Question about electrostatic equilibrium?

[jle1092]

Two fixed charges, 3.0 uC and 5.0 uC, are 0.40 m apart. The 3.0 uC is to the left of the 5.0 uC charge. Where should a third charge of 2.0 uC be placed to put the system of three charges in electrostatic equilibrium? The answer should be measured with respect to the 5.0 uC charge.

What equation do I use?

 

[gneill]

Coulomb's law looks appropriate. Start by estimating where you think the third charge will have to go in order for the net force on it to balance out to zero. Hint: there are three choices of "region" that it might be placed: to the left of the two fixed charges, between the two fixed charges, and to the right of both fixed charges. Which region looks most promising?