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Homework Help: Electric Potential

  1. Feb 7, 2010 #1
    1. The problem statement, all variables and given/known data
    Why is the change in the electric potential between two capacitors V=ED ?

    2. Relevant equations


    3. The attempt at a solution

    Isn't the change in electric potential equal to the negative of the product of the electric field and displacement?
    So, why is it that for a capacitor the change in potential is not V=-ED ?

    Thanks for any help.
  2. jcsd
  3. Feb 8, 2010 #2
    You mean the change in potential between the 2 plates of a capacitor?

    Actually the equation [itex] \Delta V = -E \cdot D [/itex] is satisfied

    [itex] E \cdot D [/itex] is the dot product of the vectors of the electric field and the displacement.
    It's +|E||D| if they are in the same direction and -|E||D| if they are in opposite directions.

    (|E| is the magniture of the electric field |D| is the magnitude of the displacement)

    In a capacitor, the electric field points from the positive plate to the negative, so from the higher potential to the lower. If you move in the direction of the electric field, then both E and D have
    the same direction, but you're moving to a lower potential, wich is as it should because [itex] -E \cdot D [/itex] is negative
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