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Homework Help: Finding r using point charges and work done by an electric force

  1. Jan 29, 2013 #1
    1. The problem statement, all variables and given/known data
    A positive point charge (q = +8.9 10-8 C) is surrounded by an equipotential surface A, which has a radius of rA = 1.3 m. A positive test charge (q0 = +2.7 10-11 C) moves from surface A to another equipotential surface B, which has a radius rB. The work done by the electric force as the test charge moves from surface A to surface B is WAB = -8.3 10-9 J. Find rB.

    2. Relevant equations

    3. The attempt at a solution

    I don't seem to understand the concepts behind this question and cannot come up with the correct answer. I'm really lost. Any help explaining this would be appreciated.
  2. jcsd
  3. Jan 30, 2013 #2

    Simon Bridge

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    Do you know what electric potential is?
    Do you know how potential relates to potential energy and to work?
    Do you know what an equi-potential surface is?

    Consider: what would be the equation for the work needed to lift an object mass m from height y1 to height y2 close to the surface of the Earth?
    How about if y1 and y2 were not close to the Earth?
  4. Jan 30, 2013 #3


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    Welcome to PF!

    Hi Rae_4! Welcome to PF! :smile:
    The surfaces are imaginary.

    It just means that the test charge moves from distance 1.3 to distance rB, but not necessarily in a straight radial line. :wink:

    Show us what you've tried. :smile:
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