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Homework Help: Voltage across Capacitors in Parallel

  1. Jun 3, 2012 #1
    1. The problem statement, all variables and given/known data
    A 10μF capacitor is charged to 5V and then isolated. It is then connected across a second uncharged capacitor of capacitance 15μF. The potential difference, in V, across both capacitors is A) 0.2 B) 0.5 C) 2 D) 3

    2. Relevant equations

    C= Q/V Ct=C1+C2

    3. The attempt at a solution
    Finding the charge of the first capacitor and trying to use this to find charge of the other to find voltage. Find the energy stored in the first capacitor and assuming this is the charge available to the second capacitor. Various other shifting about and substituting of equations. Nothing worked!
  2. jcsd
  3. Jun 3, 2012 #2
    What shall remain constant when you connect both the capacitors? :wink:
  4. Jun 3, 2012 #3
    The charge..? But you don't have the charge for the second capacitor, how do you find it?
  5. Jun 3, 2012 #4
    Initially, the second capacitor doesn't have charge. Only first capacitor posses charge.
    When you connect both, the charges will flow and come to a steady state. Till how long the charge should flow? What should be the condition so that charge should stop flowing between the capacitors?
  6. Jun 3, 2012 #5
    The charge will flow until the other capacitor is charged up?
  7. Jun 3, 2012 #6

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    So how much charge is there to begin with?

    In the end the voltage across the 1st capacitor will have to be the same as the voltage across the 2nd capacitor.
    So if we call this as yet unknown voltage V, can you give the formula of how much charge is on each capacitor?
  8. Jun 3, 2012 #7
    Aaaaaah I think I just got it,
    If the overall charge is the same, and the voltages across the two capacitors have to be the same, Qtotal = C1V + C2V = V(C1 +C2) so the answer is 2?
  9. Jun 3, 2012 #8

    I like Serena

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  10. Jun 3, 2012 #9
    Yay! Thank you so much for your help!
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