Wikipedia:Reference desk/Archives/Science/2012 December 22

= December 22 =

Generating electricity from a hypothetical magnetic monopole
Hypothetically, suppose I have a stable magnetic monopole and I want to generate electricity from it, according to the modified Faraday's_law_of_induction from Magnetic_monopole: $$-\nabla \times \mathbf{E} = \frac{1}{c}\frac{\partial \mathbf{B}} {\partial t} + \frac{4 \pi}{c}\mathbf{j}_{\mathrm m}$$

Assuming that it's a permanent magnet, then B is constant w.r.t. time and it simplifies to:

$$-\nabla \times \mathbf{E} = \frac{4 \pi}{c}\mathbf{j}_{\mathrm m}$$

According to this, all I would have to do is wound enameled wire around the magnetic monopole to generate electricity, does that sound about right?

No motion is required at all, right?

And it shouldn't require more than one monopole, right?Dncsky (talk) 01:35, 22 December 2012 (UTC)


 * $$\mathbf{j}_{\mathrm m}$$ is a current, so motion is required. -- BenRG (talk) 07:10, 22 December 2012 (UTC)
 * Could you elaborate a bit? What kind of motion would be required? Does the right hand rule still apply here? I was under the impression that $$\mathbf{j}_{\mathrm m}$$ is a current flowing out of the monopoly in all directions (to God knows where). I'm having a really hard time conceptualizing a current that doesn't travel in a loop, hence the question. Dncsky (talk) 12:37, 22 December 2012 (UTC)


 * No, $$\mathbf{j}_{\mathrm m}$$ is the magnetic current generated by the motion of the magnetic charge - analogous but independent from the electric current generated by the motion of electric charges. Dauto (talk) 12:53, 22 December 2012 (UTC)
 * I see. So would a generator built using magnetic monopoles be any more efficiency than one built using magnetic dipoles? Or would their efficiency be the same? Dncsky (talk) 13:26, 22 December 2012 (UTC)


 * There is no reason to think they would be more efficient. Dauto (talk) 13:42, 22 December 2012 (UTC)
 * Thank you. I got this out of a SF story. Guess the author didn't do his research.Dncsky (talk) 14:43, 22 December 2012 (UTC)
 * There may be an interesting twist of interest though: Use of a material that conducts magnetic monopoles (just like a wire conducts electrons) in place of the magnetic core would presumably allow a transformer to operate at DC, not only AC. Perhaps an interesting application for spin ice? — Quondum 10:30, 27 December 2012 (UTC)

Dry ice fog
The article on dry ice is rather weak on the detail of how dry ice + water = clouds of "smoke". I gather the "smoke" is condensed water vapour, but does this come from the surrounding air or from the water? What is the exact mechanism of its production? Also, why does the "smoke" sink? Is it because carbon dioxide is heavier than air (as I read in one place) or because the vapour is cold (as I read in another)? — Preceding unsigned comment added by 86.160.86.1 (talk) 03:09, 22 December 2012 (UTC)
 * Most likely this is the same phenomenon as sea smoke, which occurs when very cold air (or in this case, CO2) lies over warmer water. From the article, Evaporation from exposed water surface depends upon its vapor pressure. If the water temperature is greater than that of the nearby air, the evaporation continues faster than the air can absorb the water vapor, even though the cool air's relative humidity is 100%. This further evaporates immediately and re-condenses as visible fog.
 * The reason why the "smoke" sinks, is that the ice crystals in the "smoke" are heavier than the air/CO2 mixture. --NorwegianBluetalk 15:51, 22 December 2012 (UTC)

charcoal and salt related to earthing
why charcoal and salt should be used around the earth conductor in the earthing pit — Preceding unsigned comment added by 188.135.7.2 (talk) 09:03, 22 December 2012 (UTC)


