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Homework Help: Parallel Capacitors

  1. May 22, 2012 #1
    1. The problem statement, all variables and given/known data

    A capacitor of capacitance 9 μF is fully charged from a 200 V dc supply. The
    capacitor is now disconnected from the supply and connected in parallel with an
    uncharged 3 μF capacitor. Once the 3 μF capacitor is fully charged, it is removed
    and discharged. a) What charge remains on the 9 μF capacitor? b) How many
    times would this process have to be repeated in order to reduce the charge on the 9
    μF capacitor to below 50% of its initial charge? c) What would be the pd between
    the plates of the 9 μF capacitor now be?

    2. Relevant equations


    3. The attempt at a solution

    a) I said that the 3μF capacitor will be fully charged when the voltage across both capacitors is the same,let's say Vx.

    Since energy has to be conserved then Ei=Ef therefore

    C1V^2 = C1Vx^2 + C2Vx^2 <=> Q1' = 1.5588*10^-3 C . Am i doing this correct ?
  2. jcsd
  3. May 22, 2012 #2


    User Avatar

    Staff: Mentor

    Unfortunately, much as in elastic collisions of masses, connection of capacitors in parallel is not an operation that conserves energy; some energy is lost when the charges get shuffled into their new configuration.

    What is conserved through the connection operation, though, is charge.
  4. May 22, 2012 #3
    Thank you very much !!!
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