# Electricity and charge question, equilibrium, help!

1. Aug 31, 2008

### briteliner

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

a charge of q is fixed on a plane at the origin of an xy coordinate system and a charge 3q is fixed at (3cm, -3cm). where must a charge of -2q be placed at rest for it to be in equilibrium so that it remains at rest? is the equilibrium stable?

2. Relevant equations

coulomb's law

3. The attempt at a solution

2. Sep 1, 2008

### tiny-tim

Welcome to PF!

Hi briteliner! Welcome to PF!

How far have you got?

Can you work out what line it must be on, and whether it's between or outside the charges?

3. Sep 1, 2008

### briteliner

i don't really know where to start, all i know is that since the -2q charge is negative it should be in between the charges. i think?

4. Sep 1, 2008

### tiny-tim

Hi briteliner!

Yes, it is repelled by both the other charges, and so it must be between them.

ok … there's a line joining the two other charges … which side of that line must the third charge be on?

5. Sep 1, 2008

### briteliner

why would it make a difference which side it is on?