 * What sort of earthing do you have in mind? Electrical HV distribution earthing?  Domestic residence electrical earthing?  Radio station earthing?  Telecommunications (phone company) eathing?  Earthing for other purposes such as corrosion control?  I have been involved in earthing for HV substations, radio stations, and phone company infrastructure, and I have not seen nor heard of charcoal being used.  I have not seen, but have heard of salt being suggested, but it is not generally used, as it will give only a temporary improvement.
 * The main purpose of added substances around electrical earthing electrodes in the ground is to improve conduction to earth by making the surrounding matetial hygroscopic, as the mineral components of sand and soil are electrical insulators. Electrical conductivity in sand and soil is provided by interstitial water  (in desert areas with very little moisture in the soil, getting a decent earth is extremely difficult, at least unless you can drive earth electrodes down into the water table, which, if the soil is dry, will probably mean drilling down great depths and going thru a rock layer).  A secondary factor in some areas may be to reduce soil chemical action/corrosion of the earth electrode(s).  Bentonite is the usual material used.  Sometimes a small amount of salts in a proprietry mix is added to the bentonite when the bentonite is sold specifically for eathing purposes, and is claimed by suppliers to improve conduction still more.  However, any benefical effect must be short term, and in any case it can be shown by mathematical analysis that any such improvement over damp bentonite is marginal anyway.  The improvement for added salts comes from improving conduction in the water in teh soil/bentonite, but any added salts, not already in the soil, will gradually leach away until diffusion equilibrium is reached.  Conversely, if you back fill with pure bentonite or sand, over time salts and other conduction-promoting substances in the soil will gradually diffuse in.  So if your electrical earth is good enough electrically upon installation, over time it will typically get a bit better, providing you do NOT add salts.
 * With radio transmitter earthing, usually no soil treatment is done, as it is more effective to just bury sufficient radial wires to make a ground plane.
 * With domestic electrical earthing in the Multiple Earth Nuetral system, no soil treatment is done. The safety and operation of the MEN system does not depend on any single residence earth - it depends on a multitude of earths working together, so the performance of any one earth is unimportant.
 * You will get a better answer if a) you make your question more specific, and b) you append your question with a pen-name. Its a free world in Wikipedia, and you don't have to, but you will get better answers if you do.  Keit 121.221.223.13 (talk) 10:01, 22 December 2012 (UTC)


 * Also, in case you heard that pure water is an insulator, and are thinking that the salt was added for that reason; I don't think so. Yes, pure water is an electrical insulator, but you won't encounter water that pure in the ground, so it's not an issue here. StuRat (talk) 19:06, 22 December 2012 (UTC)


 * Googling "charcoal earthing" returns some websites discusing it, but they all seem to be posted question-and-answer type sites of very dubious value. Charcoal will be more costly than bentonite and can be shown by analysis that even assuming the charcoal is simply a graphite cylinder around the earth electrode, any improvement over damp bentonite will be slight.  Wickwack 120.145.48.199 (talk) 14:14, 24 December 2012 (UTC)

Scratching a cat's spine
There's a spot maybe 3/4 the way down the spine, extending to the base of the tail, on at least some house cats, that, when scratched, causes the cat to behave very strangely, like having an epileptic fit or an orgasm (making jerking, repetitive motions and noises). I imagine there's a nerve bundle there, but:

1) What precisely is it called ?

2) Why does it have this effect ?

3) Do all cats behave this way ?

4) Do other felines ?

5) Do other animals ? StuRat (talk) 19:00, 22 December 2012 (UTC)


 * My first thought is that it's related to female sexual activity, in that, in my cats at least, scratching this spot produces lordosis. Good question and I shall have fun trying to track this down. Thanks. --TammyMoet (talk) 19:31, 22 December 2012 (UTC)


 * Don't have too much fun... 86.171.174.84 (talk) 20:23, 22 December 2012 (UTC)


 * I think Tammy is on to something, but (WP:OR) some male cats do it too, and certainly not all cats give a noticeable reaction. I've also seen some dogs act weirdly when scratched there, but it is more of a ticklish type response. SemanticMantis (talk) 22:23, 22 December 2012 (UTC)


