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Van de Graff Problem

  1. Oct 7, 2007 #1
    A van de Graaff generator puts negative charge on a metal sphere.

    Suppose the radius of the sphere is a = 6.9 cm, and the charge on the sphere is Q = -1.0×10-8 C. Determine the electric field strength at a point 1.0 cm from the surface of the sphere (outside the sphere).

    I've done this problem over and over and am not getting the correct answer.
    I've used E = F/q and made F = kq/r^2
    ...from there, I don't know what to do. Please help. Thanks
  2. jcsd
  3. Oct 7, 2007 #2


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    I think they are aking you to use a test charge (like an electron) and find the force on it at r (F = kq'q/r^2). Then find the field E at this point.
  4. Oct 7, 2007 #3
    step-by-step please

    OK, but I am still unsure what to do.. Do you add .069m and .01m for r?? Can someone "walk" me through this step-by-step PLEASE.
  5. Oct 7, 2007 #4


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    The field outside the sphere is the same as thought the entire charge were concentrated at a point at its center. So r would be 0.069 + 0.01.
  6. Oct 7, 2007 #5
    Got it. Thanks.
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