Conservation of energy in magnets

  1. Hi,

    With permanent magnets I have been puzzling over how they can obey the conservation of energy.

    If I make a permanent magnet then I can use it, expending negligible energy myself, to attract, and thus give kinetic energy to, many other metallic objects.

    So, when I create my magnet, do all of the objects in the universe suddenly gain 'magnetic potential energy', in which case I would have to provide an unrealistic amount of energy to produce the magnet?

    So how does COE work with permanent magnets?
  2. jcsd
  3. How do you make a magnet?
  4. jfizzix

    jfizzix 463
    Science Advisor
    Gold Member

    the conservation of energy works the same for permanent magnets as it does for everything else. The universe doesn't gain or lose any energy by assembling atoms into a bar magnet for the same reason that the universe doesn't gain or lose any energy when two bar magnets are put together to make a stronger magnet. (atoms can be treated as very small bar magnets of varying strengths and directions)

    Though you could say that the potential energy of an object changes in such a case, the potential energy decrease is equal to the energy required to assemble the big magnet from smaller magnets, so the total energy remains constant.
  5. jfizzix

    jfizzix 463
    Science Advisor
    Gold Member

    you make a magnet by orienting the spins of the electrons in atoms to point all or mostly in the same direction. the spin of an individual electron creates a small magnetic field just like a loop of current does. When you add all these spins up, you can get a strong magnetic field as in a bar magnet.
  6. You seem to think that the energy in the magnetic field is infinite. It is not. The magnetic field has an energy density which is proportional to the square of the field. Integrating this density over space gives a finite total energy.
  7. BruceW

    BruceW 3,600
    Homework Helper

    yeah. There's two main points in my opinion. 1) the magnetic field takes time to spread out through space. So once you make the magnet, it does not instantly produce a magnetic force on a wire which is very far away. 2) Even when the magnetic field does spread out into space, the strength of this magnetic field decreases with distance, such that the magnetic force on a wire which is further away, is smaller.
  8. How many? The number isn't unlimited, at least not without removing some as well. That takes energy.
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