Wikipedia:Reference desk/Archives/Science/2008 January 17

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

January 17

Macaw training

I've seen videos on YouTube of macaws and other parrots rolling over, lying on their backs with their feet in the air, relaxing and allowing their owners to tickle their bellies. How can I train my Hyacinth Macaw to do this too? I've tried to very gently get my hand underneath and flip her over when she's playing on the floor but she absolutely hates it. —Preceding unsigned comment added by 84.69.34.141 (talk) 00:05, 17 January 2008 (UTC)

Itchy ears

When Ive had earphones in for a while, the insides of my ears start to itch. Any reasons?--TreeSmiler (talk) 01:37, 17 January 2008 (UTC)

Conjecture: they vibrate enough to tickle you. --Anon, 05:49 UTC, January 17, 2008.

It may be that they are bad fit. But wearing earphones (or hearing aids) for long periods is not a good idea. They encourage a buildup of moisture in the ear which can lead to infection. In any case you should clean and disinfect the earphones regularly. Headphones are safer and give much better sound quality.--Shantavira|^{feed me} 09:54, 17 January 2008 (UTC)

Unless, of course, they are those Bose earphones. 206.252.74.48 (talk) 13:38, 17 January 2008 (UTC)

Momentum Of A Chicken

Hey,it`s Me the physics magazine guy again sorry you all got upset over the last two questions.I made up the whole prize thing.But,I really need you guys to answer another question for Me.I tried doing,this one twice.Iam,supposed to figure out the momentum of a chicken. 1.75 seconds. —Preceding unsigned comment added by 68.161.69.238 (talk) 01:43, 17 January 2008 (UTC)

0. It isn't moving. Or are you talking about a different chicken? Algebraist 01:56, 17 January 2008 (UTC)

I would think that MOMENTUM_CHICKEN = MASS_CHICKEN x VELOCITY_CHICKEN. (EhJJ) 01:59, 17 January 2008 (UTC)

Assuming, of course, that it's a non-relativistic chicken, i.e. that $\gamma _{\mathrm {chicken} }\approx 1$. —Keenan Pepper 03:16, 17 January 2008 (UTC)

Depends on whether you're in the frame of reference of the chicken, or the road it's crossing. Confusing Manifestation(Say hi!) 03:26, 17 January 2008 (UTC)

Since momentum relies on velocity and the velocity of the chicken can be referenced to any object in the universe, then the momentum of a chicken can take on any value you wish to give it (quoting the datum point of course)--TreeSmiler (talk) 05:34, 17 January 2008 (UTC)

It might help if you told us the actual question (yet somehow I doubt it).--Shantavira|^{feed me} 09:45, 17 January 2008 (UTC)

Rocket in vacuum

I know it has been asked here before, but I can't found it using google. I also can not found in wikipedia's articles. So the question is how rocket could work in vacuum? roscoe_x (talk) 03:42, 17 January 2008 (UTC)

Rockets in space are working in a vacuum and I don't know all the physics behind it, but a rocket carries its own fuel and its own liquid (compressed) oxygen to burn that fuel. When it does so, the burning gas is hot and expands and forces itself away from the rocket. This pushes the rocket in the opposite direction (this phenomenon is known as conservation of momentum - you can't have something being pushed left without an equal force being applied in the opposite direction (right)). The fact that it's a vacuum is even better, because then the rocket doesn't have to work against air-resistance like everything below the Earth's atmosphere must. For rockets that are still leaving the atmosphere, air-resistance can be a real drag. ----Seans Potato Business 03:56, 17 January 2008 (UTC)

Excellent summary. Also note that solid fuel rockets can blend the oxygen directly into the fuel without carrying a separate tank. No practical difference from a physics standpoint, and frequently less practical (you can't kill the valves on a solid fuel engine). I'll also note that the expansion of exhaust isn't so much a function of heat as one of state transition -- a gas expands to fill far more volume than a liquid simply by being a gas. While a hotter gas exerts more pressure than a cooler one, this is a comparatively minor effect for rocketry. — Lomn 05:17, 17 January 2008 (UTC)

