Question about magnetism

[Spiffor]

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

[Urban Ranger]

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.

[Sava]

does the magnetic property eventually decay? if not, you could make a perpetual motion engine...

[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

[Urban Ranger]

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.

[alva]

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

Absolutely motionless?

[Urban Ranger]

Without an external force, yes. It's just like putting a book on a desk.

[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.

[Geronimo]

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.

[Az]

You could place such a system on orbit, and make it spin indefinetly.

[Dauphin]

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.

[Dauphin]

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.

[Whaleboy]

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.

[dannubis]

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

[Spiffor]

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 ?