Wikipedia:Reference desk/Archives/Science/2009 May 8

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May 8

Skin color in Hindu art

I've looked on Wikipedia but haven't found an answer to my question. In various types of Hindu and/ or Indian visual arts, various divine figures and some non-divine figures (as far as I can tell, not having much knowledge of Hinduism), are often depicted with skin tones of bluish tint, or other colors which appear to be non-naturally occuring. Is there any scientific explanation for why such skin tones are used? What is the symbolic/religious meaning? Thank you. --71.111.205.22 (talk) 01:04, 8 May 2009 (UTC)

Well from a scientific point of view there's not much i can say about this. But from a religious point of view, many hindu gods, as you, said, are depicted blue. Vishnu and many of his avatars are portrayed blue, which is what you might have been. This might have something to do with the fact that the sea, highly regarded as a symbol of power by the hindus, is blue. But that's not saying much... some questions really don't have a comprehensive answer...Rkr1991 (talk) 08:24, 8 May 2009 (UTC)

Krishna is often portrayed in blue. Hinduism uses a lot of symbolism to depict its deities and blue, amongst other things, denotes transcendentalism. Remember, the blue depiction of god is in our material universe and on our planet, the sky is blue. The gods are in the spiritual abode and the entire material universe is a cloud in the sky of the spiritual abode. So, do you get the symbolism now? Sandman30s (talk) 08:41, 8 May 2009 (UTC)

That helps a lot. Thanks for all your assistance! --71.111.205.22 (talk) 15:14, 8 May 2009 (UTC)

Among the Hindu Trimurti (see File:Brahma Vishnu Mahesh.jpg):

• Vishnu and his avatars Rama, Krishna etc. are typically depicted blue-skinned (often with yellow dhoti); in fact the word Krishna literally means "black; dark; or dark blue"
• Shiva is depicted white-skinned (with a blue tongue)
• Brahma is depicted either red-skinned and/or wearing a red dhoti.

Other deities have different associated colors and symbols; see this very incomplete table for some more information. It is possible to interpret all these associations symbolically but one should be aware that the symbolic meaning is likely to vary with time, place and philosophical leanings of different sects. Abecedare (talk) 18:27, 8 May 2009 (UTC)

Shiva's blue throat is a result of the poison he holds there. Jay (talk) 11:38, 11 May 2009 (UTC)

FWIW Osiris, the ancient Egyptian god of the underworld, was commonly depicted as a green (the color of rebirth) or black (alluding to the fertility of the Nile floodplain) complexioned pharaoh in mummiform. Cuddlyable3 (talk) 07:24, 13 May 2009 (UTC)

Gravitational potential of an 'eccentric' object

I'm trying to find the gravitational potential of the 3-dimensional object described by a sphere of constant density $\kappa$ and radius 1 centered at the origin (0,0,0) - 'A', but hollow in the sphere of radius 1/2 centred at (1/2,0,0) - 'B' - and hollow in the sphere of radius 1/4 centred at (-1/4,0,0) - 'C' - for some point x outside of A- effectively a full sphere of density $\kappa$ plus 2 spheres of density $-\kappa$ as described previously. Can we just effectively 'add' the potentials for the respective spheres so that, if M is the mass of the large sphere 'A', we have $\phi ({\textbf {\underline {x}}})={\frac {GM}{|{\textbf {\underline {x}}}|^{2}}}{\textbf {\underline {x}}}-GM({\frac {1}{8}}{\frac {{\textbf {\underline {x}}}-{\textbf {\underline {b}}}}{|{\textbf {\underline {x}}}-{\textbf {\underline {b}}}|^{2}}}+{\frac {1}{64}}{\frac {{\textbf {\underline {x}}}-{\textbf {\underline {c}}}}{|{\textbf {\underline {x}}}-{\textbf {\underline {c}}}|^{2}}})$, where b and c are the respective sphere centers? Is this at all valid?

If so, is there any point in C where a particle could theoretically remain at rest? I think not due to the asymmetry along the x-axis but I'm not sure how to show it...

