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Homework Help: Capacitor Network (Please help )

  1. Sep 7, 2010 #1
    Capacitor Network (Please help!!!)

    1. The problem statement, all variables and given/known data

    In the circuit below, consisting of a battery and several capacitors, some quantities are known and some are not. The capacitors were completely uncharged before the battery was attached and we are considering the static situation.

    Circuit: http://img690.imageshack.us/i/showmepl.gif/

    Q1= 8.5 × 10^-6 C
    C1= 7.6 × 10^-6 F
    C2= 2 × 10^-6 F
    Energy stored on C2 is W2= 4.2 × 10^ -6 J

    What is the charge Q3 on capacitor C3 ?

    3. The attempt at a solution

    I've figured out that V1 = 1.12, V2 and V3 = 2.05...but I have no idea how to find Q3 because we don't know C3
  2. jcsd
  3. Sep 7, 2010 #2


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    Homework Helper

    Re: Capacitor Network (Please help!!!)

    In the series combination, Charges are the same in all the capacitors.So the charge on C1 is the same as the charge on the parallel combination of C2 and C3. Hence

    Q2 = C1V2 and Q2 + Q3 = Q1.

    Now find Q3.
  4. Sep 7, 2010 #3
    Re: Capacitor Network (Please help!!!)

    Thank you man!!

    I forgot to add the parallel C2 and C3 and turn it into one circuit, then assume that they are in series with C1.....I really appreciate the help
  5. Oct 10, 2010 #4
    Re: Capacitor Network (Please help!!!)

    How to know whether capacitors are in series or parallel in a circuit which contain many capacitors which are connected in tricky ways. Any thumb rule is there?
  6. Oct 10, 2010 #5


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    Re: Capacitor Network (Please help!!!)

    Consider two plates 1 and 2 from C1 and plates 3 and 4 form C2.
    In series combination, 2 and 3 plates are connected by a wire and there is no branching in the wire. In parallel combination, 1 and 3 plates and 2 and 4 plates are connected. Then this combination is connected to other source.
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