04-07-2003 09:06									#1
	Question about magnetism Inflate your Upload Space

Well, I utterly suck in science, so my question may be completely naive...

When I'm playing with magnets, I enjoy seeing two magnets repel each other. I always put a magnet on the table, to see the other one "levitate" for a second or two before falling on the table, next to (but not on) the repelling magnet.

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 ?

Thanks

	04-07-2003 09:09									#2
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It depends.

Suppose you have a large magnetic disc sitting on a table. You can float a very small magnetic disc above it for an indefinite amount of time IIRC.

	04-07-2003 09:10									#3
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does the magnetic property eventually decay? if not, you could make a perpetual motion engine...

	04-07-2003 09:15									#4
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In a couple of thousand years, the fields will reverse.
Not that this in anyway answers your question, just showing I watch scientific programs

	04-07-2003 09:19									#5
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quote: Originally posted by Sava does the magnetic property eventually decay? if not, you could make a perpetual motion engine...

1. The field does not decay, as least not according to classical EM theory.

2. No, you cannot make a perpetual machine out of it, because the small disc just sits there.

	04-07-2003 09:23									#6
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quote: 2. No, you cannot make a perpetual machine out of it, because the small disc just sits there.

Absolutely motionless?

	04-07-2003 09:29									#7
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Without an external force, yes. It's just like putting a book on a desk.

	04-07-2003 09:36									#8
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If you spin it (so that it still stays exactly above the same point on the magnetic table) it will spin for quite a while. Friction from the air will eventually cause it to stop, but if you remove friction by putting the entire room in a vacuum then the disc will spin forever. Assuming no outside interference of course.

	04-07-2003 13:05									#9
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and of course if it doesn't have a good deal of spin it will always just flip over and stick to the large magnet.

	04-07-2003 13:10									#10
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You could place such a system on orbit, and make it spin indefinetly.

	04-07-2003 17:24									#11
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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.

	04-07-2003 17:26									#12
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quote: Originally posted by Skanky Burns If you spin it (so that it still stays exactly above the same point on the magnetic table) it will spin for quite a while. Friction from the air will eventually cause it to stop, but if you remove friction by putting the entire room in a vacuum then the disc will spin forever. Assuming no outside interference of course.

Spinning interacting magnetic fields are not that simple.

	04-07-2003 17:30									#13
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Skanky: If you really want to split hairs, theres Einsteinian gravitation waves that carry energy away from all moving objects, so in truth, there can be no such thing as a perfect perpetual motion system.

The small magnet falls to the side because (a) it is not perfectly placed in the centre of the field and (b) other externel forces such as air currents would push it off the centre if it was there.

	04-07-2003 17:35									#14
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it will not turn forever...
it in fact will be very difficult to make it turn because electric currents are induced in both magnets (see the electric device (dynamo) on your bike)
but it will hover

	04-07-2003 19:29									#15
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Now, if we imagine a planet with a very strong magnetism. If we put a magnet on it (any size, but small enough not to compete with the planet's gravity), will it hover forever ?

	04-07-2003 19:53									#16
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Possibly, though it would have to stay perfectly situated.

	04-07-2003 21:40									#17
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quote: Originally posted by Urban Ranger 1. The field does not decay, as least not according to classical EM theory. 2. No, you cannot make a perpetual machine out of it, because the small disc just sits there.

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.

	04-07-2003 22:48									#18
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I've looked a bit into the matter, and from what I understand, the biggest problem with hovering magnets is instability: the magnetic field tries to upturn the hovering magnet, to have attracting poles facing each other.

You can find cool videos on this site :


Is it possible to build magnets with only one polarity ?

	05-07-2003 00:06									#19
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quote: Originally posted by Spiffor Is it possible to build magnets with only one polarity ?

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.

	05-07-2003 00:24									#20
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Any decent explanation as to WHY a magent will retain both poles even if you cut it and slice into small pieces?

	05-07-2003 00:25									#21
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quote: You can find cool videos on this site :

Umm. How did they make the frog hover?

	05-07-2003 00:27									#22
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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?

The entire magnetism thingie happens because the atoms inside the magnet are "ordered", so all the moving electrons create a magnetic field to one direction, instead of cancelling each other out like in normal matter.

If you cut the magnet to parts, they atoms will still be ordered correctly and will still create a magnetic field.

	05-07-2003 00:30									#23
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quote: 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

You watched that film with the magnetic lock didnt you? *tries to think of title*, it was similar to the Indiana Jones type but I CANT THINK OF THE ******* NAME!!!

	05-07-2003 00:32									#24
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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?

	05-07-2003 00:32									#25
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quote: Umm. How did they make the frog hover?

With the supraconductivity method. Unfortunately, I didn't understand anything about it.

	05-07-2003 00:34									#26
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quote: With the supraconductivity method. Unfortunately, I didn't understand anything about it.

	05-07-2003 00:52									#27
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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?

At 0 K i believe that electrons won't be able to move, so there will be no magnetic field
...
i think...

Anyway when you cut the magnet atoms don't have to rearrange i think...

	05-07-2003 01:31									#28
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quote: Umm. How did they make the frog hover?

Superconductivity is only remotely connected to it. Frogs (and likewise Yanks, Tommies and Krauts) are diamagnetic. Which means they are expelled by a magnetic field. If you shape your magnetic field accordingly (say, you'll shape the polar pieces of a large electromagnet like a banana, the ends pointing upwards), the frog is expelled by the field and cannot move to the sides either because there the field is stronger.
Superconductivity is simply the means to make magnets which are strong enough.

	05-07-2003 01:38									#29
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quote: Originally posted by Sava does the magnetic property eventually decay? if not, you could make a perpetual motion engine...

I was asked this very same question by somebody else recently...

Magnetism does decay eventually. The hotter your magnet is, the quicker it does so. Above the "Curie point" of the material, it no longer exhibits ferromagnetic properties at all.

Reason is that large-scale ferromagnetism is caused by thousands of aligned magnetic "regions". Their own magnetic field holds them in place, but eventually the vibration caused by temperature dealigns them...

	05-07-2003 01:39									#30
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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

What you've probably heard of, is the earth's magnetic field, which indeed reverses its sign in about that time span.

A normal permanent magnet will keep its field much longer (unless heated to the so-called Curie temperature, where a ferromagnet loses its ferromagnetism; in the case of iron it's around 770 °C). In most cases the fully magnetised state is not the most stable one, and so the magnetisation has a tendency to decay (very slowly). The stable state is one in which there are small volumes of the material magnetised homogeneously (magnetic domains) whose overall magnetisation compensates each other.