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Homework Help: Electric Potential Energy of A System of Point Charges!

  1. Sep 28, 2010 #1
    A Particle of charge q=7.5x10^-6 Coulombs is released from rest at .6m on the x axis. The particle moves due to charge Q = -20x10-6 C. What is the Kinetic Energy of the particle the instant it has moved .4 meters if Q stays fixed at the origin?

    Here I used KE1 + U1 = KE2 + U2
    Which equals 0 + (kqQ)/.6 = KE2 + kqQ/.2
    I got 2.2475 = KE2 - 6.7425
    so KE2 = 8.99 or 9 Joules.
    The answer is 4.5 J in the book however.

    Where did I go wrong? My answer for the first part of the question, with Q =positive 20 micro Coulombs, was correct. Is the KE for attraction supposed to be multiplied by 1/2 as opposed to repulsion? Please help!

    My k value by the way is correct, at 8.99x10^9.
  2. jcsd
  3. Oct 1, 2010 #2
    both potential energies should have the same sign. In your equation, one of them is positive the other is negative.
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