 10 joules into the paper at a very small point; indeed you could. The question is that whether those 10 joules can be kept in one place long enough to raise the temperature of the paper to its flashpoint.  As I said, those 10 joules per second coming in can be easily counteracted by disipating either into the air, or into the rest of the paper itself.  You can never build up, in any one spot, enough energy.  Also, a microgram of paper is of such a small surface area, you'd have to build a rig isolated from any possible purturbations; any slight motion would move the focus of the lens around and you would not be heating the same point.  At those sizes, if you BREATHED on the magnifying glass, it would move enough to make it impossible to do.  And again, those 10 joules assumes perfect 100% energy transformations.  --Jayron32. talk . contribs  21:43, 7 November 2008 (UTC)
 * I totally agree, just pointing out that the magnifying glass is not an irrelevant part of the equation and it's not just raw power output that's at issue. I'd actually be curious, thinking about this, if you could even make a liquid-type thermometer (i.e. not the dimetallic type) change temperature by focusing light from a flashlight on the bulb.  If you couldn't, that would be a quick-and-dirty way of demonstrating that you couldn't burn paper.  SDY (talk) 22:18, 7 November 2008 (UTC)


 * If I were going to try this, I would start with a small bulb like a two cell flashlight and check how much power it used. Most of the electricity will become heat and no more than about 5% (maybe 2%) will become visible light, but the infrared energy is not included in the lumens of visible light it puts out, and the IR might also contribute to heating. If a flashlight is focussed by a magnifying lens onto the bulb of a thermometer, does it cause the temperature to rise? That is a clue. Do not do experiments which might result in fires or burn injuries without adult supervision, and never aim the output at your skin or your eyes or anything else you can't afford to damage The math and concepts in measuring light are pretty complicated for a 7th grader and for me as well, so I apologize in advance for bringing in numbers, and even more so if I get any calculations wrong. Bright sunlight focussed to the smallest achievable spot will quickly start paper on fire or ignite a match. A 100 watt lightbulb will be a small fraction of the brightness of sunlight. An ordinary flashlight with 2 D cells and a normal flashlight bulb will be a small fraction of the 100 watt light bulb. See . A PR2 flashlight bulb , for 2 D cells, uses .5 ampere at 2.38 volts, or 1.14 watts, or 1.14 joules per second, but remember that less than 10 % is likely to be beamed through the lens as visible light energy and infrared, and the glass of the lens might block the infrared. The light output is .8 mean spherical candlepower or 10 lumen. With fresh alkaline batteries the voltage, current, and brightness would be somewhat higher. A 6 volt lantern bulb PR15 , would draw .5 amp at 4.82 volts, with 2.0 mean spherical candlepower or 25.14 lumen, from an electrical input of 2.41 watts. A 600 watt projector flood lamp for 120 volt operation would get hot enough to cause serious burns to anyone touching it, and would draw 5 amps at 125 volts, putting out 17000 lumens, or about 1350 mean spherical candlepower. Such a bulb should not be handled with the bare fingers even when cold, since fingerprints cause it to fail in use. The article Sunlight discusses how bright sunlight is, and how much infrared it contains. Bright sunlight hits the earth with as much as 1413 watts per meter. 1000 watts per meter is a common rule of thumb for solar energy calculations at the surface. Sunlight has a luminance of approximately 100,000 candela per square meter at the Earth's surface. A 100 watt light bulb emits about 120 candela or 1500 lumens. Thus the sun is way brighter than a 100 watt bulb which is way brighter than a flashlight bulb. A magnifier just collects the light energy falling on its surface and concentrates it in a smaller area. I have one which is 13 cm across, so it has an area of 13267 square mm. If it did not absorb or reflect any light, and it concentrated the light in a 2 mm spot, which would have an area of about 3.2 sq mm, then the light would be 4145 times as bright in that spot. If the magnifier has an area of 13267/1000000 square meter, or .013 square meter, then it receives about 13 watts of sunlight, and if it concentrates it by 4145, then the little 2 mm spot where the sun's image is formed on the paper is getting hit by enough energy to set it on fire. In reality optical defects, absorption and reflection would reduce the brightness of the spot considerably. A magnifier concentrating or focussing the light from a flashlight bulb or light bulb would similarly be concentrating the light and infrared energy falling on the large area of the lens down to a very small area, the image of the  the light bulb. If the amount of energy falling on the lens is small, then even concentrated it will not raise the temperature much.  Edison (talk) 22:14, 7 November 2008 (UTC)


