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Homework Help: Net force of electric charge

  1. Jul 5, 2011 #1
    1. The problem statement, all variables and given/known data

    In a region of two-dimensional space, there are three fixed charges: +1 mC at (0, 0), −2 mC at (15 mm, -2 mm), and +3 mC at (-5 mm, 15 mm). What is the net force on the −2-mC charge?

    ? magnitude
    ? direction ° counter-clockwise from the +x-axis

    3. The attempt at a solution
    3mc = point 1
    1 mc = point 2
    -2 mc = point 3

    length (1-3) = 26.25 mm
    length (2-3) = 15.133 mm
    length(1-2) = 15.811 mm

    angle at point 3 is 32.5 degree using the sss triangle formula

    F(1to3) = k *q1* q3 / l^2
    Fx13 = F(1to3) Cos(32.5)
    Fy13 = F(1to3) sin(32.5)

    F(2 to 3) = k *q2* q3 / l^2
    Fx23 = F(2to3) Cos(32.5)
    Fy23 = F(2to3) sin(32.5)

    Fx13 + Fx23 = Fxnet
    Fy13 + Fy23 = Fynet

    rad (Fxnet ^2 + Fynet^2 ) = Fnet which is the magnitude but it seems like i'm off by few points the answer has to be pretty close since i have to plug it into the system. Does anyone know where i made the mistake?
  2. jcsd
  3. Jul 5, 2011 #2


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    Homework Helper

    It's not 32.5 degrees. Point 3 is at (15,-2), which means it's at an angle of atan(-2/15). Try drawing out the three points to get a better feel for the configuration.

    You seem to be assuming that the angle from 2 to 3 is the same as the angle from 1 to 3, but there's no reason for this to be the case.
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