Both of those responses are great. I just wanted to add an illustration to maybe make this easier to understand. Think of something exploding in space - like for instance the famous scene in the first Star Wars movie when the Death Star blows up. As it explodes it projects stuff outward in all directions into space. Now, what is taking place inside a rocket is basically a controlled explosion. The rocket's fuel is exploding, and it wants to project outward in all directions. However, because of the engineering of the rocket the explosion is only allowed to escape in one direction. The resulting pressure on the rocket in the other direction means that the rocket is pushed away from the explosion going on inside of its engine in the direction away from its nozzle. Hope that helps, and if it's any consolation, this very question used to bother me a lot, too. -- Saukkomies 08:40, 17 January 2008 (UTC)

Would it be correct if we used a depiction of two people that push each other, one move to front and the other move backward. The one who move to the front is the rocket and the one move backward is the fire. roscoe_x (talk) 15:46, 17 January 2008 (UTC)

Yep, that's exactly the principle. It's even more elegant if you can put them on skateboards, roller skates, or some other free-rolling platform (to prove that it's the push and not just two people walking), but the illustration is correct regardless. — Lomn 15:56, 17 January 2008 (UTC)

all of the above makes sense - but would the rocket move faster if there was some 'mega platform' - completely immovable (due to magic, mirrors and/or duct tape for the sake of argument) - for the exhaust to push against? 83.104.131.135 (talk) 17:10, 17 January 2008 (UTC)

No. Rockets do not work by "pushing against" their surroundings. Suppose you have a rocket, which releases a molecule of exhaust with momentum Δp. Due to conservation of momentum, the rocket's momentum changes by −Δp; that is, the rocket gets a kick in the opposite direction of the exhaust. Now, if at some later time, that molecule of exhaust hits a barrier, or whatever, what effect does it have on the rocket? None. The rocket's gone on its way. What happens to the exhaust after it leaves the rocket is immaterial. -- Coneslayer (talk) 17:26, 17 January 2008 (UTC)

Just to head off a possible objection or follow-up question.. If the surroundings allow pressure to build up behind the rocket, that pressure will push it, much like the pressure behind a bullet pushes it down the barrel of a gun. However this is a separate action from the actual rocket propulsion. Friday (talk) 17:36, 17 January 2008 (UTC)

Yeah, I think that would be internal ballistics, not rocketry. -- Coneslayer (talk) 17:41, 17 January 2008 (UTC)

The big launch pads at NASA are designed to avoid pressure building up behind the rocket, even though it would provide some slight assistance. The hot exhaust impinging back on the rocket could damage it. --Anonymous, 07:46 UTC, January 18, 2008.

The bigger concern is reflecting acoustic waves from the engines, which can damage the rocket. This happened on STS-1. anonymous6494 22:12, 19 January 2008 (UTC)

Camping lantern - glowing white netting

I found a gas camping lantern and over the holes from which the gas comes, is a sort of white netting. When I touched part of the netting, it disintegrated (possibly a function of age or maybe it was always fragile). The function of the netting appears to be to glow white when it gets hot. What is this netting? Why is it fragile? Since the gas appeared to be burning with a blue flame, is it likely that it was burning with a clean, CO-free flame? ----Seans Potato Business 03:47, 17 January 2008 (UTC)

It's called a mantle. I believe it's a surface catalyst on which the fuel gas burns more efficiently and with a brighter, whiter flame. Read the article for how it works -- I had no idea. —Steve Summit (talk) 03:58, 17 January 2008 (UTC) [edited 04:03, 17 January 2008 (UTC)]

Years ago Coleman lanterns (fueled by white gas) had a thorium mantle which looked like a little cloth drawstring bag. It would be lit initially and burn to ash, which retained the shape. Then the mantle would glow when the gas burned, producing far greater brightness than the mere burning gas vapor. After it was initially burned, it was very brittle and would disintegrate when touched, Untouched, it would work for a long time. Something like that seems to stilll be used [1]. The mantles reportedly switched from thorium to less radioactive substances which produce about 20% less light. The mantle is actually an incandescent light (though not electrical) which dates back to the Welsbach mantle of 1884 [2]. Edison (talk) 01:15, 18 January 2008 (UTC)

Coleman has generally replaced thorium with yttrium in its mantles; this actually gives a whiter, brighter light than the thorium did. Cheers Geologyguy (talk) 01:34, 18 January 2008 (UTC)

