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Homework Help: Potential energy

  1. Apr 14, 2008 #1
    1. The problem statement, all variables and given/known data

    Two point charges are arranged as shown on the x axis. q1 = +7.00 nC,
    and q2 = −5.00 nC. a = 25.0 cm.
    Calculate the position on the x axis, outside of the charges, where the
    electric potential is zero


    (ignore the periods)

    3. The attempt at a solution


    kq1/d = -kq2/d

    k(7nC)/d = -k(-5nC)/(d-25)

    then solve.

    I get the correct answer but my question is, why is it d-25 and why is it under q2? My first approach I had.

    k(7nC)/(d+25) = -k(-5nC)/d
    Last edited: Apr 14, 2008
  2. jcsd
  3. Apr 14, 2008 #2
    It depends on what d is. your first approach used d as the distance from q2, and the secontd approach used d as the distance from q1. And in the equation kq1/d = -kq2/d you use both at the same time. This should hae been kq_1/d_1 = -kq_2/d_2

    I would have used x for the position of the point with zero potential, the positions of the charges are x_1 = 0, and x_2 = 25
    would have d_1 = |x - x_1| and d_2 = |x - x_2| (|x| is the absolute value of x)
    if you look for a point to the right of q_2 you can simply have
    d_1 = x - x_1 = x
    d_2 = x - x_2 = x - 25
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