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Electric Potential Energy Question

  1. Sep 16, 2009 #1
    A test charge of +1.0 x 10^-6 C is 40 cm from a charged sphere of 3.2 x 10^-3 C.

    a) How much work was required to move it there from a point 1.0 x 10^2 m away from the sphere?
    b) How many electrons were gained or lost from the test object.



    Relevant equations:
    Ee = kq1q2/r
    N = q/E

    I know how to find b) but for a), I'm not sure if i had to add the two distances together then subtract the charges and divide it by the new distance. anyways, the help would be much appreciated thanks.
     
  2. jcsd
  3. Sep 16, 2009 #2

    rl.bhat

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    Find the potential at 0.4 m and 100 m due to the charged sphere.
    Find the potential difference between these two points. Multiply this with test charge to find the work done.
     
  4. Sep 16, 2009 #3
    just to clarify, i need to use E = kq1q2/r for both distances, and i use the sphere and test charge for both calclations. and then i subtract the answers, and multiply it by the test charge?
     
  5. Sep 16, 2009 #4

    rl.bhat

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    If you use the potential energy formula, i.e. E = kq1q2/r, you need not multiply the answer by the test charge.
     
  6. Sep 16, 2009 #5
    actually for b), just to make sure. I use N = q/e
    the calculation would be:
    N = (1.0 x 10^-6 C) / (1.6 x 10^-19 C)
    = 6.25 x 10^12 electrons gained or lost
     
  7. Sep 16, 2009 #6
    the question is b) How many electrons were gained or lost from the test object to create the charge.
     
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