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Homework Help: Resulting charge, parallel capacitors disconnected from battery and polarity reversed

  1. Sep 30, 2009 #1
    1. The problem statement, all variables and given/known data
    Capacitors of 11.1uF and 1.9uF are charged as a parallel combination across a 333V battery. The capacitors are disconnected from the battery and from each other. They are then connected positive to negative plate and negative to positive plate. Find the resulting charge on the first capacitor in units uC.

    2. Relevant equations
    1. Ceq = C1 + C2 parallel capacitance C1=11.1uF C2=1.9uF
    2. Qtotal=V * Ceq

    3. The attempt at a solution
    1. Ceq = 11.1uF + 1.9uF = 13uF
    2. Qtotal = 333 * 13uF = .004329 C

    i tried using an equilibrium theorm:
    (Qtot / Ceq) = (Qtot - q1) / 11.1uF -> .004329 / 13e-6 = (.004329 - q1) / 11.1e-6
    with q1 = .0080253C but it's bigger than q= 333V * 11.1uF = .036963 so i don't think it's correct because that would be the max charge, right?

    after this point do i treat it as capacitors in series or what?

    part B of the problem states they are charged in series and then disconnected from the battery and connected positive plate to positive place and negative to negative plate

    sorry if i'm overlooking something obvious.
  2. jcsd
  3. Sep 30, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Re: resulting charge, parallel capacitors disconnected from battery and polarity reve

    Find the original charge on each capacitor. When they are reconnected, what's the total charge on both capacitors? When reconnected, what's the same across each capacitor? Use that to figure out how the charge rearranged itself upon reconnection.
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