 * Some mistakes in this thread:
 * Paper doesn't autoignite at 451°F. I don't know where Bradbury got that idea. It's not only inaccurate but ridiculously overprecise, given that he doesn't even mention the type of paper. Autoignition temperature gives 450°C (not °F). The source it cites says 450°C for rayon fiber, 475°C for cotton.
 * The brightness in candelas or lumens is irrelevant here, since those units are based on the sensitivity of the human eye, and human eyes don't figure into this question at all (hopefully!).
 * The focal length does matter because the light source has a non-zero size—see below.
 * As you said, your 13000 mm² magnifying glass only captures about 13 W of sunlight, and I think a magnifying glass that size is more than capable of burning a hole in a piece of paper. Capturing a comparable amount of power from a bright incandescent bulb might be possible. The tricky part is focusing it. The rays coming from the Sun are approximately parallel, and an ideal lens will focus them approximately to a point. But to capture enough power from an incandescent bulb it'll need to be close to the lens, its angular diameter will probably be 100 times the Sun's, and the lens will form a correspondingly larger image. The image diameter is $$\tfrac dr ( \tfrac1f - \tfrac1r )^{-1} = \tfrac{d}{(r/f)-1}$$, where d is the diameter of the object, r is the distance to it, and f is the focal length. For the Sun that works out to about 0.01f. Our article claims a typical focal length for a magnifying glass is 25cm, so the image size is around 2-3mm. To get a similar image from an incandescent bulb (5cm diameter?) you'd need r/f ~ 20, which is going to be tricky since r needs to be small. You might be able to do it with a succession of fairly powerful lenses, say two or three with r/f ~ 4. (And come to think of it, you could do better with a clear glass bulb—I was thinking of the frosted type for some reason.) -- BenRG (talk) 02:06, 8 November 2008 (UTC)


 * Odd, I've seen some other stuff (on the internet, not from reliable sources) that agrees with the 451°F. It's possible that the 450 and 451 (i.e. greater than 450) are from the same tests and Bradbury just used the wrong unit.  SDY (talk) 02:19, 8 November 2008 (UTC)


 * Well, it would be simple enough to test the claim by putting a piece of paper in a standard household oven, if anyone wants to. They usually go up to about 500°F.  Of course you want to keep it small, maybe a 1 cm square or even less, so it doesn't make much smoke even if it does burn.  Or course, try this at your own risk!  --Anonymous, 03:23 UTC, November 8, 2008.


 * I have set my oven to 550 degrees (F) and have placed a small corner of the Oregon measures voter information booklet for 2008 on a pizza tray. We shall see in a little while whether Bradbury got it wrong.  SDY (talk) 03:29, 8 November 2008 (UTC)


 * For reference, see this picture. The paper did not burn with the oven set at 550 degrees, though it did darken in color.  SDY (talk) 03:39, 8 November 2008 (UTC)

Popcorn and Cell Phones
What exactly is happening here?: http://www.dailymotion.com/relevance/search/portable/video/x5odhh_pop-corn-telephone-portable-microon_news Isn't this a health hazard? --Emyn ned (talk) 18:58, 7 November 2008 (UTC)


 * It's a fake viral video that was aimed to sell Bluetooth devices. Mythbusters did a bit about it on one of their viral video episodes. No need to worry! -- MacAddct1984 (talk &#149; contribs) 19:19, 7 November 2008 (UTC)


 * Fake video to advertise bluetooth headsets. More information from Snopes.APL (talk) 19:26, 7 November 2008 (UTC)


 * Since a cell phone has ~1/1000 the power of a microwave oven, I wonder if a few thousand cell phones in an appropriate enclosure might be able to cook something? I'm sure that 3-4 (like shown in those videos) has no effect.  Dragons flight (talk) 19:31, 7 November 2008 (UTC)


 * How on earth did the company behind these manage to avoid censure? It's blatantly obvious what they're trying to imply in order to sell their products. --Kurt Shaped Box (talk) 19:42, 7 November 2008 (UTC)


 * And bluetooth involves electromagnetic radiation anyway! Graeme Bartlett (talk) 21:18, 7 November 2008 (UTC)


 * Perhaps you don't realize it, but Bluetooth operates on the same 2.4 GHz microwave frequency as microwave ovens (but with ~1/100000 the power). The comparison is appropriate except that there isn't nearly enough power involved to damage anything.  Dragons flight (talk) 21:35, 7 November 2008 (UTC)

