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Stored energy among a capacitor arrangement

  1. Mar 14, 2015 #1

    BEFORE YOU READ THE REST: Problem (f) is the only one I need help with. Thank you! I have already confirmed all my other values as correct in my online program!

    1. The problem statement, all variables and given/known data

    In the figure a potential difference V = 150 V is applied across a capacitor arrangement with capacitances C1 = 14.8 µF, C2 = 4.27 µF, and C3 = 4.22 µF. What are (a) charge q3, (b) potential difference V3, and (c) stored energy U3 for capacitor 3, (d) q1, (e) V1, and (f) U1 for capacitor 1, and (g) q2, (h) V2, and (i) U2 for capacitor 2?


    C1 = 14.8 µF
    C2 = 4.27 µF
    C3 = 4.22 µF
    V1 = 33.5 V
    V2 = 116.4 V
    V3 = 150 V
    q1 = 496.5 µC
    q2 = 496.5 µC
    q3 = 633 µC
    U1 = (NO IDEA) I keep getting anywhere from .00830 to .00831 and .00832
    U2 = .0289 J
    U3 = .0475 J

    2. Relevant equations

    q = V*C

    1/CT = 1/C1 + 1/C2 + ... + 1/Cn (for a series)

    CT = C1 + C2 + ... + Cn (for parallel)

    U = (1/2) q^2/C = (1/2)QV = (1/2)CV^2

    3. The attempt at a solution

    So basically, I have solved for everything except problem (f), which I don't understand because my values for U2 and U3 were correct.

    I have tried every variation of the stored energy equation, and have tried all forms of significant figures (my program does uncertainty +/- 1 from the 3rd significant figure).

    I have also wondered if maybe the fact that the electrons travel through C2 first, before reaching C1, that it may have some effect upon the stored energy of C1, and I have tried adding, subtracting, you name it, the two values.

    I'm kind of stuck at this point.
  2. jcsd
  3. Mar 14, 2015 #2


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    Staff: Mentor

    Your graphic isn't showing, it's probably behind a login:password. You can take a screenshot, clip it to just the circuit, and attach that.
  4. Mar 14, 2015 #3


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    2017 Award

    Staff: Mentor

    The value of 33.5 V for capacitor 1 has a rounding issue that has a relevant impact. If you want to use this to get the energy, keep more digits. Apart from that, it looks fine, it is just a rounding issue.

    (I don't see the image either, I reverse-engineered the setup based on the calculated values :D)
  5. Mar 15, 2015 #4
    Sorry about that guys, I was going to change the image but it said my thread was deleted so I couldn't find it until I got mfb's notification for some reason.

    Anyway, I wonder why the program didn't place my value of 33.5 V as incorrect because it is based upon significant figures?

    Here is the image anyway:

  6. Mar 15, 2015 #5


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    Staff: Mentor

    Probably due to this reason.
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