# Conservation of energy and magnets

1. Dec 12, 2011

### nhmllr

Things with a magnetic field constantly shed light. This is why a precessing magnet will slow down until it stands straight up, giving it the lowest potential energy. But if the magnet is standing straight up, it has no way to have less potential energy, but I would think that it would continue to have a magnetic pull, which would necessitate it the keep shedding photons/energy. But then what about the conservation of energy?

Thanks

2. Dec 12, 2011

### Pythagorean

If the magnet is aligned in it's magnetic field ("straight up" in your reference frame) then there is a "pull" yes, in that there are forces on it. But the forces are symmetric, they cancel out; there is no net force on (and thus no acceleration of) the magnet. A static (non-moving) magnet does not produce a change in flux, so there is no electromagnetic radiation (i.e. no light).

3. Dec 12, 2011

### nhmllr

So... The magnet is emitting and absorbing photons equally? Makes sense.

4. Dec 13, 2011

### Pythagorean

I'm not quite sure there's any argument for emitting in the first place; at least not in a classical scenario. The flux pertains to magnetic field lines, not electromagnetic radiation.

5. Dec 13, 2011

### nhmllr

But the force has to be transmitted somehow, so light would seem like a way to do that.
Although if this is getting out of the range of classical mechanics then I'm content with the given answer.

6. Dec 13, 2011

### nhmllr

Oh wait, light's only emitted when there's a CHANGE in the magnetic field? OHHHHHH OH OH OH I see. That actually makes sense (such as with induction)

Okay I think I get it.

7. Dec 13, 2011

yes :)