A positive point charge (q = +7.2 10-8 C) is surrounded by an equipotential surface A,

Question:

A positive point charge (q = +7.2 × 10-8 C) is surrounded by an equipotential surface A, which has a radius of rA = 1.8 m. A positive test charge (q0 = +4.5 × 10-11 C) moves from surface A to another equipotential surface B, which has a radius rB. The work done as the test charge moves from surface A to surface B is WAB = 28.1 × 1029 J. Find rB.