Traditional ship hulls and maximum speed of these
A traditionally shaped ship, propeller-based propulsion and with the distinguishable sleek form, rarely reaches speeds over 30 knots. In fact, it seems outright impossible for any ship of traditional design to reach speeds over 35. See, for the sake of having examples, British battleships and cruisers from the second world war, destroyers of the same (and this) era, indeed any imaginable merchant- or warship of the time. Fast patrol boats, E-klasse, Elco and so forth, have significantly higher speeds. What are the hydrodynamic effects that constrain ship speeds to such an arbitrary range as the 30-35kt one? Note of course that I am fully aware of faster ships, and the aforementioned FPBs may owe their speeds to having so little of their hulls actually down in the wather; jumping on the waves more than plowing through them. Answer, if you can, the question of if there is at all a way to get some Iowa-class battleship to accomplish a speed of 50 knots. :) I thank you for any interest in this matter, and more so for any replies. 80.202.246.253 (talk) 20:08, 7 November 2008 (UTC)


 * Ocean waves travel at approximately $$ 2.4 \sqrt \lambda$$ knots, where lambda is the wavelength in meters. For a 100 meter wavelength, this translates to 24 kt.  A ship traveling through the ocean creates its own wave train with a wavelength approximately 1-2 times the length of the ship.  For a ship 100 m long, that implies it's wave train is moving ~25-35 kt.  As you approach or try to exceed the velocity of your own wave train, you encounter rapidly increasing drag that means further increases in thrust can offer you very little in increasing speed.  So, as a practical matter, most large ships are designed not to try an exceed this limit as it eats too much fuel for very little gain.  Dragons flight (talk) 21:06, 7 November 2008 (UTC)
 * The articles on Hull speed and Froude number are relevant here. At hull speed, a ship is basically trying to climb its own increasingly steep bow wave. You can force a higher speed than that by sheer power (essentially forcing the ship into planing). But for large ships, the cost is prohibitive. It's cheaper to just make the ship longer and/or to add a bulbous bow. -- Stephan Schulz (talk) 23:45, 7 November 2008 (UTC)
 * Exceptionally happy with the answers. :) Thank you! 80.202.246.253 (talk) 14:32, 8 November 2008 (UTC)

Metals for Spaceships
What kind of metals would the spaceships of the future be made out of?

How abundant are these metals on Earth?

Could they be found on the Moon and other satellites? What about asteroids? —Preceding unsigned comment added by 24.125.56.9 (talk) 20:37, 7 November 2008 (UTC)


 * Titanium is likely of value, but it's not particularly rare here on earth. Like aluminium, the difficulty and expense of titanium is mostly in extracting it and working with it.  The "cutting edge" of aerospace nowadays is moving away from metals in general and towards ceramics and carbon fiber composites, which offer better heat resistance or strength for the weight.  There are hybrid compounds like Cermet, which uses nickel (common on the earth), molybdenum (not too rare, has been found on the moon), and cobalt (not too rare).  The noble metals (e.g. gold) have useful properties, but aren't likely to be used in large quantities.  SDY (talk) 21:00, 7 November 2008 (UTC)


 * Spaceships for the forseeable future will likely be made out of the same metals that are abundant on Earth. Of the 80 or so metals on the Periodic table of elements, only a few are commonly used for structural purposes. Structural metals are ones that are strong enough, but not too brittle, and should be relatively inert. Aluminum, iron, chromium, titanium, cobalt, and nickel are probably your best candidates; though depleted uranium has come into fashion due to its resiliance and strength, it still remains controversial. The Space Shuttle is primarily made of aluminum and titanium, and it seems to work fairly well. --Jayron32. talk . contribs 21:04, 7 November 2008 (UTC)


 * Why should they made of metal? Might as well make them of ice or some other material that can be used as a fuel and would also provide protection from cosmic rays. Dmcq (talk) 21:45, 7 November 2008 (UTC)