 * The only cat of mine that has a strong response in this matter is a male - scratching here makes him flop over and show me his belly. I never understood why. Someguy1221 (talk) 23:48, 22 December 2012 (UTC)


 * Well, something odd definitely happens involving that spot, whatever the behavior exhibited by each particular cat. StuRat (talk) 01:45, 23 December 2012 (UTC)


 * Our cat (gib), responds by standing on his hindlegs and resting his front on the ground, as if he can't decide whether to stand or lie down, so he ends up doing both concurrently. It seems to be an enjoyable experience for this particular individual, other cats dislike this type of attention, and will respond aggresively after a period of endurance. Plasmic Physics (talk) 05:49, 23 December 2012 (UTC)


 * One of my cats, male, has gone through periods of hypersensitivity, where if you touch that spot he will compulsively lick his front paws. It seems to cause him some distress so I try not to do it.  He hasn't seemed to have the problem recently; I don't know what makes the difference (could be that I've been more diligent about treating him for fleas). --Trovatore (talk) 05:53, 23 December 2012 (UTC)

While we are on the topic of cats, why does our cat fetch one of us to stand around while it feeds? Plasmic Physics (talk) 07:53, 23 December 2012 (UTC)


 * Because it can! --TammyMoet (talk) 10:26, 23 December 2012 (UTC)


 * You may be standing around but the intention may be to have you join the meal. Bus stop (talk) 20:11, 24 December 2012 (UTC)


 * That is silly. Just because I can pat my head and rub my stomach, does not mean that I do it on a daily basis. Plasmic Physics (talk) 20:38, 23 December 2012 (UTC)


 * Animals can be a bit vulnerable while eating, both because they are distracted and because the "kill" may attract unwanted attention from other animals out to steal their prey and maybe kill them in the process. So, in social animals, having one stand guard while another eats makes sense.  Cats aren't naturally social (although they are related to lions, which are), but seem to have taken on social aspects when they were adopted by people. StuRat (talk) 22:57, 23 December 2012 (UTC)


 * Some cat species are social, however. They may still hunt alone, but they will eat and sleep as a group. Someguy1221 (talk) 20:19, 24 December 2012 (UTC)


 * Which cat species ? StuRat (talk) 20:21, 24 December 2012 (UTC)


 * Lions, for instance. I have also consistently found feral housecats moving in pairs (same pair repeatedly for over a week), but these may be ephemeral mating pairs. The are anecdotal reports of large feral "colonies" being found by animal control. Someguy1221 (talk) 20:27, 24 December 2012 (UTC)


 * To answer the OP, I found an article which says that it's "an invitation to come closer and interact" when done in the context of human/feline interaction, but that suggests it's initiated by the cat, whereas I see it almost always initiated by the human. --TammyMoet (talk) 10:29, 23 December 2012 (UTC)


 * That describes what lordosis is, but not what the spot on the spine is. StuRat (talk) 19:34, 24 December 2012 (UTC)

Perceiving numbers
Are there any scientific studies on how people perceive different numbers which are essentially the same?

1) For instance, 2 kg vs. 2000 g, or 1 Euro vs. 100 Yen.

2) Related, but slightly different, would a European perceive the difference between 1 and 2 Euro in the same way as Japanese the difference between 100 and 200 Yen?

3) Third question: Would a European perceive the difference between 1 and 2 Euro in the same way as same European the difference between 100 and 200 Yen? bamse (talk) 20:18, 22 December 2012 (UTC)


 * I numbered your Q's for ease of responders:


 * 1) I believe a larger number universally makes people think of a larger quantity, regardless of the unit. So, 2000 g sounds like more than 2 kg. StuRat (talk) 20:48, 22 December 2012 (UTC)


 * StuRat's unreferenced beliefs aside, the place to look would be market research studies, as product manufacturers are intensely interested in which fashion would be preferred by customers. Someguy1221 (talk) 05:19, 23 December 2012 (UTC)


