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Net Force Exerted by Two Charges on a Third Charge (Coulomb's Law)

  1. Jan 29, 2013 #1
    This problem has been posted before with different values for each variable and I
    did as best I could to complete it similarly. However, when I attempt to input my final
    answer I get it wrong every time. I just wanted to see where I am going wrong, thanks.

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

    Coulomb's law for the magnitude of the force F between two particles with charges
    Q and Q' separated by a distance d is:

    |F| = K(|QQ'|)/d2

    Consider two point charges located on the x axis:
    one charge, q1 = -14.5nC, is located at x1 = -1.745m
    the second charge, q2 = 36.5nC, is located at the origin.

    What is the net force exerted by these two charges on a third charge
    q3 = 45.5nC, placed between q1 and q2 at x3 = -1.085?

    Your answer may be positive or negative, depending on the direction of the force

    2. Relevant equations

    K = 1/4[itex]\pi[/itex][itex]\epsilon[/itex]0

    3. The attempt at a solution

    I found the distance between q1 and q1 to be 0.660m
    I found the distance between q3 and q2 to be 1.085m

    I began by finding the charge q1 exerts on q3

    |F1| = K*[|(-14.5*45.5)|/(0.6602)] = 1.362E13

    (At this point in the problem there is a hint section asking for this force, when I submit the answer I calculated I am told it is wrong.)

    And then I found the charge q2 exerts on q3 by:

    |F2| = K*[|(45.5*36.5)|/(1.0852)] = 1.268E13

    (Again, there is a hint section asking for this value, which also tells me it is wrong)

    From those two calculated forces I take F1 - F2 to get a final answer, which is ultimately deemed incorrect.

    Any help would be appreciated!
  2. jcsd
  3. Jan 29, 2013 #2


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    Science Advisor
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    Gold Member

    Units! The charges are quoted in nC, not C.
  4. Jan 29, 2013 #3
    Gah! I knew it would be something small. It always is. Thanks!
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