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Homework Help: Circuit with capacitors , need to find Ceq, charge on each and total stored energy

  1. Oct 26, 2009 #1
    1. The problem statement, all variables and given/known data

    For the cicuit shown, the voltage is 200V, C1= 4microF, C2= 15microF and C3= 12microF
    (i drew it on paint)
    a) the equivalent capacitance between the terminals
    b) the charge stored on each capacitor
    c) the total stored energy

    2. Relevant equations
    Ceq= 1/c1 + 1/c2 +1/c3
    Ceq = C1+C2+C3


    I think this is the right equation for the energy stored in a capacitor:
    U= QV/2=CV^2/2

    3. The attempt at a solution

    Ok i'm really not sure because i'm having a lot of trouble with figuring circuits out. I think C1 and C2 are in series but in parallel with C3. For a) what i did is :

    Ctot = 1/Ceq + C3
    1/Ceq = 1/C1 + 1/C2
    Therefore Ct = 3.15microF + 12microF
    Ct= 15.15microF

    b) I would use C= Q/V
    so Q1 = 8*10^-4 C
    Q2= 3*10^-3 C
    Q3= 2.4*10^-3 C

    c) U=CV^2/2
    my equivalent capacitance was 15.15microF and my voltage was 200v, therefore my U should be 0.303J

    It seems to me that b and c were too easy to get. Besides, i'm not even sure my a) part is right. I'm so confused....

    the picture i drew is at the bottom, it's just pending approval.

    Attached Files:

    Last edited: Oct 26, 2009
  2. jcsd
  3. Oct 26, 2009 #2


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    Re: Circuit with capacitors , need to find Ceq, charge on each and total stored energ

    In series combination of capacitors, charges in each capacitors must be the same.
  4. Oct 27, 2009 #3


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    Hi cristina045! :smile:

    (have a mu: µ and try using the X2 and X2 tags just above the Reply box :wink:)
    Your a) is fine,

    (except of course it should be Ceq + C3 in the first line :wink:)

    (and personally I'd have used something like C12 instead of Ceq)
    Your Q3 is fine, because the voltage across C3 is 200 V.

    But the voltage across C1 (or C2) is not 200 V.

    The total voltage across both of them is 200 V, so use that (and the fact that, as rl.bhat :smile: says, their charges must be the same).
  5. Oct 28, 2009 #4
    Re: Circuit with capacitors , need to find Ceq, charge on each and total stored energ

    thank you guys soooo much
    i'm really having trouble understand these things
    it's so much easier to understand and solve with clear explanations :)
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