If two stick two magnets the mass will be less than sum of the individual magnets. They lose certain mass while they stick together. Probably, they release some energy. My question is: when the magnets are separated, whether their original mass is restored to its original level? If that is right then after performing sticking and separating several times of the two magnets, they should vanish.
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asked Aug 8, 2016 at 6:37
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If performed carefully the process is, with exception of a small amount of heat generated by eddy currents and hysteresis of the magnetization curve, completely reversible. That heat requires a little bit of additional work during separation that we can't get back by work done by the approaching magnets on us. So, yes, whatever energy we take out of the magnetic field we can put back. If it wasn't this way, building permanent magnet motors and generators that work reliably for decades would be much harder, if not next to impossible.
answered Aug 8, 2016 at 6:44
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$\begingroup$ If it is true that magnets loose a bit of mass when they stick together (which I seriously doubt) then that difference in mass would be extremely small. So if one generates heat by separating the magnets, then that heat would completely dominate the difference in mass. So then how can one claim that the process is reversible? $\endgroup$ Commented Aug 8, 2016 at 10:54
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$\begingroup$ @flippiefanus: You take 1J out and lose 0.01J in the process, so you have to put 1.01J back in. The 0.01J have to come from the mechanical work done on the separation, not from the magnetic field or the magnet. If that wasn't the case, a permanent magnet motor/generator would be without a magnetic field in no time and stop working. They don't stop working but are just as good after decades of use and potentially billions of cycles. Would a magnet be a good system to test mass defect? No. $\endgroup$ Commented Aug 8, 2016 at 13:09
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$\begingroup$ Not sure I understand your point. Doesn't the 0.01J show that the process is not reversible? Does it matter where the 0.01J comes from? When a motor or generator operates for decades, does that exclude the possibility for loss of energy due to heat? The way I understand reversibility is that you can reverse the process and end up with the original state without any heat generated. $\endgroup$ Commented Aug 10, 2016 at 4:44
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$\begingroup$ The process with regards to the state of the magnet is perfectly reversible. It goes back to exactly the same magnetization that it had before the cycle, i.e. no energy was taken out or put into the magnetic field overall. $\endgroup$ Commented Aug 10, 2016 at 6:35