05-07-2003 01:39									#31
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quote: Originally posted by alva In a couple of thousand years, the fields will reverse. Not that this in anyway answers your question, just showing I watch scientific programs

Uh uh. You're thinking of the earth's magnetic field. Regular ferromagnets just slowly lose their magnetization.

	05-07-2003 01:42									#32
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quote: Originally posted by Angelo Scotto I know this one! No, absolutely not, you can't and it was a direct consequence of Maxwell equations (but don't ask me why because i don't remember...) edited Ok, i couldn't resist, i checked, it was the second one (also called Gauss' Law for Magnetism), and as you can read here: the form of Gauss' law for magnetic fields is then a statement that there are no magnetic monopoles.

So far as we know there aren't. Maxwell's equations can be self-consistent when the divergence of the magnetic field is not uniformly zero.

	05-07-2003 01:43									#33
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quote: Originally posted by Big Crunch quote: Now, I wonder: if I put a large magnetic surface on the table, and I let a tiny magnet "levitate" above it., then will my levitating magnet end up falling anyways ? Will the levitation last longer ? Will it last on long term ? Ever ridden a maglev? Anyway, the only reason the magnets fall off is because the magnetic field spreads out as it comes out of the magnet. That is to say that apart from directly above the magnet the repulsive forces push sideways to some extent. It is akin to have a ball at the top of a hill. Its only stable at the very top. If it is slightly on the slope it will fall off. Having a surface that is totally magnetic strightens the magnetic field lines to go straight up, and so there are no forces pushing sideways. The hill slope in the analogy has been levelled.

Only if the table is infinite.

	05-07-2003 01:44									#34
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quote: Originally posted by Big Crunch Spinning interacting magnetic fields are not that simple.

Now there's the understatement of the century.

	05-07-2003 01:47									#35
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quote: Originally posted by Solver Any decent explanation as to WHY a magent will retain both poles even if you cut it and slice into small pieces?

Because the simplest unit of magneitc source is a dipole, not a monopole.

	05-07-2003 01:50									#36
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quote: Originally posted by Sava You could fashion a wheel of small magnets, then put the larger one under the one side so it spins constantly. If the field didn't decay, there would be magnetic power plants... but there aren't.

This makes even less sense than most of your posts...

	05-07-2003 01:58									#37
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quote: Originally posted by Solver quote: If you cut the magnet to parts, they atoms will still be ordered correctly and will still create a magnetic field. And what if I cut the magnet in temperature of absolute null, whatever it's called in English, but you get it. I realize it's theoretical, but then the atoms would have no chance to rearrange... would it be a monopole magnet, or would it cease to emit the magnetic field?

1) Each single electron already is a magnet, due to its property "spin". Contrary to what is usually said (and what the name suggests) spin has nothing to do with movement at all - it's simply that mathematics is similar to that of a rotation. So electrons don't need to rearrange if you cut at 0 K (apart of the fact that you almost certainly will put some energy into the system when you try to cut at 0K).

2) Matter being at absolute zero means that there is no state in which it could lose energy. So, if you cut a block of iron at (its momentary) absolute zero, and the two fragments have a way to relax to a lower energy state, they will do so. This is actually observed quite often also at higher temperatures, and is called surface reconstruction. As very often the ferromagnetic state already is the ground state for the magnetic subsystem, cutting a magnet into two pieces results in having two magnets. This works even better at lower temperatures. At room temperature, particles of a magnetic material with sizes of less than a few tens of nanometres will become paramagnetic (i.e. they aren't "magnetic") because thermal fluctuations are bigger than the energy set free by magnetic ordering.

	05-07-2003 02:11									#38
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quote: Originally posted by KrazyHorse So far as we know there aren't. Maxwell's equations can be self-consistent when the divergence of the magnetic field is not uniformly zero.

KH, do you like cross-posting?

Maxwell equations are usually quoted with div B = 0.

div B = magnetic charge would be a different equation for me . With the Maxwell equations written as four-vectors I see even less a reason for div B != 0.

God said
dF = 0
dG = 4 pi J
G = *F
and there was light.

	05-07-2003 02:14									#39
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Ad, I didn't say they were written like that or that I thought monopoles existed. I said that from a classical EM standpoint there is no inconsistency caused by inserting a magnetic source density in the divB equation...

And yes, with relativistic effects magnetism is completely explained rather easily...

	05-07-2003 02:21									#40
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quote: At room temperature, particles of a magnetic material with sizes of less than a few tens of nanometres will become paramagnetic (i.e. they aren't "magnetic") because thermal fluctuations are bigger than the energy set free by magnetic ordering.

You mean, only at room temperature?

	05-07-2003 02:31									#41
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quote: Originally posted by KrazyHorse Only if the table is infinite.

What if the "table" is in fact a celstial body with its own gravity ? or what is the "table" recovers earth ?

	05-07-2003 02:33									#42
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Trajanus, to be more precise, exactly between 18.4 °C and 20.1 °C.

No seriously, the higher the temperature, the larger the particles are when they lose magnetism. You can simply say that for very small particles, the Curie temperature is size dependent, and for the smallest falls below room temperature.