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Magnets and energy

  1. Sep 19, 2004 #1
    When a magnet is used to lift up a paperclip, clearly it gains gravitational potential energy. Where does this energy come from? Does the magnet lose energy to the paperclip? Does the magnet regain this energy when the paperclip is removed from the magnetic field? I have never heard of the phrase "magnetic potential energy"! :confused:
    Thanks in advance for any insight anyone can provide!

    Chris Carter
     
  2. jcsd
  3. Sep 19, 2004 #2

    krab

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    Nevertheless, there is such a thing. Magnetic charges (monopoles) do not exist independently; the most elementary magnetic charge is a dipole. Since dipole fields are more complicated than say the electric field from a single charge, they, and their energy formulas don't appear in the most elementary textbooks. So most people do not run into the concept of magnetic potential energy.
     
  4. Sep 20, 2004 #3
    Thanks krab.
    So is this situation (paperclip in magnetic field) analagous to that of a mass in a gravitational field, in terms of energy transfers?
     
  5. Sep 20, 2004 #4
    Macroscopically, almost...

    Magnets might cause the paperclip to rotate, (but not gravity). So there is extra potential energy if the paperclip is previously magnetized, and if the paper clip and magnet are not aligned. The position of both poles (whether you use a bar or a horse shoe for example) might also come into play.
     
    Last edited by a moderator: Sep 20, 2004
  6. Sep 20, 2004 #5

    krab

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    To amplify on Gonzolo: The proximity of the magnet to the paper clip changes the paper clip, lining the magnetic domains up. In this sense, the situation is not analogous to gravity. The reason is that the paper clip is a "soft" ferromagnet; it magnetizes when another magnet is nearby, but demagnetizes if it is taken out of the external magnetic field. Gravity is more analogous to a situation of two permanent magnets. (Permanent magnets are "hard".) Except that in gravity there are only monopoles, while in magnetism therefore only dipoles. You can get round this problem by considering two very long bar magnets arranged N-S to N-S so they are almost touching. If they are sufficiently long, then you can neglect the outboard poles and consider the almost-touching poles alone. These will act like monopoles, and for short distances will have a force law similar to two gravitating bodies.
     
  7. Sep 20, 2004 #6
    Thanks guys! :smile:
     
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