The equipment looks quite old, and they're still making thorium mantles today so there is a chance it's radioactive and I touched it and turned it to powder and naturally, thought nothing over it (figured the netting material had just decayed). Should I do anything about it? I happen to be on a short radiology course right now and might be able to take the thing into university but if it is radioactive, transporting it might be stupid. Still, if they manufacture them nowadays, it's probably not a threat... according to Cecil Adams though, thorium is an alpha emitter and that would make the powder quite dangerous. --Seans Potato Business 10:14, 18 January 2008 (UTC)

The article on Gas mantle has a section about thorium safety. For comparison, see background radiation. -- SEWilco (talk) 17:53, 18 January 2008 (UTC)

The safety section says the dose from ingestion of a whole mantel is 2 mSv (twice the maximum yearly allowance for a member of the public in Europe), but I never ingested the whole thing, maybe only a tiny fraction. I guess the responsible thing to do, after the gas is finished, is to just chuck it in the bin. ----Seans Potato Business 21:09, 18 January 2008 (UTC)

This type of lantern is very widely used by campers, Scouts, etc., worldwide. My Scout group has several. They give a better light than electrical lanterns, and a canister of gas lasts much longer than a battery. Replacement mantles are readily available at any camping and outdoor shop. DuncanHill (talk) 10:27, 18 January 2008 (UTC)

A proton and electron in an atom

Why don't they just stick together?199.76.154.127 (talk) 04:46, 17 January 2008 (UTC)

the answer to this very innocent question lies in quantum mechanics. I don't quite have a layman's explanation for it. (by the way, the explanation that it's in "orbit" around the nucleus is absolutely wrong) —Preceding unsigned comment added by 18.96.6.229 (talk) 05:00, 17 January 2008 (UTC)

The "simplest" (ha!) explanation is that electrons don't completely behave like "particles" in the common-world-experience sense. There isn't precisely a negatively-charged object to stick to a proton—there's a lot more going on. See Atomic orbital model#Current theory for some introduction to the situation. DMacks (talk) 05:10, 17 January 2008 (UTC)

That sad truth is that you can only understand the complete answer once you know how to prove such an atomic configuration is not a stable solution of the Schrödinger equation. As the first lie on the step to that understanding, you might be interested in the Bohr model. Someguy1221 (talk) 06:05, 17 January 2008 (UTC)

A hydrogen atom in the ground state is just a proton and an electron "stuck together". The electron doesn't orbit the proton, it's simply superimposed on it. They don't collapse to a point because the uncertainty principle forbids it. A decrease in position uncertainty beyond a certain point implies an increase in momentum uncertainty which leads to an increase in position uncertainty. Instead they settle into an equilibrium state where these two effects balance out. The electron ends up with a much larger uncertainty of position because its much smaller mass means that a small uncertainty of momentum counts for much more in terms of velocity. That's why the atom looks like a small nucleus of positive charge surrounded by a big cloud of negative charge. For larger atomic numbers the exclusion principle also comes into play. -- BenRG (talk) 13:01, 17 January 2008 (UTC)

Speaking of lies: Ben, you have given the lie to 18.96.6.229, DMacks, and Someguy1221, who said it couldn't be done. That was a beautiful, simple explanation. —Steve Summit (talk) 16:41, 17 January 2008 (UTC)

Thanks for the inputs everyone. So I guess the reason an electron doesn't come into contact with a proton is due to its fundamental make-up of the electron, with the behavior that obeys the uncertainty principle. The proton, on the other hand, does not have the behavior described by the uncertainty principle (correct me if I'm wrong). That begs another question: Why does the electron have such a weird behavior? 128.163.174.150 (talk) 23:33, 17 January 2008 (UTC)

The proton does obey the uncertainty principle. Remember that ΔxΔp>h/2 (p = mass*velocity) or however you want to represent it. Since the proton happens to be 2000 times as massive as an electron, its uncertainty in velocity or position can be a lot smaller than the electron's. Someguy1221 (talk) 23:43, 17 January 2008 (UTC)

