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Two opposite polarity charged capacitors in series

  1. Nov 16, 2013 #1
    1. The problem statement, all variables and given/known data

    W7KpVbQ.png

    2. Relevant equations
    Q=C


    3. The attempt at a solution
    a) I want to say that since both have the same potentials then it is similar to a parallel circuit where the charges can differ on each capacitor. If i think of point A as an input and point B as an output then the potential difference would have to be 100V, but im not sure.

    b)
    since both are at the same potential they will no longer transfer charge.

    Q1= 10^(-6) * 100=10^(-4) C

    Q2 = 3*10^(-6) * 100 = 3 * 10^(-4) C

    Is this right?
     
  2. jcsd
  3. Nov 16, 2013 #2
    The equation should be Q=CV but I see that you did that in your calculations.. Looks alright to me though. for a), voltage doesn't chance for capacitors in parallel. Although it does seem counterintuitive, that is the case.
     
  4. Nov 16, 2013 #3

    haruspex

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    The polarities are opposite. When the switches close, there will be a flow of charge through each until both capacitors have the same potential difference again, but now with the same polarity.
     
  5. Nov 18, 2013 #4
    How would that happen though? If a negative plate transfers its charge to the positive plate of the other capacitor then it just causes a chain reaction around the loop. I'm having a hard time seeing how everything will balance out.
     
  6. Nov 18, 2013 #5

    haruspex

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    Let's just do the algebra and see if it becomes clearer to you.
    Let the initial charges be +Q1 and -Q1 on C1, +Q2 and -Q2 on C2. Write equations for those. When the switch is closed, assume a quantity of charge Q flows from the +ve of C1 to the +ve of C2. What charge will flow on the negative side? What equations can you write now?
     
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