# Conservation of energy question with demagnetized magnets

1. Mar 3, 2015

### Zet

I have a conservation of energy question I’ve asked about elsewhere but I didn’t get a fully satisfying answer.

I hoping someone here can help.

TIA

http://wedgecircles.com/images/384_newmpe01.gif​

Say you have two magnetically aligned magnets close to one another but some distance apart. There is a certain amount of potential energy between them. If they are allowed move they will set each other into motion due to mutual attraction. There will be an increase in kinetic energy and an equal decrease in potential energy.

http://wedgecircles.com/images/384_newmpe02.gif​

Now, say, there are two chemicals in a packet nearby and when these chemicals are exposed to one another thermal energy is generated (there is a decrease in chemical potential energy and an equal increase in thermal energy). And, say, the amount of thermal energy generated is greater than the Curie temperature of the two ferromagnets. (There are no other strong magnetic fields nearby in this closed system.)

If the chemicals are exposed to one another before the magnets are allowed to set each other into motion then the amount of potential energy between them is gone. However, if the chemicals are exposed to one another after the magnets have set each other into motion the kinetic energy (that came from an equal decrease in potential energy) is not gone.

So, in the end, after ferromagnets have been raised above their Curie temperatures, there must be more of another form of energy (such as thermal energy) in the first case and less of another form of energy (such as thermal energy) in the second case.

Is there more thermal energy in the end in the first case and less thermal energy in the end in the second case?

(One possible way to resolve this is to set the amount of potential energy between the two magnets at the start at 0. This way, in the first case, when the potential energy between them is then gone, 0 amount of potential energy becomes no potential energy. However, this does resolve the conservation of energy analysis in the second case because the positive amount of kinetic energy remains but the offsetting decrease in potential energy (negative potential energy) is then gone.)

So, when two ferromagnets are some distance apart and then raised above their Curie temperatures is more heat generated when they are further apart or is more heat absorbed when they are closer together?

It seems to me one or the other must be the case for energy to be conserved.

Is it the case?

?

Last edited: Mar 3, 2015
2. Mar 3, 2015

### Simon Bridge

Welcome to PF;

Potential energy has no absolute so you intuition about setting the initial potential energy of the magnets to zero is a good one.
Configurations have potential energy only in relation to some other configuration.

But you may have more luck thinking about whether a magnet is harder to demagnetize when it is inside a strong magnetic field.
If it is in a heat bath with temperature much higher than the Curie temperature, then the random jiggling of the magnetic domains would still be constrained by the magnetic field acting on those domains right? So the domains will tend to (try to) remain aligned to the field.

Also consider: the Curie temp is not the temp at which magnetism vanishes, but where the type of magnetism changes.

Last edited: Mar 3, 2015
3. Mar 4, 2015

### A.T.

But each magnet is in the magnetic fields of the other magnet. So why do you think they should demagnetize?