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Homework Help: Finding magnitude of two charge given midpoint

  1. Apr 28, 2017 #1
    1. The problem statement, all variables and given/known data
    Two positive point charges, A and B are 6m apart. The electric field intensity at point C, midway between the two charges is 5 N/C directed towards B and the potential at the same point is 45V. Find the magnitude of A and the magnitude of B.

    2. Relevant equations
    E = kQ/r2

    3. The attempt at a solution
    From what I visualize it:


    Using E = kQ/r2 where Q=Er2/k

    Q = (5 N/C)(3 m)2 / (9x109 Nm2/C2) = 5x10-9C

    which is the answer for A and B I suppose? But the answer key tells me that the answer in A is twice the B. How come? I'm quite confuse on how this works.
  2. jcsd
  3. Apr 29, 2017 #2


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    Homework Helper

    The equation you applied is valid for the electric field of a single point charge. You have two charges, QA and QB, both contributing to the electric field and to the potential at point C.
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