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Homework Help: Electric problem

  1. Jan 19, 2009 #1
    Note: ^i means carrot on top of variable

    1. The problem statement, all variables and given/known data
    A particle with charge 4 µC is located on the x-axis at the point 10 cm, and a second particle with charge -3 µC is placed on the x-axis at 4 cm. What is the magnitude of the total electrostatic force on a third particle with a charge 4 µC placed on the x-axis at -2 cm? Answer in units of N.

    3. The attempt at a solution
    1) I converted all my variables to µC = C , and cm = m
    q1 = 4 µC = 4e-6
    q2 = -3 µC = -3e-6
    q3 = 4 µC = 4e-6
    x1 = 10 cm = 0.1 m
    x2 = 4 cm = 0.04 m
    x3 = -2 cm = -0.02 m

    2 By using Coulom's law in vector form:
    F13= ke(q1q3/r2)^r13
    where ^r13 is a unit vector directed from q1 to q3; i.e., ->r13 = ->r3 - ->r1

    x13 = x3 - x1
    (-0.02m)-(0.1m) = -0.12m
    x23 = x3 - x2
    (-0.02m)-(0.04) = -0.06 m

    ^x13 = x3-x1/ √(x3-x1)^2 = -1 or -^i
    ^x23 = x3-x2/ √(x3-x2)^2 = 1 or +^i

    Since the forces are collinear, the force on the third particle is the algrebracis sum of the forces between the first and third and the second and third particles.

    ->F = ->F13 + ->F23
    = ke [q1/ r213 X ^r13 + q2 / r223 x^r13 ] q3

    After putting in the variables, I got 33.9444444445 N as my answer, but was wrong. What did i do wrong?
    Last edited: Jan 19, 2009
  2. jcsd
  3. Jan 20, 2009 #2


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    Homework Helper

    First of all find the force between 4micro C at -2 cm and 4 micro C at 10 cm. It is repulsive force. Next find the force between 4micro C at -2 cm and -3 micro C at 4 cm. It is attractive force. Now find the net electrostatic force on the third charge.
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