# Electric Forces

1. Sep 13, 2013

### dwn

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

Two point charges are 3 m apart and their combined charge is 20µC. If they
repel each other with a fore of 0.075 N, what are the two charges? Ans: 15 μC and 5μC  

2. Relevant equations

F=k(q1*q2)/r^2

(could someone mention in their response how we use the math text in our posts, so that I can make it more convenient for the reader)

3. The attempt at a solution

I'm really not so sure where to begin here, but I made a few attempts by manipulating both q2 and q2. I will post one of them below:

9*109 (q1+q2)/32=0.075N

2. Sep 13, 2013

### Staff: Mentor

Good.

Check out the FAQ, under "SITE INFO/Frequently Asked Questions" at the top of every page.

The one mistake you made was to use q1+q2 instead of q1*q2.

The second equation you'll need is q1+q2 = ??

3. Sep 13, 2013

### dwn

I tried that in one of my attempts, but I don't see the "bridge" between the two equations.

If I solve Coulombs equation for q1*q2, I don't see where I'm supposed to plug in q1+q2 = 20 into the other equation....?

4. Sep 13, 2013

### Staff: Mentor

You'll have two equations and two unknowns. Solve them simultaneously, using any method you like. (Such as substitution.)