Thanks very much! Mathmos6 (talk) 04:27, 8 May 2009 (UTC)

You can just add the potentials since Newtonian gravity is linear, but the answer you wrote down isn't correct because GM/r is the potential of a point particle, not a sphere of constant density. (Also it has the wrong sign.) Your potential is fine (aside from the sign error) if you're only interested in the |x| > 1 region. There is an equilibrium point somewhere in C because every potential function, no matter how misshapen, has to have a minimum somewhere. The minimum in this case is a ring around the x axis. There's also a saddle point (unstable equilibrium) on the x axis. -- BenRG (talk) 11:42, 8 May 2009 (UTC)

Sorry, I just noticed I didn't read your question very carefully. I think the saddle point is in C (the hole of radius 1/4). It shouldn't be hard to prove that, but you'll need to write down an inside-C, inside-A, outside-B potential. -- BenRG (talk) 12:23, 8 May 2009 (UTC)

Oh, also, the potential is a scalar, so you want $-{\frac {GM}{|{\textbf {\underline {x}}}|}}$, not ${\frac {GM}{|{\textbf {\underline {x}}}|^{2}}}{\textbf {\underline {x}}}$. Funny I didn't notice that before. -- BenRG (talk) 12:31, 8 May 2009 (UTC)

Fair enough, I was only interested in the region outside the largest sphere yes, although I can't say I'd know how to calculate the potential inside the big mess in A - also, I see your point about the scalar, but don't we need to take the fact that the potential isn't radially symmetrical into account somewhere though? Or do the 'x-b'/'x-c's do that anyway? Thanks a lot! Mathmos6 (talk) 03:51, 10 May 2009 (UTC)

The |x-b| and |x-c| takes care of that. There are four regions (inside B, inside C, inside A but outside B and C, and outside A) and you'll have a different formula for each one. Or you could write down a single formula with max/min or some such, but you'd end up having to split into four cases to prove anything interesting anyway. If you can write down the potential inside and outside a single spherical object then you can write down the potential in all four regions, since it's just a sum of the inside/outside potentials of A, B, and C (with the B and C potentials negated). -- BenRG (talk) 22:42, 10 May 2009 (UTC)

How to ensure 100% RDA with supplements?

Dietary supplements usually contain 100% RDA of some micronutrients, but some ridiculously high or low value of many others, if they contain them at all. Given that it is prohibitively complicated to find out how much of each micronutrient we actually get in our food each day, and given that it probably won't hurt to get a little bit more than 100%, wouldn't it be nice if there were a supplement on the market that had simply 100% of every micronutrient, just to be safe? Why is there no such "100% supplement" available? Or is there? Mary Moor (talk) 06:02, 8 May 2009 (UTC)

You don't say where you are, but in the UK there are such supplements available: Centrum makes one [1] --TammyMoet (talk) 08:37, 8 May 2009 (UTC)

Yep, I've got a bottle of '100% everything' in my own cupboard. I got mine from Boots, I think - though I've seen similar in most pharmacies and health food shops that I've been into... --Kurt Shaped Box (talk) 10:31, 8 May 2009 (UTC)

Thank you; maybe I have to look some more. I'm in the US, in the state of Washington. Mary Moor (talk) 15:46, 8 May 2009 (UTC)

A word of advice. Don't pay extra for products that claim to contain 500% (or more) of your RDA. You don't need *that* many micronutrients and vitamins. I don't think that your body can even do anything useful with the excess.--Kurt Shaped Box (talk) 00:21, 9 May 2009 (UTC)

Not only do you not need that level, high levels of some micronutrients can make you quite ill. Franamax (talk) 09:11, 9 May 2009 (UTC)

Unless you've got a very unhealthy diet, vitamin pills will not do you any good. Overdoses of vitamins in fact do you harm - more is not better. I used vitamin pills for many years until I started reading scientific research about them, which showed that overdoses reduced longevity, increased cancer rates, kidney damage, etc. Now I never use them (with the exception of Vitamin D in the winter as I am in northern latitudes). Better to put your time and money into eating a healthy varied diet - plenty of vegetables and fruit etc. Its very likely that there are many as yet undiscovered micro-nutrients that have long term effects, and these are only found in healthy foods, not pills. If you want to get particular vitamins or minerals then its best to find out which foods are rich in them and eat those. For example B12 - sardines. Selenium - brazil nuts. And many others. Sardines are also rich in calcium and Omega3, which illustrates how natural foods give you extra nutrients for free. 78.146.190.197 (talk) 13:00, 16 May 2009 (UTC)

neutrons, protons and electrons

During my one year of chemistry class at school, I had other things on my mind. But now I try to learn on my own...
My very elementary question is: are all neutrons, protons and electrons the same?? That is, can you see for instance, this is a typical oxygen electron and that is a typical sodium electron? Or is there no such difference? Lova Falk (talk) 06:17, 8 May 2009 (UTC)

