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Homework Help: Storage of capacitors in series

  1. May 3, 2010 #1
    1. The problem statement, all variables and given/known data
    Two identical capacitors that have been discharged are connected in series across the terminals of a 100V battery. When only on of the capacitors is connected across the terminals of the battery, the energy stored is U. what is the total energy stored in the two capacitors when the series combination is connected to the battery? a) 4U, B)2U, C)U D)U/2 E)U/4

    2. Relevant equations
    U = 1/2 CV2
    1/Ceq = 1/C1+1/C2

    3. The attempt at a solution
    The only attempt I can think to make is to get U with one capacitor in terms of C1 and then get U2 in terms of C1 and C2 and then find the ratio of them. When I do this though I keep getting U/U2 = C2/(C1+C2) Which is not any of the answers. So I'm not really sure where to go from here.
  2. jcsd
  3. May 3, 2010 #2


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    Homework Helper

    Re: Capacitors

    U/U2 = C2/(C1+C2)
    In the problem, it is given that C1 = C2.
    Ceq = C/2
    U1 = 1/2CV^2
    U2 1/2*C/2*V^2
    So U1/U2 = ....?
  4. May 3, 2010 #3
    Re: Capacitors

    How did you know that C1 = C2? With that bit of knowledge I got D) (U/2) as the answer. Thanks for your help.
  5. May 3, 2010 #4
    Re: Capacitors

    It says two identical capacitors.
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