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Static magnet attached to a fridge

  1. Aug 12, 2012 #1
    Hi all,

    I had a thought earlier about magnetic forces.

    If there's a magnet attached (at rest) to my fridge, then there is a magnetic force on the magnet towards the fridge and vice-versa. This force stops the magnet from falling because of the frictional force between the magnet and the fridge it induces.

    Here's where I get confused. If the magnet isn't moving, then there is no energy expenditure while the magnet is at rest. The magnet gained energy from being placed at a point of lower potential. But since it is now at rest, an outside force (my hand, for example) must have taken away whatever energy it gained going from higher to lower potential.

    Right? But then, where does the magnetic force come from? Why does a force exist indefinitely between the magnet and the fridge? Is energy trivial in this case? Or is the energy it gained still present and, presumably, causing our force?
     
  2. jcsd
  3. Aug 12, 2012 #2
    The magnet still possesses potential energy even when stuck to the fridge.

    The magnet lost potential energy and gained kinetic energy when it was brought closer to the fridge. Your hand did work Against the gain in KE of the magnet.
     
  4. Aug 12, 2012 #3

    russ_watters

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    Staff: Mentor

    Applying a static force transfers no energy.
     
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