# Will a load-bearing permanent magnet weaken over time?

Let's say a permanent neodymium magnet is used to suspend an iron weight from a ceiling, as crudely depicted here:

Does the magnet gradually lose strength and eventually drop the weight, or will the weight stay suspended forever? If the latter case, wouldn't that be a violation of conservation of energy, in that it should take a certain amount of force to keep the weight suspended against gravity, and the energy to do that has to come from somewhere? And also in that case, would it make any difference if instead of leaving the static load on the magnet, the weight is instead repeatedly detached and then gradually brought towards the magnet from below until it 'jumps' up to contact the ceiling (presumably making the weight travel upwards expends some energy, from somewhere)?

I suppose the underlying question is, if a permanent magnet is used to do work, where does the energy involved come from and can it be exhausted?

• Work done can be defined as the force times distance, since the magnet is stationary the distance is zero. So their is no work done, otherwise tying a a rope to the weight would also violate conservation of energy. I do not know for sure for whether a magnet loses strength. Often magnets do lose strength at too high temperatures (Curie temperature), but I am not sure if it loses any at lower temperatures. – fibonatic Aug 20 '16 at 14:21

## 1 Answer

If the latter case, wouldn't that be a violation of conservation of energy, in that it should take a certain amount of force to keep the weight suspended against gravity, and the energy to do that has to come from somewhere?

No.

Contrary to what might be intuitive, it does not take energy exert force. You might be tempted to think that magnet "gives" your weight some energy by pulling it. While the weight would indeed have more gravitation potential energy, it would also have less magnetic potential energy.

In other words, you have to do the work to remove the weight from the ceiling, the magnet won't simply drop it just because you want it. :)

Permanent magnets can lose some of their magnetism over time, but this has nothing to do with the conservation of energy.