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Charge induced on capacitor plates

  1. Dec 18, 2014 #1
    1. The problem statement, all variables and given/known data
    Three conducting capacitor plates are kept horizontally, above each other with a gap 'd'. If a charge ##+1\mu C## is kept on the middle plate what is the charge on the plates?

    2. Relevant equations
    (This question is theoretical)

    3. The attempt at a solution
    My textbook gave the charge of the plates (starting from the upper face of the topmost plate) : ##+0.5\mu C##, ##-0.5\mu C##,##+0.5\mu C##,##+0.5\mu C##,##-0.5\mu C##,##+0.5\mu C##.

    My question is: when we have opposite charges on the opposite faces of a plate (here the topmost and the lowermost, due to induction), won't there be an electric field created inside the capacitor plate (due to the opposite charges on the opposite faces, starting from positive face to negative)? And as it is a conductor is this allowed?

    //Sorry that I couldn't upload a picture
    Last edited: Dec 18, 2014
  2. jcsd
  3. Dec 18, 2014 #2


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    If you consider the top plate, it will have positive charge on one face and negative charge on the other face. These charges on the top plate would produce a nonzero electric field inside the conducting material of the top plate. However, what is the net electric field inside the material of this top plate due to all of the charge in the system (including the charges on the faces of the other two plates)?
  4. Dec 18, 2014 #3
    Oh yes, the mechanism of the equilibrium is like this: The middle plate has a net positive charge, this creates an electric field in the conductor, so to counter this electric field the top plate rearranges its charges as said above. Am I right?
  5. Dec 18, 2014 #4


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    Yes, that's right.
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