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Homework Help: Capacitor Problem

  1. Mar 8, 2007 #1

    21385

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    1. The problem statement, all variables and given/known data
    The potential across a 3uF capacitor is 12V when it is not connected to anything. It is then conntected in parallel with an uncharged 6uF capacitor. At equilibrium, the charge q on the 3uF capacitor and the potential difference V across it are
    a)q=12uC, V=4V b)q=24uC, V=8V
    c)q=36uC, V=12V d)q=12uC, V=6V


    2. Relevant equations
    q=CV
    Parallel connections
    C(total)=C1+C2


    3. The attempt at a solution
    The answer is a) but I am not sure why. The first part of the question confuses me. I don't really know what it means when they say that it is not connected to anything. Thanks.
     
    21385, Mar 8, 2007
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  3. Mar 8, 2007 #2

    berkeman

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    Staff: Mentor

    Write the equation Q=CV for the initial situation, and then for the final situation. The initial situation is just the 3uF cap charged to 12V. The final situation is as you indicate, with the total capacitance the sum of the two caps. What is the final voltage across the two caps? Use that final voltage, which is across both the 3uF and 6uF capacitors now, to calculate the final charge that is on the 3uF cap.
     
    berkeman, Mar 8, 2007
  4. Mar 8, 2007 #3

    Vagrant

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    The common potential across both the capacitors after connection is given by
    << exact answer deleted by berkeman >>, i.e. total charge upon total capacitance. And for the first part I assume it means that it was charged by connecting to a battery, after which the battery is removed thereby making the charge accumulated on the plates of the capacitor constant.
     
    Last edited: Mar 8, 2007
    Vagrant, Mar 8, 2007
  5. Mar 8, 2007 #4

    berkeman

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    Staff: Mentor

    shramana, please do not post exact answers to homework questions. Our task here is to provide tutorial help, not the answers. That is stated clearly in the PF rules.
     
    berkeman, Mar 8, 2007
  6. Mar 8, 2007 #5

    21385

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    Thank you guys!
     
    21385, Mar 8, 2007
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