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Stupid question on energy

  1. Feb 28, 2015 #1
    I pry two magnets apart. The magnets attract each other. Say they exert a constant force F on each other. I exert an equivalent force F on each in the opposite direction and pull them apart a distance d (pull each of them d/2 distance). So I have done positive work Fd in total and increased the potential energy of the arrangement by Fd. The magnets have each done negative work -Fd/2 on each other.
    Me: lost Fd energy
    Left magnet: lost Fd/2 energy from work done on other magnet
    Right magnet: lost Fd/2 energy from work done on other magnet
    The potential energy of the arrangement rose by Fd.

    Anyone tell me where I messed up? Because conservation of energy doesn't work out.
  2. jcsd
  3. Mar 1, 2015 #2

    Simon Bridge

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    The magnets have not lost any energy - you have done work storing energy in the magnetic field between the magnets.
    The amount of energy stored is the same as the work you did.
  4. Mar 1, 2015 #3


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    Τhe middle statements about the left and right magnet are wrong. The left magnet doesnt lose Fd/2 energy instead it gains Fd/2 potential energy. Similar right magnet gains Fd/2 potential energy. It is a gain in potential energy rather than a gain in kinetic energy that we would have if there was no magnetic field. You view that "possible" gain in kinetic energy as a loss which isnt realy an energy loss, it is a gain in potential energy instead.
  5. Mar 1, 2015 #4
    Thanks, you two. Ok, I think I get it now
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