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

1. Jan 29, 2013

### PHYSteve

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$\pi$$\epsilon$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
by:

|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. Jan 29, 2013

### haruspex

Units! The charges are quoted in nC, not C.

3. Jan 29, 2013

### PHYSteve

Gah! I knew it would be something small. It always is. Thanks!

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