# Electric Potential Energy Question

1. Sep 16, 2009

### wilson_chem90

A test charge of +1.0 x 10^-6 C is 40 cm from a charged sphere of 3.2 x 10^-3 C.

a) How much work was required to move it there from a point 1.0 x 10^2 m away from the sphere?
b) How many electrons were gained or lost from the test object.

Relevant equations:
Ee = kq1q2/r
N = q/E

I know how to find b) but for a), I'm not sure if i had to add the two distances together then subtract the charges and divide it by the new distance. anyways, the help would be much appreciated thanks.

2. Sep 16, 2009

### rl.bhat

Find the potential at 0.4 m and 100 m due to the charged sphere.
Find the potential difference between these two points. Multiply this with test charge to find the work done.

3. Sep 16, 2009

### wilson_chem90

just to clarify, i need to use E = kq1q2/r for both distances, and i use the sphere and test charge for both calclations. and then i subtract the answers, and multiply it by the test charge?

4. Sep 16, 2009

### rl.bhat

If you use the potential energy formula, i.e. E = kq1q2/r, you need not multiply the answer by the test charge.

5. Sep 16, 2009

### wilson_chem90

actually for b), just to make sure. I use N = q/e
the calculation would be:
N = (1.0 x 10^-6 C) / (1.6 x 10^-19 C)
= 6.25 x 10^12 electrons gained or lost

6. Sep 16, 2009

### wilson_chem90

the question is b) How many electrons were gained or lost from the test object to create the charge.