Yes, as far as we know, all protons are identical to each other. None of them have nicks, bumps, or customized parts hanging off of them. Same with neutrons, electrons, etc. An electron is an electron - there's no difference between oxygen electrons and sodium electrons. We have an article (of course), Identical particles, but that's rather advanced. Clarityfiend (talk) 06:59, 8 May 2009 (UTC)

Thank you! This is actually a bit shocking. In the articles on neutrons, protons and electrons, the article Identical particles is not mentioned in "See also". Do you think I should put a link there? Lova Falk (talk) 07:17, 8 May 2009 (UTC)

Probably not, because this is a generic property of all quantum particles (and even atoms and small molecules). Maybe subatomic particle should link to it. -- BenRG (talk) 21:53, 8 May 2009 (UTC)

See no-hair theorem for the black hole version of this identity issue. --Sean 14:06, 8 May 2009 (UTC)

They're not quite the same. For example: The two electrons in a helium atom have opposite spin. It's easy to change these differences, however. — DanielLC 15:45, 8 May 2009 (UTC)

No, Daniel, you are wrong. Single-electron states may have opposite spin, but the electrons occupying these states are still identical. If they were not identical (i.e., distinguishable), then the helium atom would have had many more quantum states than it actually has. Both atomic physics and statistical mechanics would have looked very differently if at least some of the electrons were distinguishable. --Dr Dima (talk) 18:25, 8 May 2009 (UTC)

It's more than that. "One electron up and one down" is the same quantum state as "one left and one right" or any other pair of opposite spin directions, so the electrons lose their identities in a more profound way than mere swappability. You don't have two electrons with opposite spins, you just have a compound state with zero spin (a singlet state) that doesn't divide into electrons any more uniquely than a two-liter bottle of soda divides into liters. Still, I think what DanielLC said was reasonable—you can have one electron over here and one over there and none in between, and though they are exchangeable in the wave function they aren't going to swap places with each other by crossing the intervening space. That's a kind of distinguishability, though it's not the technical sense in which the word is used in quantum mechanics. -- BenRG (talk) 21:53, 8 May 2009 (UTC)

Surface tension

What will be the Condition of spreading of one liquid over another and its expression ?Supriyochowdhury (talk) 06:51, 8 May 2009 (UTC) —Preceding unsigned comment added by Supriyochowdhury (talk • contribs) 06:50, 8 May 2009 (UTC)

I'm sorry, but I had trouble understanding your question. Could you try again with a little more detail? Thanks. --Sean 14:08, 8 May 2009 (UTC)

I think Supriyochowdhury is asking for a word for situations like having a layer of oil on top of water, and for the conditions that permit this to happen. I don't know the answers, though. Looie496 (talk) 15:58, 8 May 2009 (UTC)

== I want to about spreding coefficient $ba (where liquid b spread over liquid a)of a liquid and how can I derived.Supriyochowdhury (talk) 17:54, 8 May 2009 (UTC)

See spreading coefficient. --Heron (talk) 19:27, 8 May 2009 (UTC)

British bird identification/song question

There is a bird outside one of the places I reside frequently which makes a "doo-deladoo-delaadoo" (sorry for the crappy transcription) song over and over again, with the pitch going up and then it making a thrush sort of strangled noise, for long periods of time, most of the day. It will occassionally switch a bit, but this is what it does at least 90% of the time. It's been going on for a few months, at least, even into night sometimes. The place is a housing estate, but it backs out onto some wild scrubland/wooded area. First guess would be a blackbird but if so, why does it seemingly have such a restricted singing voice? Thanks for any answers 82.26.198.108 (talk) 07:38, 8 May 2009 (UTC)

Sounds a bit like a wood pigeon to me, check out the audio file in the article, it certainly has a very repetitive call. Mikenorton (talk) 07:58, 8 May 2009 (UTC)

