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Homework Help: Magnitude of a Point Charge on a Line

  1. Sep 2, 2013 #1
    1. The problem statement, all variables and given/known data
    The drawing shows a positive point charge +q1, a second point charge q2 that may be positive or negative, and a spot labeled P, all on the same straight line.
    The distance d between the two charges is the same as the distance between q1 and the spot P.
    With q2 present, the magnitude of the net electric field at P is twice what it is when q1 is present alone.
    Given that q1 = +1.19 µC, determine the magnitude |q2| when q2 is positive. Determine the magnitude |q2| when q2 is negative.


    2. Relevant equations
    Actually my problem is I don't know which equation I'm supposed to use. All I know is that E = kq/r2

    3. The attempt at a solution
    I tried making an equation, given that the net field at P is twice what q1 is alone. So I would then have something like:
    2(kq/r2) = kqQ/2r2
    I'm not sure if that's right though. I would just get 4 as an answer. And then I have no idea what to do about -q2...
  2. jcsd
  3. Sep 2, 2013 #2


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    Homework Helper
    Gold Member
    2017 Award

    The basic concept in this problem is the principle of superposition. The net electric field vector at point P when both charges are present is the sum of the electric field vectors from each charge.

    ##\vec{E}_{net} = \vec{E}_1 + \vec{E}_2##.

    You will need to think about the directions of the vectors and how to express the distance ##r## for each charge in terms of the distance ##d## given in the figure.
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