# Homework Help: Electric Field Question

1. Sep 8, 2014

### Rastamonstah

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

Two point charges that are 0.600m apart experience a repulsive force of 0.400N. Sum of the two charges equal 1.00 x 10^-5 C. Find the values of the two charges

2. Relevant equations

3. The attempt at a solution

My only reference on how to start on this problem is the section of my textbook on Electric Field lines, the textbook outlines the steps on how to get the strength of a field from its field lines, the formula given is completely unrelated to this problem given so i don't know how to start. I don't need an answer i'd just like some help on starting on it.

2. Sep 8, 2014

### Staff: Mentor

Surely your text must supply the basic formula for the force between two charges (Coulomb's Law)?

3. Sep 8, 2014

### Rastamonstah

Yes F= k (q)(q')/r^2

Would i rearrange this equation to find the solution?

4. Sep 8, 2014

### Staff: Mentor

Yes, but you have two unknowns: q and q'. So what does that tell you?

5. Sep 8, 2014

### Rastamonstah

Does that mean i need to set one variable to zero and solve for the other?

6. Sep 8, 2014

### TheTank

No then you would be cheating nature. When u have an equation with 2 unkowns, u need 2 equations to solve. If u have 3 unkowns, u need 3 equations etc...

7. Sep 8, 2014

### Rastamonstah

Okay that makes sense, how would i go about setting up these two equations? Am i using Columbs Law twice? Sorry for my ignorance but i haven't been able to grasp much from my lecture.

8. Sep 9, 2014

### TheTank

No, the second equation is q + q' = 10^-5 C..

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