Electric Forcs and Work (Algebra based physics)

[Seikon85]

Charges q1=+7.15 x 10-05 C and q2=-7.80 x 10-04 C are placed at rest 71.5 cm apart. How much work must be done by an outside agent to move these charges slowly and steadily until they are 31.0 cm apart?

Ok so I tried a few things on my own, and got stuck. So I referred to a previous post from this morning for guidance:

http://answerboard.cramster.com/physics-topic-5-199660-0.aspx

however, I am still getting the wrong answer.


I am using F = kq/r initially. The r value i am using was obtained by figuring out the distance one charge moved. (.715-.31)/2. Then subtracting that distance from .715. So my r = .513m here.

I get F = 968J.

W = F*d = 968J * (.31m) = 300J.


Am I missing something obvious. Any insight?

Thanks.

 

[Shooting Star]

Have you learned about potential energy between two charges?