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Extra energy from capacitor problem

  1. Oct 3, 2007 #1
    a parallel plate capacitor is charged by a battery and then disconnected from the battery. the separation between the plates is doubled...

    the E-field stays the same since charge is constant
    the voltage doubles since V=Ed
    the energy doubles since U=0.5QV

    I was wondering, where does this extra energy come from?

    does total energy increase?
     
  2. jcsd
  3. Oct 3, 2007 #2

    Hootenanny

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    Hint: There will be a force of attraction between the two plates...
     
  4. Oct 3, 2007 #3
    the force should stay the same since it's F=EQ

    how does that play into the increase in energy?
     
  5. Oct 3, 2007 #4

    learningphysics

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    It takes work to increase the distance between the plates. You have to do work on the plates to get them apart further.
     
  6. Oct 3, 2007 #5
    so the work done on the plates translate to energy stored in the plates? so the total energy does increase.
     
  7. Oct 4, 2007 #6

    Hootenanny

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    Energy stored in the electric field increases, the energy isn't actually stored in the plates. And yes, the total energy stored increases.
     
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