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This is of course a purely theoretical question and it would take energy to get the magnet moving in the first place but once it's moving in the vacuum of space, with no gravity or magnetic field nearby, could it spin nearly forever (as in billions of years) producing a magnetic field from which you could get electricity?

If this wouldn't work, why not?

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  • $\begingroup$ Apply energy conservation to the problem. Where does the energy that can be extracted from a spinning magnet with a coil come from? $\endgroup$ – CuriousOne Sep 8 '14 at 0:02
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    $\begingroup$ To produce electricity there would have to be something nearby to have a current induced. Once the current is induced an opposing magnetic field would be setup and energy would be lost to resistance. $\endgroup$ – Brandon Enright Sep 8 '14 at 0:08
  • $\begingroup$ Unfortunately, conservation of energy works every time... $\endgroup$ – Floris Sep 8 '14 at 0:54
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But once it's moving in the vacuum of space, with no gravity or magnetic field nearby, could it spin nearly forever (aka billions of years) producing a magnetic field,

No. A rotating magnet creates a changing magnetic field. Similar to an oscillating electric field, it will radiate electromagnetic energy. This energy will come from the rotational energy of the magnet. The generation of this radiation will slow down the magnet.

for which you could get electricity from?

Only a finite amount. Even if the rotation didn't radiate any energy spontaneously, any attempt to pull energy from the system would slow the magnet's rotation. After extracting a finite amount of electricity, the magnet would stop spinning and there would be no more energy available.

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    $\begingroup$ I am sorry but a rotating magnetic dipole loses energy by radiation too, so eventually it will stop rotating . link.springer.com/article/10.1007/BF01001080#page-1.. also If the magnetic dipole is inclined by some angle 0 from the rotation axis, it emits low-frequency electromagnetic radiation. from cv.nrao.edu/course/astr534/Pulsars.html $\endgroup$ – anna v Sep 8 '14 at 4:08
  • $\begingroup$ Yes, you're correct. The EM losses aren't relevant to the actual argument, but it does make my phrasing incorrect. I'll edit it. $\endgroup$ – BowlOfRed Sep 8 '14 at 4:19
  • $\begingroup$ A physics teacher of mine really drove this home with an example I saw. It was simply a hand cranked generator. Attached to nothing it was easy to crank. Hook it up to a light bulb and instantly it becomes really hard to crank. Generating electrical current is hard! $\endgroup$ – Vectorjohn Jun 13 '18 at 22:16
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A spinning magnetic dipole produces electromagnetic radiation.

The radiation carries both energy and angular momentum away from the dipole.

As a consequence the dipole will lose rotational kinetic energy.

Exactly this is thought to be responsible for the spin-down of pulsars.

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Depends on how you spin in.

Consider a cylindrical bar magnet.

If you spin it end over end (perpendicular to the axis of symmetry), then the dipole is changing and you will slowly radiate away energy and the spin will slow down.

If you spin it along the axis of symmetry, the dipole (or any higher moment) is not changing in time, so no energy will be radiated away. However this will create an electric field. It would cost you energy to start it spinning, but it doesn't cost any energy to continue "producing" the electric field.

Of course, if you try to use this electric field to accelerate charges and light a flourescent light or something, then of course, by conservation of energy, you would not be able to do this forever.

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  • $\begingroup$ Whoever downvoted this answer - could you please explain why? $\endgroup$ – lesnik Oct 9 '17 at 8:03
  • $\begingroup$ @lesnik no idea, but in case it is because there is doubt that spinning along the axis will create an electric field, here is a related question on that physics.stackexchange.com/questions/6581/… $\endgroup$ – BuddyJohn Oct 9 '17 at 8:19
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If there were solar panels to act as a back up supply to provide power to keep the magnet going despite the wear of radiation, then theoretically, yes

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