On a much simpler level (for those unfamiliar with quantum mechanics), the reason an electron does not stick to the nucleus of an atom is similar to the reason that the earth does not stick to the sun. The answer is energy. The earth orbits the sun (rather than falling into it through gravity) because it carries a lot of energy. If this orbital energy disappeared, the earth would become part of the sun. Similarly, the electron carries energy (you could think of this as orbital energy in the Rutherford model, but quantum theory treats it differently) which causes it to maintain a distance (probability distribution of position) from the nucleus. If this energy disappeared (in a different state of matter) then the electron really would "stick" to the proton. dbfirs 10:00, 18 January 2008 (UTC)

This is not correct, it's actually the opposite of the case here. There is insufficient energy for them to stick together, not too much. If there was too much you could just emit that extra energy (as a photon). If they stick together they need to become a new particle. There simply aren't any particles that have a mass of a proton plus an electron - a neutron is slightly heavier, and that energy has to come from somewhere. And to make things every more complicated, I believe the mass of a hydrogen atom is LOWER then an electron plus a proton, due to the binding energy involved (ionizing hydrogen back to it's component parts takes energy). Basically an atom is NOT like a planet orbiting the sun, it's rather more complicated (the electron doesn't even orbit the proton! It completely surrounds the proton like a blanket of sorts - remember it's also a wave, not just a particle, the wave wraps around the proton.) Ariel. (talk) 09:43, 22 January 2008 (UTC)

We can also turn everything upside down and ponder why sometimes they do stick together. DMacks (talk) 04:47, 19 January 2008 (UTC)

They don't stick together because they don't have enough energy to do so. Take a look at the article on Neutron decay. If a proton and electron did stick together what would be the result? Well, the charges cancel out, so presumably a neutron. Now, take a closer look at the neutron decay article - you will see that when the neutron decays to a proton and electron, it releases energy! (Plus a neutrino, but that doesn't matter much - it the energy that's important.) So to cause an electron to stick to the proton you have to give it extra energy. This doesn't happen normally. But it DOES happen in the sun read here: Proton-proton chain reaction. The sun creates neutron by sticking proton and electrons together, then adding some extra energy to them.Ariel. (talk) 09:28, 22 January 2008 (UTC)

LASER mixing questions

Let's say, hypothetically, I managed to attach three different gain mediums to one device, each emitting a different color (Red, Blue and Yellow, unless green really is a better color for this), and used a lens to focus them into one point. What is the resulting color, or does it fail entirely? Silverfireshadow (talk) 06:15, 17 January 2008 (UTC)

I assume you're talking about just superimposing beams upon one another, and not frequency mixing. You'll get a new color (in appearance only. In fact, you just have three superimposed waves), and you might be able to predict it from the visual color wheel. Someguy1221 (talk) 06:23, 17 January 2008 (UTC)

You're probably want to try it with Red Green and Blue, if you're trying to mix lights to produce (visually) white light, (or vary the intensities to get a full visual spectrum.) A company called Microvision has announced a pocket-sized digital projector based on this technique. APL (talk) 14:17, 17 January 2008 (UTC)

This has been done for a very long time. I visited a place (Magic-Stage if memory serves) in N. Carolina's research triangle that used tri-color lasers to produce 3D holograms on a stage that interacted with humans. The holograms looked very real. They just didn't cast shadows. The primary clients came from Las Vegas (magicians and concert stage shows). It is very easy to make your assistant disappear when she is just a hologram. -- kainaw™ 14:52, 17 January 2008 (UTC)

I'm in a closely related industry, and I've never seen a hologram anywhere near that level of sophistication. Are you sure they weren't just projecting video onto a plate of glass? Or perhaps even a Pepper's Ghost style illusion? APL (talk) 17:36, 18 January 2008 (UTC)

They were crossing computer-controlled laser beams (low quality - like laser-pointer lasers). Initially, they only had red. They made things like apples appear by drawing lines in space using computers to rapidly move the lasers. Then, they got a green and a blue laser system and improved it to have color. Then, they added motion sensors on the humans so the computers could track them and they drew the holograms in space coordinated to the humans' movement. There was no glass or smoke to capture the light. It was purely scattered light caused by crossing laser beams. If you got close enough, you could see the vector graphic lines being drawn. I remember that the original name was "Magic Stage". I do not know what name the company goes by now. -- kainaw™ 23:09, 21 January 2008 (UTC)

