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Capacitors with switch

  1. Feb 8, 2009 #1
    1. The problem statement
    The capacitors in the figure are initially uncharged and are connected, as in the diagram, with switch S open. The applied potential difference is Vab = + 210 V.

    What is the potential difference Vcd?

    YF-24-60.jpg

    2. Relevant equations
    (series) 1/C = 1/c1 + 1/c2 + ...
    (parallel) C = c1 + c2 + ...
    C=Q/V

    3. The attempt at a solution
    Trying to find Vcd:
    I first found the equivalent capacitance of the series, which is 4*10-6 F. I calculated the charge on the capacitors using Q = CV, which was 8.4*10-4. Then, I tried to find Vad and Vac by using V=Q/C. I don't think this is the right way to do this problem though.
     
    Last edited: Feb 8, 2009
  2. jcsd
  3. Feb 8, 2009 #2

    LowlyPion

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    Homework Helper

    That looks like the way to do it as far as you went.
     
  4. Feb 8, 2009 #3
    Ah, I figured it out. The upper and lower series each have half the value of the charge that I calculated. Then using V=QC, Vad=140V and Vac=70V, so Vad-Vac=70V.
     
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