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Homework Help: Potential Difference across Capacitor.

  1. Sep 12, 2009 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    Not sure which equations to use but I know that we'll have to use the answers from part a and b to answer the questions.

    3. The attempt at a solution

    What I did was that I said that C1 and the 6 microfarads are parallel to C2 and the 2 microfarads capacitor and said that the voltage would just be halved. Really confused on what to do.
  2. jcsd
  3. Sep 12, 2009 #2
    remember that voltage is not 'split up' to components in parallel, this only occurs to components in series, which is the second half of the problem
  4. Sep 12, 2009 #3
    hmmm thanks that helps to clarify that point. So its 90 volts to C1 and the 6 microfarads capacitor and to the C2 and 2 microfarads capacitor. What equation do I need to use? I have a couple here. Looking at PE=(.5)(C)(deltaV^2) is that the right one? I think the main problem here is that I dont know what equation to use or how to look at the problem.
  5. Sep 12, 2009 #4
    hmm i'm afraid not, that's the equation for working out the energy stored in the capacitor, so you'll need it later, however

    V = Q / C voltage = charge / capacitance , you know the Q & C for each capacitor, so try to work out V
  6. Sep 12, 2009 #5
    hmmm....Im still getting some wrong answers. So lets say for C2, I know the charge on that capacitor is 120 microCouloumbs. Im not sure what the Capacitance for it is. Would the capacitance for C2 be the sum of (1/C1)+(1/C2)=(1/Ctot)? so it would be that V=120/1.5?

    Just figured out this part. Now onto the total energy stored by group?

    the equation used would be PE=(.5)(C)(deltaV^2) ?
    Last edited: Sep 12, 2009
  7. Sep 12, 2009 #6
    correcto, but look

    let's take C1, you worked out earlier that charge was 120 * 10^-6
    and you also know that it's capacitance is 2 * 10^-6 -it tells you on the top of the picture!

    so now you have Q and C, and V = Q / C , they seem to work, try them again
  8. Sep 12, 2009 #7
    Problem solved!

    Thanks a million.
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