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Homework Help: 2 charges located in the x-y plane

  1. Oct 4, 2011 #1
    1. The problem statement, all variables and given/known data

    Consider 2 charges located in the x-y plane; q1 of magnitude 5 mC (millicoulombs) is located at the origin, and q2 of unknown magnitude is located at (x,y) = (0.1, 0) (in metres). The electric field E at position (0.03, 0.05) is found to be entirely in the +y direction (i.e. the x-component is zero). Determine the sign of charge q1 and the magnitude and sign of charge q2.

    2. Relevant equations

    |q| = 5mC (microCoulombs)

    F= qE ; q= charge, E= electric Field
    E(r) = ƩEi(r)

    3. The attempt at a solution

    E1(for one charge)= 1/(4*pi*ε°)*q1/r2
    = 1/(4*pi*ε°)* (0.000005 C)/0.0015m
    E= 2.996*107

    Then I went and did,
    rearrange for q2 and,
    q2= [E(4*pi*ε°)+(r2/q1)](r-r2)2
    q2= -29.9997 C. (This seems way to high to be correct)

    So q1 is positive to make E in the + y direction.
    Last edited: Oct 4, 2011
  2. jcsd
  3. Oct 4, 2011 #2


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    If q2 is negative, and q1 is positive, then at a location having it's x coordinate between that of the two charges, the electric field would definitely have a non-zero x component.
  4. Oct 4, 2011 #3
    Thanks for that. What do you think of the other part of the problem? I'm not quite sure if I'm missing something or did something wrong.
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