 * Aluminum is 8.3% by weight of the Earth's crust, thus quite common. It must be separated from elements it is in compoundss with, oxygen and silicon. Bauxite is the ore it is commonly obtained from. Aluminum is also abundant in the lunar surface material, as is iron, magnesium, manganese and titanium. The abundance of these metals varies at different lunar locations. Asteroid mining discusses the possibilities of getting useful materials from asteroids. A 1 km asteroid might contain as much iron ore as is used on earth in 2 years. The lunar surface also contains water, as do some asteroids, which could be used to create fuel and extracted for life support, by the use of external energy sources such as solar. See also Space mining,  Space manufacturing , Space-based industry , and In-situ resource utilization. Robots could be useful for such work. Present technology is not quite there yet to unleash robots and have them extract raw materials and build and fuel spaceships and habitats. One drawback is that the robots, if their artificial intelligence were advanced enough to function semi-independently, might decide they should be working for themselves rather than us. Edison (talk) 21:46, 7 November 2008 (UTC)


 * If you're not going anywhere (asteroid mining stations, orbital colonies) or if you've got energy to spare (Orion-type nuclear propulsion), steel is a likely choice. It's quite abundant and easy to work with. --Carnildo (talk) 22:15, 7 November 2008 (UTC)

Moon Colony
How dependent would a moon colony be on Earth in terms of being provided materials (fertilizers,food,water)? Obviously it depends on its capacity (plant biodomes), but would it be possible to make it completely independent? Thanks for the replies to the last question, by the way. 24.125.56.9 (talk) —Preceding undated comment was added at 23:09, 7 November 2008 (UTC).
 * I guess it'd be possible to create an independant colony on the moon with exceptional amounts of time and resources initially. It would need a power source (if you want independant, it's got to be solar, hydroelectric or someting similar), food supplies (in a biosphere you can grow everything we do here), water supplies with a way of recycling this water and recycling processes for other materials. I guess a lot also depends on the size of the colony you wish to create. —Cyclonenim (talk · contribs · email) 23:29, 7 November 2008 (UTC)


 * The Biosphere 2 article might be interesting reading as far as what's possible for autonomy. SDY (talk) 23:34, 7 November 2008 (UTC)
 * The added difficultly to get from almost self-sufficient (one small shipment of supplies every few months, say) to completely self-sufficient would be enormous and almost certainly not worth it for a lunar colony - the moon just isn't that far away. Power is easy - you just build your colony on a Peak of Eternal Light and use solar power (incidentally, how do you intend to use hydroelectric power on the moon? The energy comes from the sun evaporating water and it then falling as rain...). It's replacing the small but constant loses of water and air that you would need Earth for, but you could get those loses down to a very low level with some effort. As long as you have people travelling between the Earth and Moon reasonable frequently you can easily include some supplies along with the people. There will also be other, less essential, supplies you would need to get shipped in since manufacturing everything locally would be impossible unless your colony was enormous - compare with small island nations, they have all the food and water (and air!) they need, but still import other stuff. --Tango (talk) 01:11, 8 November 2008 (UTC)
 * Moon colony is an idea of science fiction fans and irrelevant in scientific discussion.  Otolemur crassicaudatus  (talk) 08:44, 8 November 2008 (UTC)


 * That was completely uncalled for and entirely incorrect. The creation of a moon colony is very much within the realm of science, as is anything else we might actually wish to do. If you're not going to contribute anything or at least play well with others, perhaps you should focus your efforts elsewhere. You could start by removing your head from your ass. Matt Deres (talk) 16:23, 8 November 2008 (UTC)


 * You will probably be interested in this http://en.wikisource.org/wiki/Advanced_Automation_for_Space_Missions 93.132.139.211 (talk) 11:04, 8 November 2008 (UTC)

Lunar Atmosphere
Suppose one were to break down lunar rocks with the express intent of freeing oxygen and forming a lunar atmosphere. Due to thermal losses and solar wind ablation, that atmosphere would escape over time, but I am curious what the rate of loss would be. Even at 1/6th Earth gravity, the fraction of molecules in a Maxwell distribution with a velocity above lunar escape velocity is very small (approximately 1 in 10 billion, for 30 amu molecules at 350 K). It's orders of magnitude bigger than the fraction at Earth's escape velocity, but still very small. So, that leads me to wonder, can the moon retain an atmosphere on timescales that are relevant to humans? How long would a 1 bar atmosphere last on the moon? I know it won't last forever, but is the ablation rate 1% per day, 1% per year, 1% per century, etc.?

