ElectroStatic Equilibrium problem

1. Sep 8, 2011

RandellK02

1. The problem statement, all variables and given/known data
particle 1 of charge q1 (at origin) and particle 2 of charge q2 = 3q1, are held at separation L on an x axis. If particle 3 of unknown charge q3 is to be located such that the net electrostatic force on it from particles 1 and 2 is zero, what must be the (a) x and (b) y coordinates of particle 3?

2. Relevant equations
F=k|q1||q2|/d2

3. The attempt at a solution
I assumed q1 and q2 are like charges so I put q3 in the middle. This could be my mistake since im getting incorrect answers for x, I do know y = 0.
If q3 does belong between the two charges on the X axis, then I am not setting up my d2 part of Coulombs law. Could someone point me to the right direction?
Net Force = 0
F31=k|q3||q1|/?2
F32=k|q3||q2|/?2
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Sep 8, 2011

wukunlin

if the particle is at distance d away from q1, what distance is it between the particle and q2? assuming this is a one dimensional problem

3. Sep 8, 2011

RandellK02

Okay say, if distance between q3 and q2 =d than distance between q3 and q1 is L - d correct? So...
kq3q1/(L-d)2=kq3q2/d2
=>q1/(L-d)2=q2/d2

If the above steps are correct... is it safe to assume to use the fact: q2=3q1 to eliminate the two charges?

4. Sep 8, 2011

yes :)