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Homework Help: Electron field around sphere

  1. Jan 24, 2007 #1
    1. The problem statement, all variables and given/known data
    A 2.40 -mm-diameter glass sphere has a charge of + 1.70 nC.
    What speed does an electron need to orbit the sphere 1.40 mm above the surface?

    2. Relevant equations
    Electric Field of Sphere: E=k*Q/(r^2)

    3. The attempt at a solution
    It seems pretty straight forward but I must be doing something wrong with the r values. Could someone tell me what the radius is considered. For the electric field is it the radius of sphere + the distance away or what and same for the other equation. Ive tried pretty much all the combos I can think of and still am getting it wrong so not sure where Im going wrong.
  2. jcsd
  3. Jan 24, 2007 #2
    It says above the surface so you would have to add the radius of the sphere to the radius of the orbit.
    Last edited: Jan 24, 2007
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