A doorbell that rings itself

As if kids pulling pranks wasn't bad enough, my parents have a doorbell that rings itself - what can be done? They and one neighbor - me - each have the same type of wireless doorbell, as does a third person across the street. For some reason, for the last few years, wehenver we've had the same type of doorbell, when one of ours rings, the other rings - and sometimes the one across the street does, too. We don't know if they've replaced theirs, but when one of us replaced a doorbell, then when both replaced, the same thing happened. Last night, both our doorbells suddenly started ringing "off the hook." (Old fashioned term for when phones still were put on hooks :-) So, we unplugged them. This had happened before if one of ours got a short because of tons of rain, but we have not had rain for several days, so they were not wet. Is there a long-term solution to these doorbells that seem to pull pranks? Why are our houses so inexorably linked by doorbell?Somebody or his brother (talk) 14:05, 17 January 2008 (UTC)

Long term solution. DuncanHill (talk) 14:09, 17 January 2008 (UTC)

Usually wireless things like this have a way of setting "channels" so that you can't accidentally set someone else's off with the same frequency. You'd have to consult their manuals though. --24.147.69.31 (talk) 14:27, 17 January 2008 (UTC)

Even with separate channels, you'll get interference on wireless doorbells that causes them to ring. My father-in-law lives in one of the heavily pre-planned neighborhoods where very house is identical and all have identical wireless doorbells. There were many complaints about doorbells ringing and they called out an electrician to try and figure it out. After a few months, it was discovered that the doorbells were ringing as the police drove through the neighborhood doing nightly patrols. Something (who knows what) was coming from the patrol car and causing the doorbells to ring. They switched out all the doorbells to a different model of wireless doorbell and the problem was solved. Personally, I would have dumped the whole wireless thing and put a wired one in. They aren't more expensive and they are very easy to install. Of course, that is only if you want a doorbell. I have a Victorian-style turnbell on my door. No electricity needed. Just turn the butterfly handle and it rings. -- kainaw™ 14:50, 17 January 2008 (UTC)

Sorry to give you a question rather than an answer, but what is the point of a wireless doorbell? I feel like I'm missing something here. Algebraist 15:02, 17 January 2008 (UTC)

To save you the effort of running (and concealing) wiring from the button to the chime. -- Coneslayer (talk) 15:08, 17 January 2008 (UTC)

(ec) In retrofitting a home that didn't have a doorbell, going wireless saves the homeowner/installer the trouble of drilling holes and pulling wire through walls. In new construction, the walls are open anyway, so the 'ease of installation' argument is weaker. As well, it allows one to relocate either the button or the bell component fairly easily. While it's unlikely that the front door will move very often, a homeowner might want to have the bell component located far from the door: in the basement near the television, for example, or up in the attic near the home office, or out in the garage workshop. A wireless system avoids long cable runs, allows the 'bell' to be relocated easily, and some systems are designed to permit the easy installation of more than one bell. TenOfAllTrades(talk) 15:15, 17 January 2008 (UTC)

My thanks to both of you. Algebraist 16:59, 17 January 2008 (UTC)

Thanks to all; interesting about the police cars, I actually was thinking while walking the dog of how our college radio station would interfere with the physics department's lab, and wondered if there was some outside interference. I'll have to see if the manual says anything about changing channels.

As for the movability of the bell, yes, not only was there no bell, when I bought this house it didn't even have a side door - and the front was stuck shut. (Poor widow didn't keep it up well, & always left out the garage. Sad situation, but I got it for a good price, and after some fixing up it's been really nice.) —Preceding unsigned comment added by DTF955 (talk • contribs) 15:24, 17 January 2008 (UTC)

what you do is you open up the push switch and ringer (with a screwdriver) and you'll see about eight tiny switches inside each of them set identically for each (up or down). what ya do is change some of the switches from up to down etc and make sure you do the same for both. this chnages the frequency it transmits and receives on and now you wont get no interferience from your neighbors. good luck mate! xxx User:Hyper Girl 15:25, 17 January 2008 (UTC)

To pick a nit, the switches probably change the binary code transmitted by the doorbell, but not the frequency. If the problem is your bell rings when the neighbor's button is pushed (and vice-versa), this will fix it. If the problem is radio frequency interference that's "faking out" your doorbell receiver, this probably won't fix it.