 * I agree with Stu, I certainly have that bias, the same way that I imagine a sour taste when I watch someone eat a lemon. I'm also biased towards a higher level of precision, regardless of whether that is true. I have to convince myself that I don't have the information neccessary to differentiate between the two. Plasmic Physics (talk) 05:37, 23 December 2012 (UTC)


 * Yes, and it's the reason why salesmen don't volunteer to tell you the full price, but just what an installment costs. Many people don't do the math, and lower numbers sound better. StuRat (talk) 06:03, 23 December 2012 (UTC)


 * I'm not sure this is true. I can certainly remember beauty adverts which boast that users of the product will see the difference in just "14 days", rather than "2 weeks", possibly because the former 'feels like' a shorter length of time. --Iae (talk) 11:26, 23 December 2012 (UTC)
 * Not precisely the same thing, but related is Psychological pricing. Vespine (talk) 02:17, 24 December 2012 (UTC)
 * Any number of days seems shorter than any number of weeks or months, even if the first number is way higher than the second number. Being sentenced to 60 days in prison somehow feels not too bad; but even 1 month in prison - hell, that's a long time.  In these cases, the unit of measurement outweighs the actual numbers involved.  Who says humans aren't weird?  --   Jack of Oz   [Talk]  01:05, 25 December 2012 (UTC)


 * The radio show "Radiolab" just aired an episode this week dealing with this exact subject. See http://www.radiolab.org/ and select the audio for the most recent show, titled "Numbers".  The first segment, I believe, was about the science behind people's number sense, and was quite facinating.  -- Jayron  32  03:18, 24 December 2012 (UTC)

pipes history
I am interested in the early history of pipe manufacture. In particular, how were the first wooden pipes made? how were the first metal pipes made? Were there any related products that used the same manufacturing techniques around during their invention? 204.191.89.147 (talk) 20:37, 22 December 2012 (UTC)


 * The first wooden pipes probably didn't need to be manufactured at all. Bamboo, for example, forms hollow pipes naturally, so you can just pick up old bamboo off the ground: .  Other trees aren't quite so helpful, but the bark of some might form a nice tube, although with a split where it was removed from the trunk (they'd also need to be quite flexible to avoid breaking elsewhere).  I imagine these type of pipes were used long before it became possible to drill holes through a solid trunk. StuRat (talk) 20:52, 22 December 2012 (UTC)


 * As for the earliest metal pipes, one way might be to pour molten metal (say copper) into a vertical cylindrical form (perhaps made of sand). If you then waited the proper length of time, only the outside would have hardened, and you could drain the inside out, although you'd need to keep the ends heated so they didn't harden.  Pipe like this wouldn't be all that good, but might work to line an existing masonry sewer, preventing a collapse.  StuRat (talk) 21:04, 22 December 2012 (UTC)


 * Anything you wanted to know about early water pipes at Sewerhistory.org. Isn't the internet a wonderful thing? Alansplodge (talk) 23:05, 22 December 2012 (UTC)


 * thanks for the link. That one looks promising. 204.191.89.147 (talk) 01:07, 23 December 2012 (UTC)


 * Bamboo was not available to most of the world. I suspect the first pipes were made in the middle east and africa. Also, sand casting is plausible, but puts the time of invention later than my first guess. 204.191.89.147 (talk) 01:07, 23 December 2012 (UTC)


 * Based on what I've read from that link, it appears that clay predates metal pipes (which is so obvious when you think about it), and that at least some early pipes were cold forged then soldered. That was a really useful link. 204.191.89.147 (talk) 02:05, 23 December 2012 (UTC)


 * Yes, I hinted at that with my comment on "existing masonry sewers", but I should have been more explicit. Can we mark this Q resolved ? StuRat (talk) 06:54, 23 December 2012 (UTC)
 * I'd like to also add a link to our (unreferenced) Tap_water. It is claimed that pipes made of clay and straw were found in Mesopotamia circa 3000BC, and brass and copper pipes in Egypt in 2500BC, although the means of manufacture is not given. The Romans made their pipes out of lead sheet metal, shaping the flat sheets into the desired form. Someguy1221 (talk) 07:04, 23 December 2012 (UTC)

