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Homework Help: Charge on capacitor

  1. Jul 21, 2011 #1
    1. The problem statement, all variables and given/known data

    Two parallel plate capacitors, C1 and C2, are connected in series with a 65-V battery and a 500-kΩ resistor, as shown in the figure. Both capacitors have plates with an area of 2.1 cm2 and a separation of 0.1 mm. Capacitor C1 has air between its plates, and capacitor C2 has the gap filled with porcelain (dielectric constant of 7 and dielectric strength of 5.7 kV/mm). The switch is closed, and a long time passes.


    (a) What is the charge on capacitor C1?

    (b) What is the charge on capacitor C2?

    3. The attempt at a solution

    (a) What is the charge on capacitor C1?

    C = E0 * A / d

    q = C delta V

    C = (8.85 * 10^-12 * 2.1 cm^2 / 10000) ( to change it 2.1 cm^2 to m^2) / 0.0001m)

    q = C delta V
    = 1.8585e-11 * 65V = 1.21 e -9

    the answer is 1.06e-9

    where did i go wrong?

    (b) What is the charge on capacitor C2?

    i used C = (E0 L^2 / (2d)) (1+k) and it didn't work
    any hints?
  2. jcsd
  3. Jul 21, 2011 #2


    User Avatar

    Staff: Mentor

    The capacitors are in series, so they can't both have the full 65V -- that would total 130 V.

    Have you learned how to combine capacitors in series or in parallel to find the effective total capacitance?
  4. Jul 22, 2011 #3
    ya i got it thx
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