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Capacitors in Series

  1. Feb 21, 2007 #1
    We've been studying capacitors in series in my physics class. I have a problem tonight and I know the answer, I just don't know how to get to it.

    The problems is as follows:
    [​IMG]
    Find the voltage V1 across the first capacitor.
    Express your answer in terms of V.
     
  2. jcsd
  3. Feb 21, 2007 #2

    ranger

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    capacitors in sereis always have the same charge. They do not the same voltage unless they have the same capacitance.

    What would be your first step?
     
  4. Feb 21, 2007 #3
    The first step is finding that they all have the same charge. Then, I think, I would find the equivalent capacitance which is equal to 11/6 C. From there I don't know where I would get the voltage for each one.
     
  5. Feb 21, 2007 #4

    ranger

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    Do you know also know of a formula that relates capacitance, voltage, and charge?
     
  6. Feb 21, 2007 #5
    ofcourse, c=q/v... So Ceq=Q/Veq...
     
  7. Feb 21, 2007 #6

    ranger

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    Good. But instead of Ceq=Q/Veq (not sure what you mean by Veq). It should be Ceq=Q/V, where V is the total source voltage.

    We know what Ceq and V is simply V, whats left to be determined? Remember that this quantity would be the same for all caps.
     
  8. Feb 21, 2007 #7
    Ok by the Veq I meant the total voltage... So what we have is that the total voltage is equal to V. The C equivalence is equal to 11/6 C. And that the charge on each capacitor is Q. Now from that information, I am still at a loss as to how to determine the voltage across the first capacitor.

    The answer is 6/11 V according to the system my homework is in...
     
  9. Feb 21, 2007 #8

    ranger

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    Are you sure the Ceq = (11C)/6? Recheck it. If the charge on each cap is Q, then what is Q? Knowing Q, you will be able to find V for any cap.
     
  10. Feb 21, 2007 #9
    Ok, I see that I made the mistake there... It is 6/11 C. So we would then set that 6/11 C = Q(for each capacitor) / (Vtot).

    So Q = 6 (Vtot) (C) / 11 for each capacitor.

    Now that we have that, we could then find the voltage by knowing the q=v/c equation.

    C = (6/11 C Vtot / 11)/V1
    C V1 = 6/11 C Vtot
    V1 = 6/11 Vtot

    And that's the answer!!! Thank you for your help!!!
    The problem that I had was knowing to get the charge for each capacitor and then work backwards...
     
  11. Feb 21, 2007 #10

    ranger

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    You're very much welcome. And well done on your part :approve:
     
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