If it is in the range of 1% per year or longer then it seems like terraforming of the Moon to create an environment where humans can live unprotected might even be possible. Obviously there would be a lot of difficulties with that, and a great many resource challenges to overcome, but I like the idea of people one day looking up at a moon that is green and blue rather than gray. Dragons flight (talk) 02:21, 8 November 2008 (UTC)
 * The moon's magnetic field is much smaller than Earth's so the impact of the solar wind on the proposed lunar atmosphere will be significant. See Atmosphere of the Moon. Rmhermen (talk) 04:54, 8 November 2008 (UTC)


 * Sci fi writers have gone with domes containing the air, rather than trying to surround the moon with a breathable atmosphere. They may just have been on to something. Edison (talk) 04:56, 8 November 2008 (UTC)


 * Maybe they just lacked sufficient imagination. Dragons flight (talk) 05:10, 8 November 2008 (UTC)


 * Without doing the math, because of the Moon's lower gravity the atmosphere would have to be hundreds of miles (maybe 1000s?) thick to approach 1 atm at the surface (the Earth's is about 100 miles deep). The escape velocity would be much reduced at those elevations.  That's also a lot of gas.  Domes could be much cheaper.  Saintrain (talk) 15:38, 8 November 2008 (UTC)


 * Of course, one could have a pure oxygen atmosphere at lower pressure. (For American readers, the Mercury and Gemini space capsules had an internal pressure of only 3 psi (about a fifth of an atmosphere) but contained pure oxygen. That gives the same partial pressure of oxygen as sea-level air on Earth.)  I haven't done the math either, but needing only a fifth of the pressure makes life a lot easier.
 * It gets even better &mdash; you could populated the Moon with Earthlings adapted to high-altitude conditions. La Paz, Bolivia is a major city (nearly a million residents) sited 3650 m (12,000 ft) above sea level.  That puts them above more than a third of the atmosphere, so now we're down to a measly 2 psi of pure oxygen: less than a seventh of sea-level air pressure.  TenOfAllTrades(talk) 16:09, 8 November 2008 (UTC)


 * The escape velocity at 500 km above the moon is only 10% lower than at the surface. Escape velocity does not change rapidly with height.  99.9% of the Earth's atmosphere is below 60 km, by analogy, 99.9% of a lunar atmosphere should be below 360 km.  Of course it's a lot of gas.  A large fraction of an Earth atmosphere if you really want 1 bar, but who wants cheap.  Dragons flight (talk) 16:26, 8 November 2008 (UTC)


 * I think the key problem isn't maintaining the atmosphere, it's getting it there in the first place. It will escape, but you're probably talking (I'm guessing here, if someone has accurate figures please speak up) 100s or 1000s of years (next to nothing on a geological timescale, but plenty on a human timescale). However, the shear amount of gas required, even if you limit it to pure oxygen, is enormous. Most methods I can think of to build up that kind of atmosphere (chemical reactions with lunar rock, etc) would do so gradually, probably over the same kind of timescale as the atmosphere escaping, so you wouldn't get anywhere. Domes are far, far simpler. An alternative to domes is to build your colony underground - you may need to do something to make the lunar rock above you air tight, but that's not too difficult (easier than building a dome, since the rock can handle the pressure, you just need to fill in the gaps). --Tango (talk) 17:00, 8 November 2008 (UTC)


 * Actually, making oxygen is straightforward (if ridiculously energy intensive). Melt 100 kg of lunar soil at 2500 C and you release ~8 kg of oxygen gas (which is about 20% of the oxygen in the rock).  There are probably ways to make that more efficient and less energy intensive, but the basic idea is there.  So to make 1 kg of oxygen we only need to melt say 5 kg of rock.  Done uniformly, that would mean smelting a 5 m deep layer over the entire moon.  Or a 2 km deep layer over 0.25% of the moon.  Oh, and that would require the energy equivalent of the Earth's entire electricity generating capacity for the next 3000 years.  So there are some practical problems.  :-)  But maybe in a few millenia mankind will have solutions to those problems.  Dragons flight (talk) 18:52, 8 November 2008 (UTC)
 * So, like I say, it's going to take in the order of 1000s of years to generate the oxygen and that's roughly the same rate at which it will be escaping, so you don't get anywhere. Until you can generate oxygen significantly faster than it escapes, you're not going to get an atmosphere. --Tango (talk) 20:11, 8 November 2008 (UTC)
 * Would lunar soil, after over 6000 years of solar radiation, be fully reduced or fully oxidized? That is, how much of the new O2 is just going to oxidize the rocks?  Saintrain (talk) 01:36, 9 November 2008 (UTC)