Or try [2]. Bazza (talk) 13:29, 8 May 2009 (UTC)

Common Blackbird#Behaviour mentions the fact that some of them imitate noises and other things they heard in their song. (OR: There used to be one near my aunt's home that did a bit of a Mozart tune followed by a motor saw.)71.236.24.129 (talk) 13:25, 9 May 2009 (UTC)

It probably is a blackbird. Maybe it's just not very good at music. Nothing else sticks out of the songs I tried on the RSPB site and it's definitely not a wood pigeon. Thanks for the help everyone! 82.26.198.174 (talk) 19:15, 9 May 2009 (UTC)

Now that I read your description of the song as if sung by a blackbird, then it does sound like a blackbird. They have different calls - this is not their alarm call, but sounds like the call I recall hearing when all is peaceful. 78.146.17.231 (talk) 17:03, 16 May 2009 (UTC)

Identifying a tree

Can someone identify the species of the large tree growing directly in front of the building in this image? Thanks! LANTZY^TALK 08:04, 8 May 2009 (UTC)

Podocarpus perhaps? Where was this photograph taken?CalamusFortis 15:32, 8 May 2009 (UTC)

Detective work says at Gujurat University in Ahmedabad, India. Looie496 (talk) 16:05, 8 May 2009 (UTC)

Podocarpus seems very plausible. Thanks! LANTZY^TALK 17:16, 9 May 2009 (UTC)

Magnatite and hydrochloric acid

All of my searches to date indicate that magnatite dissolves slowly in hydrochloric acid, but no time frame. Does anyone now the time to dissolve, cold and warm —Preceding unsigned comment added by 86.137.11.144 (talk) 08:59, 8 May 2009 (UTC)

Speed of dissolving will depend on much more than the identity of the substances involved. For example, ground magnetite will dissolve much faster than a solid chunk, and a stirred solution will dissolve much faster than one left to sit idle. It will also depend on the concentration of the HCl, and as you thought, on the temperature. There are just far too many factors to give a definitive answer, variations of any one of these could change the speed of the reaction by orders of magnitude. --Jayron32.talk.contribs 00:05, 9 May 2009 (UTC)

From testing in lab: 3M HCL will dissolve magnetite powder almost instantly at RT, useful for cleaning. But 1M HCl has to be left at least a few hours, and more dilution solutions can take weeks, to the extent that it is no problem for washing to remove basic residues.YobMod 10:17, 14 May 2009 (UTC)

Random number

Can humans generate perfectly random numbers between any two arbitrary limits? - DSachan (talk) 09:43, 8 May 2009 (UTC)

Our page Random_number_generator_attack#Human_generation_of_random_quantities may help. 194.221.133.226 (talk) 10:19, 8 May 2009 (UTC)

It all comes down to what you define "random number" to be. --98.217.14.211 (talk) 10:23, 8 May 2009 (UTC)

(ec):No humans can't. It is well known that when humans are asked to pick numbers at random, they are strongly biased to particualr numbers, a fact often made use of by stage magicians. Computer algorithms for generating pseudo-random numbers are not truly random, but appear to be so for all practical purposes because the algorithm is seeded by a continuously changing number such as the time and date. One of the best hardware methods for producing random numbers is to amplify thermal noise. However, even this is not truly random since the signal must necessarily be band-limited which slightly biases against large differences in successive numbers over short intervals. There is also the question of what is meant by random. If the requirement is to build a machine that can output integers within a certain range with equal probability then a machine can be built to approximate to this. If the requirement is to output any real number randomly between two limits then no finite machine is capable of doing this. The reason is that there are infinitely many numbers between any two points. Any real (finite) machine will have a finite resolution and cannot possibly represent all of them, only a finite number of them. In fact, a digital machine is only able to represent rational numbers and these have a zero probability of occuring in a true random sequence of numbers even if the machine was capable of producing any of the infinite rational number between the stated limits. Why? because there are infinitely more irrational numbers in any interval than there are rational numbers, even though the number of both is infinite. SpinningSpark 10:30, 8 May 2009 (UTC)

When discussing "true random", an example I give that tends get the point across is that a true random number generator programmed to output true random numbers between 0 and 9 should have the possibility of outputting 1 1 1 1 1 1 1 1 1 1 1 ... The point is that it is truly random. Even if every number previously has been 1, the chance that the following number will be 1 is just as high as every other number. So, a generator that ensures the distribution of numbers is equal is not truly random. -- kainaw™ 13:24, 8 May 2009 (UTC)

That's true, but actual (computer) pseudorandom number generators don't try to even things out—they're just as likely to produce long sequences of ones as real random number generators are.