Atlant (talk) 19:27, 17 January 2008 (UTC)

To Algebraist: I think a wireless doorbell could make sense in a detached house where the button is on the garden gate, and the bell is in the house. – b_jonas 17:17, 17 January 2008 (UTC)

Canalphones

I'm looking for the reason canalphones give such superb bass response (reportedly down to 2 Hz). How is it done?--TreeSmiler (talk) 15:59, 17 January 2008 (UTC)

Because of the very small volume of air between the canalphones and your eardrums, they only have to move a teeny tiny bit of air to make your eardrums move strongly.

Atlant (talk) 19:30, 17 January 2008 (UTC)

What about the extremely small enclosed volume of air behind the diaphragm that has quite a low acoustic compliance, putting up the resonant frequency of the system?--TreeSmiler (talk) 02:43, 18 January 2008 (UTC)

Electrostatic Problem

Dear All, It would be very nice of you if you can help me.Well these are two problems of Electrostatics which I cannot find an answer.The first one I do not have any idea. The second one I've got some way but I have difficulty in finding the projection of AB.Please help me out and this is not part of any homework.

1)A rod of length L lies along the x-axis with its left end at the origin.It has a nonuniform charge density k=ax, where 'a' is a positive constant. Calculate the electric potenetial at point A where x=-d?

Ans:(1/4πЄ) * a[L-d(ln(1+(L/d)))]

2)A uniform electric field of 100V/m is directed at 30 degree with positive x-axis as shown in figure. Find the potential difference V_BAif OA=2m and OB=4m?

Thanks in Advance. Oasa (talk) 16:25, 17 January 2008 (UTC)

I assume you know the basic formula for the electrostatic potential generated by a point charge. Given this, (1) is an easy integral. Algebraist 18:05, 17 January 2008 (UTC)

Yeah I know all the formulae reqrd, but i cant still understand.plz do explicate it please i am in great need.Exam starts on monday.......... Oasa (talk) 03:23, 18 January 2008 (UTC)

Why do newer rifles have a lower rate of fire (typically) than older rifles?

For example, the XM8 has a lower rof than the M16. Why? 64.236.121.129 (talk) 17:45, 17 January 2008 (UTC)

I don't know whether it's really true that this is a general trend, not following military rifles much, but I can think of some reasons why this might be true. High rate of fire isn't necessarily an advantage with a rifle. You're going to have less manageable recoil, and more heat build-up as your rate of fire increases - not to mention the potential to waste more ammo more quickly. Keep in mind, these are just rifles; they're not meant to be used as machine guns, and they certainly can't sustain a rate of fire anywhere near the maximum rate of fire. Whether the slightly lower rate was intentional or not in this particular rifle, I don't know, but I doubt it was seen as a disadvantage. Even on the venerable M16, they fairly quickly abandoned full-auto for most purposes, in favor of a 3 round burst. Full autos require more discipline from the troops using it, to be effective. Contrary to what the movies imply, just making more bullets fly around isn't generally much of an advantage. Friday (talk) 18:21, 17 January 2008 (UTC)

Unidentified Plant

Can anyone help me identify this [[3]] plant? I would be very grateful if anyone has any ideas! Thanks, Aiyda 18:04, 17 January 2008 (UTC)

No, but when someone competent turns up, it'll help if you tell them where you found it. Algebraist 18:08, 17 January 2008 (UTC)

Apologies, the picture was taken in the Ashdown Forest in East Sussex. Aiyda 18:10, 17 January 2008 (UTC)

It looks a lot like a day lily. --Milkbreath (talk) 18:53, 17 January 2008 (UTC)

Almost impossible to be specific without the presence of flowers. It certainly could be a day-lily but equally it could be some form of iris. I look forward to another photo when you revisit to record the flowers. 86.4.187.55 (talk) 07:47, 18 January 2008 (UTC)

Unfortunately, I don't think anyone will be able to give you an definite answer without seeing the flower or the roots and rhizome. If you don't want to wait until it flowers to figure out what it is, you may be able to get an answer if you can photograph or describe clearly what it looks like underground. The rhizomes of day lilies and irises look quite different. Deli nk (talk) 17:40, 18 January 2008 (UTC)

