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Homework Help: Spherical Capacitor with Charge, Potential Difference and More

  1. Oct 2, 2012 #1
    1. The problem statement, all variables and given/known data
    A spherical capacitor contains a charge of 3.10nC when connected to a potential difference of 220V . If its plates are separated by vacuum and the inner radius of the outer shell is 5.00cm .

    Part A) Calculate the capacitance.

    2. Relevant equations
    C = Q/V

    C = Aε0/d

    3. The attempt at a solution

    I tried plugging in the values into

    C = Q/V

    C = (3.1x10^-9 C)/220V
    C = 1.409x10^-11 F

    But it didn't accept this. Any suggestions?
  2. jcsd
  3. Oct 2, 2012 #2
    Maybe a matter of significant digits?
  4. Oct 2, 2012 #3
    I made a mistake with the units. It wanted the answer in pico-Farads and I gave it in Farads. The answer is 14.1 pF.

    Thanks for your help.
  5. Oct 2, 2012 #4
    I'm stuck on the second part as well, it asks:

    B) Calculate the Radius of the inner sphere

    C = Aε0/d

    where I believe that A is the area of a capacitor and d is the distance between them.

    C = 4πr^3*ε0/d = 14.1

    So I solved for r and I got

    r = Cuberoot[(14.1x10^-12 F)*0.05 m/8.85x10^-12]

    r = 0.2937 m = 29.37 cm

    It says this is wrong and I don't know where to go from here.
  6. Oct 2, 2012 #5
    This formula is valid for capacitors with plane, parallel plates.
    For spherical capacitor there is a different formula.
    You can look it up on Google if you don't have it.
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