# Coulomb's Law

1. Dec 10, 2005

### eok20

I'm stuck on the following problem: Point charges of q_1=-12nC are placed at (-4,0) cm and (4,0) cm. Determine the magnitude of the electrostatic force on a third charge of q_2=20nC placed at (0,8) cm. So because of symmetry I figured that the x-components of the forces will cancel each other and the total force will be 2 times the y-component. So I did 2*k*q_1*q_2*sin63.4/(0.04^2+0.08^2) and got 4.8e-4 N but the answer is supposed to be 3.1e-8 N. Any help would be appreciated, thanks.

2. Dec 10, 2005

### daniel_i_l

What about the distance (r) ?

3. Dec 10, 2005

### StatusX

Just check all your units. That's usually the problem when your off by several orders of magnitude.

Last edited: Dec 10, 2005
4. Dec 10, 2005

### eok20

my r^2 is (0.08^2+0.04^2). I'm pretty sure my units are right, I did 2(8.99e9)(12e-9)(20e-9)(sin63.43)/(0.08^2+0.04^2) = 4.8e-4 N

5. Dec 10, 2005

### StatusX

It looks like your answer is correct. If you copied the problem correctly, the solution must be wrong. This is clear if you do an order of magnitude estimate: the charges are of order 10^-8 C, and they are seperated by about 10^-1 m, so the force should be of order 10^10*10^-8*10^-8/(10^-1)^2, or about 10^-4 N.