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Homework Help: Question about circuits

  1. Sep 18, 2013 #1
    1. The problem statement, all variables and given/known data

    A circuit is constructed with five capacitors and a battery as shown. The values for the capacitors are: C1 = C5 = 4.1 μF, C2 = 3 μF, C3 = 6.8 μF, and C4 = 3.4 μF. The battery voltage is V = 12 V.

    Picture is here: http://s3.amazonaws.com/answer-board-image/7cae5cec-ff95-4f38-b803-29ac0b6ddae6.png

    I need to find the equivalent capacitance between points a and c.

    2. Relevant equations

    I have the answer but I am a bit confused as to how it is arrived at. See below.

    3. The attempt at a solution

    I found the equivalent capacitance between a and b as c234=(c4)+(1/c23)

    I see next that c234 is in series with c1 and c5. Why does the equation then become Cac=(1/((1/C1)+(1/C234)) and not (1/((1/C1)+(1/C5)+(1/C234))?
  2. jcsd
  3. Sep 18, 2013 #2
    You can neglect c5 since it doesn't affect the capacitance at a or b since the current is flowing clockwise.
  4. Sep 18, 2013 #3


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    Staff: Mentor

    Apparently they're only looking for the capacitance to the right of the points a and c. This is not clear in your problem description, so perhaps the problem is stated differently in the original?
  5. Sep 18, 2013 #4
    This was exactly the way the problem was written, so it left me a little confused as well
  6. Sep 18, 2013 #5


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    Staff: Mentor

    Current direction doesn't affect equivalent capacitance! Not unless there's a nonlinear circuit element like an open switch or a diode involved that would prevent "seeing" the capacitance.
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