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Homework Help: Electric potential energy and point charges problem

  1. Jan 28, 2007 #1

    1. The problem statement, all variables and given/known data

    Two point charges 2.0 cm apart have an electric potential energy -149.9 µJ. The total charge is 28 nC. What are the two charges?

    2. Relevant equations

    U = (kqQ) / r

    3. The attempt at a solution

    U = -149.9 x 10^-6 = [(9 x 10^9)(28 x 10^-9 - q)(q)] / (0.02). Is that correct? I seem to get the wrong answer.

  2. jcsd
  3. Jan 28, 2007 #2

    Chi Meson

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    This appears to be a simulateous equation problem. You don't know what either charge is, but you do know that the sum of both charges is 28 nC. (If I understand the problem correctly).

    Since the PE is negative, that means one charge is + and the other is -. Therefore, how much more is the + charge's magnitude greater than that of the negative?
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