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I have read lots of articles about permanent magnet motors, some of which claim the possibility and other which refute it. Is it possible to have a permanent magnet motor that runs on the magnetic force of permanent magnets?

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You should probably clarify the question. The phrase "permanent magnet motors" is ambiguous: there are a lot of motors that incorporate permanent magnets in their designs. I suspect that you are asking about perpetual motion machines that purport to draw their power from permanent magnets. If so it is purest bunkem. –  dmckee May 27 '12 at 18:27
    
All magnets do lose strength even though extremely slowly over time s true "perpetual motion machines" of this type are not possible. It would just be a "multi-lifetime" motion machine by not truely perpetual. –  Argus May 27 '12 at 19:09
    
These days if you buy a battery-powered electric drill, what makes it small, lightweight, and powerful, is that it has been possible to make permanent magnets that are very strong for their weight. Wikipedia –  Mike Dunlavey May 27 '12 at 22:13
    
Yes, the machines do exist. But at the end the magnets become demagnetized. So, no free gain in energy. –  user299 May 28 '12 at 3:08

2 Answers 2

Magnetic field, and interactions of atoms with magnetic field, conserve energy, i.e. no net energy gets created at any point. There is not a lot of energy in the field of even a strong permanent magnet. One could, in principle, construct a 'motor' that would demagnetise the magnets somehow, converting the field energy into motion, but it wouldn't be some effective super energy storage or the like, and would run for some time then stop.

I think the reason people turn to magnets when trying to build perpetual motion devices is that it is harder to understand energy conservation in this context, and furthermore it seems more magical. The fact is, the laws governing electromagnetic interactions are very well known, incorporate conservation of energy. There is the point of "what if the scientists are wrong?" Science, indeed can be wrong sometimes. But if this particular knowledge was off by more than utterly microscopic amount, the computer you use to read this message, the communication equipment, the power equipment, the generators, the hard drive in the computer storing this message, and so on, would not have been possible to engineer.

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"One could, in principle, construct a 'motor' that would demagnetise the magnets somehow" I believe that the magnetized state is at lower energy than the demagnetized state, because is represents the aligned state of many intrinsic magnetic dipole moments. That is, I think you should be thinking in terms of a Ising model. –  dmckee May 27 '12 at 23:46
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Put a pair of magnets together side to side, north up south down, parallel, like !! , as you'd do to make a bigger magnet. Remove hands. One of the magnets will flip over, resulting in lower energy state of !i (the ! is meant to represent up arrow and i down arrow). Yes, the lowest energy state is magnetized, but the magnetic domains are not aligned (and external magnetic field is very small). edit: found an image en.wikipedia.org/wiki/File:Powstawanie_domen_by_Zureks.png –  Dmytry May 28 '12 at 3:08
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That is a fully classical argument, but the cause of ferromagnetism is inherently quantum mechanical. You can't reproduce it with a handful of macroscopic magnets. –  dmckee May 29 '12 at 2:00
    
A ferromagnetic material consists of many small magnetic domains, which are, themselves, magnetized (quantum mechanical phenomenon). A piece of steel that is magnetized, has those domains in alignment, which is a higher energy configuration, while the piece of steel that is not magnetized, has those domains out of alignment, which is a lower energy configuration. It is very common to store some energy in the magnetic field inside the ferromagnetic material; the device which does so is called 'inductor', and most inductors use field inside ferromagnetic material. –  Dmytry May 29 '12 at 10:31
    
Yes, of course. The exchange potential goes by the size of the domain boundaries while the field goes by the volume. –  dmckee May 29 '12 at 15:30

Why, here you go, an example of permanent magnet motor - a Curie engine.

Motors that combine permanent magnets and electromagnets are common too.

Thing is magnetic field as a means of storage of energy is subject of the same thermodynamics laws as the rest of the universe. If you want the motor to move, you must change the field. To change the field you must expend energy one way or another. Something like a motor that uses just permanent magnets with no external energy sources is impossible. Now ones that use permanent magnets and little more - like in the example, a heat source - these are common.

The linked engine uses the Curie effect - metals above certain temperature cease to be ferromagnetic - attracted by magnets. And while you could theoretically build such a motor in a way that restores spent heat energy, it will still need to spend more heat to remove ferromagnetic effect from a magnetized piece of metal than from unmagnetized one.

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protected by Qmechanic Feb 8 '13 at 16:38

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