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Homework Help: Electric Field Problem -Easy

  1. Oct 7, 2007 #1
    Electric Field Problem -Easy plz help

    1. The problem statement, all variables and given/known data

    Two point charges, q1 = 4.0×10-6 C and q2 = -1.0×10-6 C, are located on the x axis at x1 = -1.0 cm and x2 = 3.0 cm.

    Determine the x coordinate of a point on the positive x axis where the electric field is zero; i.e., a test charge placed at this point would experience no force.

    2. Relevant equations


    3. The attempt at a solution

    Don't know, please help, I would like to know how to solve this.
  2. jcsd
  3. Oct 8, 2007 #2
    I am not good in electric field. So, i will ..... the problem with electrical force.
    Let this test charge is +x cm from the origin.
    Experience no force: The forces that act on this test charge by charge 1 and 2 must be the same in magnitude but in opposite directions. one will repel the test charge in one direction, the other will attract in the opposite direction.
    Considering only he magnitude of the two forces:
    k*q3*q1/(x-3)^2 = k*q3*q2/(x+1)^2 [remember to take the magnitude only especially the charge]
    solving for x, you suppose to get 7 cm and 5/3 cm .
    The answer will be 7 cm. since this test charge cannot be less than +3 cm from the origin, otherwise zero net force will not be possible since the the two forces acting on test charge are in the same direction.
    Last edited: Oct 8, 2007
  4. Oct 8, 2007 #3
    Yes, 7cm is the right answer, thank you Leong. I understand that the forces are equal yet opposite, but where did you get the equation
    k*q3*q1/(x-3)^2 = k*q3*q2/(x+1)^2 from????? What I really mean is where does (x-3) and (x+1) come from???
  5. Oct 8, 2007 #4
    x-3 is the distance between q3 and q2. x + 1 is the distance between q3 and q1. draw the diagram, put the test charge to the left of q2, thenit will be clearer.
    Last edited: Oct 8, 2007
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