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Homework Help: Two capacitors

  1. Feb 23, 2010 #1
    1. The problem statement, all variables and given/known data

    A 1.00 micro-F capacitor charged to 50.0 V and a 2.00 micro-F capacitor charged to 20.0 V are connected, with the positive plate of each connected to the negative plate of the other. What is the final charge on the 1.00 micro-F capacitor?

    2. Relevant equations

    Q = CV

    3. The attempt at a solution

    Let a subscript of 1 denote the 1.00 micro-F capacitor, and a subscript of 2 denote the other.


    Q1 = C1V1 = 50 micro-C

    Q2 = C2V2 = 40 micro-C

    When the two capacitors are connected together, the first one has charge Q1' and the second has charge Q2' with:

    Q1' + Q2' = 50 - 40 = 10 micro-C

    The total potential drop around the loop must be zero, so we have

    V1' = V2'


    Q1'/C1 = Q2'/C2

    Then when I solve for Q1', I get 3.3 micro-C, which is wrong. The correct answer according to the textbook is 45 micro-C.
  2. jcsd
  3. Feb 23, 2010 #2
    You're right. The capacitor formed from C1 and C2 in parallel with 3 microF capacitance gets a charge that is the difference of the charges on C1 and C2. This 10 microC charge will get divided between C1 and C2 proportional to their capacitance, so C1 gets 3.33 microC and C2 gets 6.66 microC

    If you would connect the positive plates together you'd get a total charge of 90 microC with 30 microC on C1 and 60 microC on C2. I have no Idea how they got 45 microC
  4. Feb 23, 2010 #3
    Thank you! Time to mark some corrections into the book!
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