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Homework Help: Parllel Plate Capacitors Connected Opposite

  1. Sep 25, 2008 #1
    1. The problem statement, all variables and given/known data

    Two capacitors are connected parallel to each other and connected to the battery with voltage 30 V. Let C1 = 9.8 micro F,C2 = 4.0 micro F be their capacitances. Suppose the charged capacitors are disconnected from the source and from each other, and then reconnected to each other with plates of opposite sign together.

    By how much does the energy of the system decrease?

    2. Relevant equations

    U = (1/2)QV

    For PPC connected in parallel,

    Q1 = C1V and Q2 = C2V

    Ceq = C1 + C2 + C3.......

    3. The attempt at a solution

    I found the charge stored on the plates initially, and was trying to find the charge stored on them after they were reconnect, but failed. I really don't know what to do. Do I have to find the new voltage too?
  2. jcsd
  3. Sep 26, 2008 #2
    Re: Parllel Plate Capacitors Connected Oppositely

    If someone could just tell me if I have to recalculate a new potential difference, that would be great. I really have no clue how to proceed from here.
  4. Sep 26, 2008 #3
    Can anyone help me with this? It's very frustrating, and I'm not quite sure how to approach it.
  5. Sep 26, 2008 #4


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    Homework Helper

    Note that when you connect the capacitors in series, they have the same charge on each other. So now you have to find the new charge on both of them and add up the stored energy in both.
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