Computers aren't limited to rational numbers. They can work with numbers of the form $a+b{\sqrt {7}}$ where a and b are rational, for example. More interestingly they can represent the whole collection of computable reals. Computable real arithmetic systems work by successive approximation, and there's no reason you couldn't add genuine uncomputable random reals to such a system if you have a random oracle available in hardware. /dev/random is supposed to give you genuine quantum randomness, so it should work. -- BenRG (talk) 14:20, 8 May 2009 (UTC)

True, but the point stands - computable numbers are still only countable (the same size as the rationals), so you can still be almost certain that a number chosen at random from a bounded interval with a uniform distribution will not be computable. I don't see how you can add uncomputable numbers to a computer system, a computer can only work to finite precision. You might be able to fiddle around and extend the set of computable numbers a bit, but you won't stop it being countable. --Tango (talk) 14:27, 8 May 2009 (UTC)

Well, there's no real number that you can prove won't occur (exactly) in an exact-real-arithmetic system running on a machine with /dev/random. It may be philosophically dubious, but I don't think randomness gets any less dubious than that. Sure, a Turing machine with a given oracle can only be in countably many states and will only reach finitely many of those, but by the same token there are only countably many theorems about real numbers in ZFC and only finitely many of those will ever be written down. Only finitely many digits of pi will ever be known to the human race, every formal system has a countable model, etc. Of course, mathematicians never actually generate random numbers, they just reason about what would happen if they did, and computers can do that too. -- BenRG (talk) 20:33, 8 May 2009 (UTC)

Only because I wouldn't be able to describe such a number, but that doesn't mean much - almost all real numbers are undefinable. (There are describable numbers that aren't definable real numbers, but I believe the set of all describable numbers is still countable, by even the broadest definitions - I could be wrong, though.) --Tango (talk) 21:41, 8 May 2009 (UTC)

Getting off-topic here a little bit, but I kind of think this is important to repeat: There is no single, mathematically precise, notion of what it means for a real number to be definable. You have to say definable how. This has been the problem from the beginning at the definable real number article, and I have never been able to come up with a truly satisfactory way of making that article both correct and compliant with WP standards. I'm not sure it can be done. The option of campaigning for its deletion has certainly crossed my mind, but that won't make the issues go away; they'll just spread out through other articles, so I don't really think that's a good idea either. --Trovatore (talk) 03:52, 9 May 2009 (UTC)

Indeed, but I think any reasonable definition gives you only countably many numbers, so it doesn't really matter in this case. --Tango (talk) 13:21, 9 May 2009 (UTC)

It also depends on what you mean by "generate", as a human certainly can build a Geiger counter which s/he can then use to generate perfectly random numbers. A human can also shuffle a deck of cards pretty well, which can also generate good quality random numbers. --Sean 14:11, 8 May 2009 (UTC)

Geiger counters are not perfectly random in the mathematical sense and humans probably cannot shuffle as well as you think they can. SpinningSpark 19:42, 8 May 2009 (UTC)

When two waves intercept at a right angle

Do they interfere with each other? If yes, what types of waves interfere with each other?--Mr.K. (talk) 11:21, 8 May 2009 (UTC)

Yes, you will get additive interference just as any other case which combines waves. If you know the wavefunction for each wave, you can do a point-by-point combination of the phase for each wave, to determine the resulting total wave amplitude. The interference pattern will depend on whether you have two plane waves, wave-packets, or something else; in the case of two plane waves at right-angles, your interference pattern will be sort of "checkerboard-like" with constructive and destructive interference. Nimur (talk) 12:24, 8 May 2009 (UTC)

It depends what you mean by "interfere". There will be interference in the sense of transient superposition, as described by Nimur, but not necessarily interference in the sense of a permanent distortion. Gandalf61 (talk) 12:29, 8 May 2009 (UTC)