Looks like the strappy leaves of an Iris. Julia Rossi (talk) 07:15, 20 January 2008 (UTC)

MESSENGER and the white spot

Two of us were looking thru MESSENGER's latest returns and were discussing the impact craters. What might the white spot be in this one (lower right side)? We're speculating its glass from a dead-on impact, since a large portion of Mercury's crust is composed of silicates. Its also interesting to note that the other white spots don't always appear to be in craters. We've disregarded volcanic activity. Inquiring minds want to know. --LeyteWolfer (talk) 20:49, 17 January 2008 (UTC)

It could be smashed up rocks, eg puverized grit, it would not be just glass, but it could be broken glass. Graeme Bartlett (talk) 22:30, 17 January 2008 (UTC)

For the moon I know that young craters often have brighter appearance that dulls over time. I would assume the same is true of Mercury. In this context "young" may include tens of millions of years. Dragons flight (talk) 00:17, 18 January 2008 (UTC)

Does wood burn faster in colder temperatures?

Tried to find the answer to this one for some time. Thought you guys might be the key. Does wood burn faster in colder temperatures, all other things being equal? Discussion started when one friend asserted that her wood burner went through fuel faster in colder temperatures. Of course in colder temperatures she uses more wood, but is the rate of combustion faster? —Preceding unsigned comment added by Tanthalas39 (talk • contribs) 21:26, 17 January 2008 (UTC)

Can't really see why; the combustion temperature of the wood is going to be the same in any event. What's a 30 degree swing in environment when you're burning at 500K? — Lomn 21:30, 17 January 2008 (UTC)

Faster convection with the outside air? Someguy1221 (talk) 21:40, 17 January 2008 (UTC)

I can imagine perhaps some relationship between how much fuel is in the burner, and the rate of burning. And, since you put more fuel in at once when it's colder... Friday (talk) 21:44, 17 January 2008 (UTC)

As an ex-firefighter for the US Forest Service, I recall that in Fire School we were taught that forest fires burned a lot more intensely on hot days. When the temperature is lower a forest fire will burn more slowly. Now, whether that means that the wood burns faster or not is unclear to me. -- Saukkomies 17:39, 17 January 2008 (UTC)

Cold air is denser, and so may provide more oxygen to a fire than a comparable volume of warm air, but it really feels like a stretch. Still, you might look at intercooler for something along those lines. --Mdwyer (talk) 23:51, 17 January 2008 (UTC)

Chimneys work better in cold weather - once the draft is established. This could explain what was happening in a wood burner. Delmlsfan (talk) 03:38, 18 January 2008 (UTC)

Can someone explain this to me?

Commutator (electric)? Seriously, that's the question, the article is too worthless to know enough to ask a more specific one! What does a commutator do? That 2nd picture on the right is very confusing too- it looks like the + and - ends of the battery are just connected up and something spins uselessly on top of the wires. Aah! --f f r o t h 21:37, 17 January 2008 (UTC)

• Just a mild observation; the article certainly doesn't look worthless to me. It looks well-written and informative. Tanthalas39 (talk) 21:58, 17 January 2008 (UTC)

The force exerted on a current in a magentic field acts in a particular direction relative to the current and the field (it's a right-hand rule, IIRC). So, if we had no commutator, and just a loop of wire connected to a battery between two magnets, then it would start turning, and then at some point the current would be going in the opposite direction than initially, and the torque would hence be in the opposite direction as well, meaning that instead of continuing to rotate, the wire would start turning backwards, and then you'd just have something that wobbled (or the wire would just turn until the torque was zero, and stay there).

