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Homework Help: Coulomb's Law

  1. Jan 22, 2005 #1
    Here's a question from my textbook:
    Two positive point charges, each of magnitude q, are located on the y-axis at points y=+a and y=-a. A third positive charge of the same magnitude is located at some point on the x-axis.
    (a) What is the net force exerted on the third charge when it is at the origin?
    (b) What are the magnitude and direction of the net force on the third charge when its coordinate is x?
    (c) Sketch a graph of the x-component of the net force on the third charge as a function of x for values of x between +4a and -4a. Plot forces to the right upward and forces to the left downward.

    I got (a) and (b) using F= k(q1)(q3)/(r^2) and components. (a) F=0, (b) F= (2kx q^2)/ ((x^2)+(a^2))^(3/2) in the +x-direction.

    However, I don't really understand (c). The x-component of the net force is F_x = (2kx q^2)/ ((x^2)+(a^2))^(3/2) , but then do I plug in 4a, 3a, ... , -4a for values of x to get the graph? What are the axes? Are the "4a, 3a, ..., -4a" on the x-axis and F_x on the y-axis?

  2. jcsd
  3. Jan 23, 2005 #2


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    Yes, that's right.

    On the Y-axis to plot the F_x in multiples of kq^2/a^2 (ie : let each division be equal to kq^2/a^2)

    On the X-axis you have different values of x, namely -4a, -3a, ..., 3a, 4a (ie : multiples of 'a' from -4 to +4)
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