# Homework Help: Finding r using point charges and work done by an electric force

1. Jan 29, 2013

### Rae_4

1. The problem statement, all variables and given/known data
A positive point charge (q = +8.9 10-8 C) is surrounded by an equipotential surface A, which has a radius of rA = 1.3 m. A positive test charge (q0 = +2.7 10-11 C) moves from surface A to another equipotential surface B, which has a radius rB. The work done by the electric force as the test charge moves from surface A to surface B is WAB = -8.3 10-9 J. Find rB.

2. Relevant equations
VB=VA-(WAB/q0)
V=kq/r

3. The attempt at a solution
kq/rB=kq/rA-(WAB/q0)

I don't seem to understand the concepts behind this question and cannot come up with the correct answer. I'm really lost. Any help explaining this would be appreciated.

2. Jan 30, 2013

### Simon Bridge

Do you know what electric potential is?
Do you know how potential relates to potential energy and to work?
Do you know what an equi-potential surface is?

Consider: what would be the equation for the work needed to lift an object mass m from height y1 to height y2 close to the surface of the Earth?
How about if y1 and y2 were not close to the Earth?

3. Jan 30, 2013

### tiny-tim

Welcome to PF!

Hi Rae_4! Welcome to PF!
The surfaces are imaginary.

It just means that the test charge moves from distance 1.3 to distance rB, but not necessarily in a straight radial line.

Show us what you've tried.