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Homework Help: Radii of eqiuipotential surfaces of a charged metal sphere

  1. Feb 5, 2013 #1
    1. The problem statement, all variables and given/known data

    A metal sphere of radius r0 = 0.29 m carries a charge Q = 0.90 µC. Equipotential surfaces are to be drawn for 100 V intervals outside the sphere. Determine the radius r of the following equipotentials from the surface.
    a) first:___m
    b) tenth:___m
    c) 100th:_____m

    2. Relevant equations

    V=kQ/r --> r=kQ/V

    3. The attempt at a solution

    I've tried plugging the numbers from the problem in the equation:

    r=(8.99 x 10^9 NM^2/C^2)*(0.9 x 10^-6 C)/(100V) = 80.91m

    and it's incorrect. I assume the radius of the sphere itself comes into play, but I'm not sure where. Can anyone help?
  2. jcsd
  3. Feb 5, 2013 #2


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    What is the potential at the surface of the ball ?
  4. Feb 5, 2013 #3
    I was able to find the potential of the ball:

    V=(9 x10^9NM^2/C^2)*(0.9 x10^-6 C)/0.29m = 27931.03V

    and was able to answer part a, by adding 100 V to the ball's potential and using the formula. So, for b, I added 1000 V to the potential and for c I added 10000 V to the potential, but the results were incorrect.
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