# Force exerted by two charges

1. Feb 1, 2010

### bphysics

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

Two point charges are placed on the x-axis as follows: one positive charge, q1, is located to the right of the origin at x= x1, and a second positive charge, q2, is located to the left of the origin at x= x2.

What is the total force (magnitude and direction) exerted by these two charges on a negative point charge, q3, that is placed at the origin? Use $$\epsilon_{0}$$ for the permittivity of free space. Take positive forces to be along the positive x-axis. Do not use unit vectors.

2. Relevant equations

Coulomb's Law: F = $$\frac{1}{4\pi\epsilon_{0}}\frac{\left|q_{1}q_{2}\right|}{r^{2}}$$

3. The attempt at a solution

http://img5.imageshack.us/img5/3753/equationu.jpg [Broken]

MasteringPhysics keeps giving me a "Check your signs error". Yet, as far as I can tell, I should be subtracting the force which is going left / the negative direction (ie, q2) from the force going to the right (ie, q1).

Any hints?

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Last edited by a moderator: May 4, 2017
2. Feb 1, 2010

### bphysics

My apologizes if the "attempt at a solution" section appeared blank for anyone -- I have changed the link so that users who are not logged in can now see the link.

3. Feb 2, 2010

### thebigstar25

u can find the electric field at the origin from the charge q1 and q2 and then u can find the force using F= q3 * E .. try this one, I hope it will work