OK, if I put two lasers each one intercepting the other at 90 degrees. Will they affect each other in any way? (even if for practical purposes, it's irrelevant).--Mr.K. (talk) 17:11, 8 May 2009 (UTC)

Did you see our article on interference? They will affect one another, in the region of intersection; but if you were to measure/observe one of the beams outside the region of intersection, you wouldn't be able to tell that it had "crossed" with another beam. If you do the mathematics of adding the sine-waves and solve the wave equation, you will find this result is physically consistent. Nimur (talk) 19:42, 8 May 2009 (UTC)

In quantum electrodynamics there is an interaction between the beams, mediated by virtual electrons (the simplest Feynman diagram is this one). It's a small effect but it's large enough to be detectable. In principle there's a very tiny gravitational attraction too. Roughly speaking, though, they just pass through each other. -- BenRG (talk) 20:15, 8 May 2009 (UTC)

If they are in a non linear material you may get non linear optics effects with modulation harmonics or mixing occurring. Graeme Bartlett (talk) 09:02, 17 May 2009 (UTC)

youtube

How does this work

http://www.youtube.com/watch?v=e3kyNGVK-hI

Thanks--Mudupie (talk) 11:21, 8 May 2009 (UTC)

I've only watched the first minute or so, but it looks genuine to me. I imagine if one were to beat box into a flute (with sufficient skill) that is what it would sound like. --Tango (talk) 11:30, 8 May 2009 (UTC)

I agree, it's probably genuine. As for how it works, well the flute works by acoustic resonance, and the beat-boxing is sort of augmented by the percussive effect of the microphone amplifier clipping. Nimur (talk) 12:27, 8 May 2009 (UTC)

mWS in a mesurement of pressure drop

What´s means "mWS" in a mesumement of pressure drop or in a indication of pressure drop of a equipment? —Preceding unsigned comment added by Gigobqto (talk • contribs) 15:31, 8 May 2009 (UTC)

It is metres of water (approx 10⁴ Pa, see Conversion of units#Pressure or mechanical stress) but I do not know what the "S" stands for. In the unit mW(g), the g stands for guage pressure, meaning the pressure is metres of water relative to atmospheric pressure, so I would surmise that mWS is measured relative to something else. Perhaps vacuum or negative guage pressure? SpinningSpark 16:19, 8 May 2009 (UTC)

Ah found it on German Wikipedia, it's metres Wassersäule which if my useless technical German is not letting me down means column of water. It seems to be used exclusively for pressure differences, such as the pressure drop along a supply pipe, and is distinguished from both absolute pressure and guage pressure. SpinningSpark 17:33, 8 May 2009 (UTC)

buffer solution

Hi I have been asked to write a symbol balanced equation for equilibrium of a buffer solution of 0.6 mol of propanoic acid and 0.8 mol of sodium propanoate. Would the answer be: CH3CH2COOH (reversable arrows) CH3CH2COO^- + H⁺? I'm guessing its not CH3CH2COO^-,Na⁺ (reversable arrows) CH3CH2COO^-+Na⁺ because the next question is asking for a balanced symbol equation for the dissociation of sodium propanoate. Please help! —Preceding unsigned comment added by 92.18.81.46 (talk) 15:39, 8 May 2009 (UTC)

Looks good to me! Not that you mentioned it, but you may find Buffer solution and Henderson–Hasselbalch equation to be relevent reading. --Jayron32.talk.contribs 20:37, 8 May 2009 (UTC)

So. Balanced symbol equation. What's that then? Even google thinks we might not know. --Tagishsimon (talk) 00:10, 9 May 2009 (UTC)

Speed of magnetic and electric fields

What are the speeds of the magnetic field and electric field? Can we treat their speeds like that of an electromagnetic wave? --Email4mobile (talk) 17:27, 8 May 2009 (UTC)

Generally speaking electric and magnetic fields are not going anywhere at all unless the source of the field is itself moving. What does move is a disturbance in the field and such disturbances are called electromagnetic waves. The speed of these disturbances is c, the speed of light. Also note that there are not really separate electric and magnetic fields, but only the electromagnetic field, and the two are united through the Lorentz transformation. SpinningSpark 17:56, 8 May 2009 (UTC)

