1989 Physics B Electro - Question #2.

[GP_Skier]

Homework Statement

Two point charges, Q1 and Q2, are located a distance 0.20 meter apart, as shown above. Charge Q1 = +8.0*10^-6C. The net electric field is zero at point P, located 0.40 meter from Q1 and 0.20 meter from Q2. Charge Q2 = -2.0*10^-6C.

Determine the coordinate of the point R on the x-axis between the two charges at which the electric potential is zero.

Homework Equations

There is an x-axis in which Q1 is located at 0, Q2 is located at 0.2, and P is located at 0.4 (all in meters).

The Attempt at a Solution

I can't figure out what formula to use in order to find the location... what should I use?

 

[Shooting Star]

What is the potential at a point at a distance of x from a point charge q?