Thread: equipotential surfaces problem

A positive point charge is surrounded by an equipotential surface A, which has a radius of rA. A positive test charge moves from surface A to another equipotential surface B, which has a radius rB. In the process, the electric force does negative work.

The positive point charge is q = +7.5 10-8 C, and the test charge is q0 = +6.0 10-11 C. The work done by the electric force as the test charge moves from surface A to surface B is WAB = -8.1 10-9 J. The radius of surface A is rA = 1.7 m.

Find rB, the radius of surface B.

Could someone help me set this problem up. I hit a wall and don't know what to do

Thanks

From Wikipedia: "The electric potential created by a point charge q, at a distance r from the charge (relative to the potential at infinity), can be shown to be ." Since the potential is the potential energy of the test charge divided by the test charge itself, you know the energy of the test charge at . Similarly, you know the energy of the test charge at as an expression of . Finally, you know the difference of these energies, which is the work done by the electric force. From here, you can find .