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Capacitor charged and disconnected then connected to capacitors

  1. Jul 21, 2012 #1
    Initially, a 15uF capacitor is charged by a 100V battery. Once charged, it is disconnected from the battery and connected to two uncharged capacitors(C_2 = 20uF, C_3 = 30uF) in parallel. What is the charge and potential difference on each capacitor.

    (The two capacitors mentioned above are in series to each other but parallel to the charged capacitor.)

    So, I found out the Q on C_1 as being 1.5mC when fully charged.

    Now since they are in parallel the voltage across the two uncharged capacitors is 100V.
    I find the equivalent capacitor of those two as 12uF and the charge being 1.2mC.

    Now I am confused about how there is 1.5mC on C_1 but just 1.2mC on the two capacitors in series.
    Shouldn't the charge across the whole circuit be the same? Or am I mistaken?
  2. jcsd
  3. Jul 21, 2012 #2


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    Science Advisor

    When you add the extra capacitors, the voltage on the combination will drop.

    I make the series combination 12 μF and the parallel combination 27 μF when you add the 15 μF.

    The charge must be constant,

    Initial charge = Q = C * V = 0.000015 Farads * 100 volts = 0.0015 Coulombs, as you said.

    So, the new voltage will be Q = C * V

    or 0.0015 Coulombs = 0.000027 Farads * V volts

    V= 55.56 volts
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