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Magnetism Question

  1. Nov 3, 2005 #1
    I place a magnet near a piece of iron. The iron bit accelerates across the floor towards the magnet.

    The magnet has done work. Energy has been expended. Where did this energy come from?

    This energy seems inexhaustible. Why isn't it used up?
  2. jcsd
  3. Nov 3, 2005 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    The energy came from the magnetic field.

    How is this any different from placing a massive object near a particle and watching the particle accelerate towards the large object, due to the gravitational attraction between them ?

    The potential energy from the field is converted into kinetic energy. There's no violation of energy conservation.
  4. Nov 3, 2005 #3
    Is the potential energy of the field therefore reduced, by being converted to kinetic energy?
  5. Nov 3, 2005 #4


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    Staff: Mentor

    Yes. The presence of the iron affects the magnetic field, in such a way that the total energy of the field decreases as the chunk of iron approaches the magnet and gains kinetic energy.
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