Thanks S Spinningspark for this explanation, but how can I describe the attraction and repple forces without accepting that these fields must move like radiation. For example If I've a giant magnet and another giant magnet was passing by very fast causing the nearest distance between them to be let's say 300,000 km. Will the maximum attraction|repple force take place at that position or after the magnet has passed by another distance proportional to its velocity? —Preceding unsigned comment added by Email4mobile (talk • contribs) 18:27, 8 May 2009 (UTC)

First of all, it is wrong to think of the magnetic field being solidly attached to the magnet and moving with it through space. Rather, the magnet is causing causing a disturbance in the field as it moves through it. That disturbance is propagated through the field and finally arrives at the second magnet. As for where the moving magnet is when the maximum effect is felt at the stationary magnet: ask yourself this, if instead of a moving magnet we have a moving star, where is the star when the maximum light from it is seen at the fixed position? SpinningSpark 19:10, 8 May 2009 (UTC)

It works surprisingly well to think of the field as being solidly attached to the magnet. When you push on a solid object the rest of it doesn't move instantly, there's a ripple effect through the object at the speed of sound. The magnetic field acts in a similar way, with the radiation being the ripple. When you stop pushing, the object eventually returns to its original shape, and the same happens to the field. I think possibly you (the original poster) are wondering how much the magnet's field lags behind the magnet when the magnet is moving. The (somewhat surprising) answer is that it doesn't lag at all, as long as the speed is constant. The maximum force between the magnets will be at the point of closest approach (which is the same from the rest frame of either magnet). The star is different—the maximum intensity of light will be somewhat before the time of closest approach. -- BenRG (talk) 21:19, 8 May 2009 (UTC)

Thank you BenRG and Spinningspark. Indeed the reason, I raised that question was that I was wondering why a conductor has to cut the magnetic lines in order for the e.m.f to be produced. If we think about massive bodies and the fields we will find that massive bodies have a relative motion (example rotating Earth and its movement around the Sun and the Galaxy, meanwhile the fields must have an absolute motion?. If so then why should the motion or disturbance be created in the conductor for example manually to get this emf?--Email4mobile (talk) 23:24, 8 May 2009 (UTC)

Carbon Dioxide and Dry Ice

Is carbon dioxide inert and is Carbon Dioxide in its solid form, Dry Ice inert? THX —Preceding unsigned comment added by Jzeilhofer (talk • contribs) 19:03, 8 May 2009 (UTC)

"Inert" is always relative:) Carbon dioxide is a common material used to extinguish fires (the fire extinguishers with a black plastic cone-shaped nozzle, for example), and they produce both the gas and solid forms. But burning magnesium reacts (quite spectacularly sometimes) with it. It dissolves in water (carbonated beverages) and dissolves in and/or binds to many biological components (carried by blood). And plants consume and chemically react CO2 (photosynthesis). Solid vs liquid is still the same chemical, just colder and "more of it" present in the same measured volume. So dry ice reacts more slowly (many reactions go slower at lower temperature) but for reactions that do occur. DMacks (talk) 19:10, 8 May 2009 (UTC)

Definitely not inert by a simple chemistry perspective, in which Oxygen is reactive and Nitrogen is inert (even if many reactions of N2 are known, they are relatively few). User:DMacks mentions Mg, but CO2 should never be used on any metal fires, due to its reactivity. It also reacts with water, forming carbonic acid, and is involved in many organic chemistry reactions (carboxylations). It will also act as a ligand in metal complexes, and can be reduced. In most lab conditions, it sublimes quickly, so while the solid form is easier to store, in terms of (non-cryonic) chemistry it is equally reactive. YobMod 11:45, 14 May 2009 (UTC)

Aluminium in tap water, tea, vegetables, and foods?

How much of the aluminium/aluminum in tea is due to the aluminium in the tap water from which it is made? And are there any vegetables which have significantly more aluminium in them than other vegetables? What about other foods? 89.240.209.79 (talk) 21:21, 8 May 2009 (UTC)

I'm not sure of tea but this site (http://www.eatwell.gov.uk/healthissues/factsbehindissues/aluminium/) has some info - it seems to suggest that it is from the water and soil. On the basis of water-based i'd guess that more water-y veg (e.g. Cucumber) would have more aluminium than 'drier' veg. ny156uk (talk) 21:30, 8 May 2009 (UTC)