Your wish is my command. Alansplodge (talk) 17:10, 23 December 2012 (UTC)
 * I have seen some wooden piped made in the early 19th century. They were produced from long logs by a boring process. Yhen the end of one was trimmed down like a pencil to fit into the end of the next. 99.140.254.238 (talk) 19:51, 24 December 2012 (UTC)

Invariant mass
At the top of the article, it says "invariant mass" and "rest mass" are the same thing, I also looked up on several other websites that says the same thing. What confusing is, there is a section in the article devote to explain about difference between invariant mass and rest mass. If they are the same thing then how could there be an entire section telling their difference? After read the section, I still have no idea what are they talking about. Apparently, the wordings are not for someone who doesn't know much about physic to understand it.184.97.227.164 (talk) 21:43, 22 December 2012 (UTC)


 * They are identical when both can be defined. The difference is that when you're talking about a system of particles - as opposed to a single particle - it may not be possible to define rest mass since the particles may be moving with respect to each other. The invariant mass on the other hand can always be defined. Dauto (talk) 23:14, 22 December 2012 (UTC)
 * Usually,AFIK, if one can be defined so can the other, thus an example where one cannot be defined would help. --Trillianthcircuit (talk) 23:49, 22 December 2012 (UTC)
 * I think it's a language thing &mdash; to some people it seems odd to refer to the "rest mass" of something that, no matter what reference frame you choose, still has parts of it moving. --Trovatore (talk) 00:03, 23 December 2012 (UTC)
 * Invariant mass and rest mass are the same thing. A beam of light, which can't be at rest, is routinely said to have a rest mass of zero. The "invariant mass vs. rest mass" section is wrong to make a distinction and needs to be changed. -- BenRG (talk) 00:34, 23 December 2012 (UTC)
 * Well, how about a rotating object? I think most people would intuitively understand its "rest mass" to be the mass after you stop its rotation, but its invariant mass is larger than that. --Trovatore (talk) 00:38, 23 December 2012 (UTC)
 * That isn't a good example. The invariant mass isn't larger, because, if I'm not mistaken, you need to subtract the energy due to the rotation to obtain the object's invariant mass. I'm not certain, but I think Ben is correct and the article needs a fix. -Trillianthcircuit (talk) 19:13, 24 December 2012 (UTC)
 * OK, I know this section has been archived now, but I think it's important to respond to this for the benefit of anyone who may be searching later. I'm afraid you are mistaken.  The invariant mass is equal to the energy (divided by c2, if you're not using natural units) in the center-of-momentum frame; that is, the inertial frame in which the total momentum is zero.  In that frame, the object is still rotating, so no, you don't subtract the rotational energy. --Trovatore (talk) 20:38, 30 December 2012 (UTC)
 * Per proton, "The rest mass of the quarks are thought to contribute only about 1% of the proton's mass. The remainder of the proton mass is due to the kinetic energy of the quarks and to the energy of the gluon fields that bind the quarks together." Interesting! 86.171.174.84 (talk) 23:44, 22 December 2012 (UTC)
 * From what I've learned rest mass usually describes a single particle. When there is interaction between particles, invariant mass is used instead since they are no longer all "at rest". Zhieaanm (talk) 02:05, 23 December 2012 (UTC)
 * Then why at the beginning of article, they describe rest mass and invariant mass are the same thing?184.97.227.164 (talk) 21:01, 23 December 2012 (UTC)
 * Perhaps because they are when one is talking about a single particle (but only then)? If so, this equivalence needs to be qualified. Thus, I presume that one should talk of the invariant mass of a spaceship rather than its rest mass. The description of the mass of a proton above only makes sense with this interpretation: The total rest mass of the particles constituting a proton and the invariant mass of the proton are very different. — Quondum 10:55, 27 December 2012 (UTC)