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Homework Help: Final charge on capacitor

  1. Sep 26, 2016 #1
    1. The problem statement, all variables and given/known data
    In the circuit shown in the figure, C1 = 1.0 µF, C2 = 1.9 µF, C3 = 2.8 µF, and a voltage Vab = 30 V is applied across points a and b. After C1 is fully charged the switch is thrown to the right. What is the final charge and potential difference on each capacitor?


    2. Relevant equations


    3. The attempt at a solution

    I found everything (V1, Q2, V2, Q3, V3) except for Q1. I tried doing Q1 = CeffV1, but the answer was wrong.
    Last edited: Sep 26, 2016
  2. jcsd
  3. Sep 26, 2016 #2
    Ok so since V1=V2+V3, I did Q1/C1 = Q2/C2 + Q3/C3. Solving for Q1, I get Q1 = C1[ Q2/C2 + Q3/C3].


    Q1=C1*q[1/C2 + 1/C3]

    Plugging in the numbers, when q = 15.9μC, I get Q1 = 1.4*10-5 C. Does that look right?
    Last edited: Sep 26, 2016
  4. Sep 26, 2016 #3


    User Avatar

    Staff: Mentor

    That looks right for the final charge on C1, yes.
  5. Sep 26, 2016 #4
    Thank you!
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