With a commutator, the loop of wire isn't directly connected to the battery, but instead it makes contact with a pair of carbon brushes that complete the circuit. When the loop turns far enough, the two halves of the commutator lose contact with their respective brushes, and instead make contact with the other brush, thereby reversing the direction of current in the loop and ensuring the torque is in the same direction as before. Confusing Manifestation(Say hi!) 22:01, 17 January 2008 (UTC)

Oh, very neat! I didn't see that the purple loop was supposed to be attached to the spinning part.. I see how it works now. And I said it's worthless because a lot of the science articles (including that one) are written for people who already know everything about it.. they document knowledge, not teach it. So I guess that's a "use" but not the use I need it for :o --f f r o t h 00:14, 18 January 2008 (UTC)

Yeah- this is frequently part of an electric motor, as the article says. That thing that "spins uselessly" isn't useless at all- electric motors have many applications. Friday (talk) 22:55, 17 January 2008 (UTC)

I always thought it as a very crude way to simulate AC for an otherwise normal AC motor. --antilived^{T | C | G} 23:41, 17 January 2008 (UTC)

why dont we heat with rocks from hot places and cool with rocks from cool placse.

Why not make a very large thermos (vacuum flask) and put something, like rocks, with a high specific heat in it from a hot place, raise it to a blimp, and slowly, slowly, ship the blimp to where it's cold. This can take a long time, but no energy, if you're willing to wait long enough for it to drift over thanks to wind or whatever, or like sailboats, you can go against the current as well. Anyway, then you let the rocks down and heat with them until they're cold and repeat. Also, you get to cool with them where it's hot. The beginning of ice cubes history really did include shipping, so why dont we do this today, but with vacuum flasks? Also, if you use water you only have to lift it when the water is cold, since hot water could rise through a series of clever tubes. What do you think? —Preceding unsigned comment added by 212.51.122.20 (talk) 23:45, 17 January 2008 (UTC)

For a typical specific heat of say 0.9 J / (kg K), and a temperature difference of say 20 K, you have an energy density of 18 J / kg of rocks. Standard heating oil has an energy density closer to 40,000,000 J / kg. In other words, the amount of rocks required to displace normal fuel sources is simply impractical. Dragons flight (talk) 00:14, 18 January 2008 (UTC)

Water has about 4 J / (g K). Per gram not per kilogram. —Preceding unsigned comment added by 84.187.108.103 (talk) 01:02, 20 January 2008 (UTC)

Waiting for blimps to sort of drift there is a terrible idea! Why not just harness the energy of the wind directly and use electric heating/cooling? --f f r o t h 00:17, 18 January 2008 (UTC)

I've read about buildings that do this by time-shifting rather than space-shifting. They let a big bunch of water freeze behind the building during the winter, then insulate it really well, then use the ice to cool that same building during the summertime. Presto! (But it obviously only works in areas that reliably get sustained hard freezes during the winter.) —Steve Summit (talk) 00:46, 18 January 2008 (UTC)

Actually, that's not obvious. They could use a heat pump during the winter to cool, maybe freeze, the water and heat the building. There are limits to that, of course. --Anonymous, 07:52 UTC, January 18.

Also, Crimes_of_the_Hot --f f r o t h 05:57, 18 January 2008 (UTC)

The heat pump idea is actually a way that some people have used to get heat from "hot" rocks and cold from "cold" rocks. There is a technique for building houses where a sub-basement is excavated and filled with largish sized rocks. The idea is that there would be plenty of air space around the rocks so that air can freely move around them. The rest of the house is then constructed on top of this sub-basement. Then a heat pump is set up to go down into the sub-basement. This could either take the form of a pipe or hose full of fluid that is looped through the rocks in the sub-basement, or it might also be an air duct that circulates the air in the sub-basement. What this does is it takes the ambient temperature of the rocks that are below ground level, and which will remain more or less constant throughout the year, and distributes this temperature to the rest of the house through the heat pump. In the cold winter months the rocks in the sub-basement will be warmer than the temperature above ground, and so the heat pump will extract the warmth from the rocks and distribute it to the home above ground, assisting the home's primary heating source in warming the structure. Then in the warmer summer months the rocks in the sub-basement will be cooler than the temperature above ground, and the heat pump will act to cool the temperature down in the home. This is a very cheap and efficient way to help save fuel costs. There are designs for this in various places, including here[4]. -- Saukkomies 08:28, 18 January 2008 (UTC)

What you are describing is a form of passive heat exchanger. A "heat pump" is a more specific kind of mechanism that uses an input of energy to transfer energy against a thermal gradient. Dragons flight (talk) 17:41, 18 January 2008 (UTC)

Thanks for clarifying that for me. Bonk. -- Saukkomies 05:25, 19 January 2008 (UTC)