Electric Potential Energy: Work Required to Move 3 Charges Out to Infinity

[ayreia]

Homework Statement

Three charges are distributed as follows:

http://tinypic.com/r/1fviq0/5

How much work must an external force do to move them infinitely far from each other?

Homework Equations

W = -$\Delta$U = (kq$_{1}$q$_{2}$)/r

The Attempt at a Solution

So what I did was find the work needed to move each individual charge out to infinity using

W = -kq$_{1}$ [(q$_{2}$/r$_{12}$) + (q$_{3}$/r$_{13}$)].

For q$_{1}$ this gave me -0.539 J, q$_{2}$ +0.862 J, and q$_{3}$ -0.861 J. I thought I could just add them together to get the total work, but this is incorrect. I'm completely stumped now as to how to proceed. Can someone point me in the right direction? Thanks a lot!

 

[tiny-tim]

hi ayreia!

you only need to move two of them to infinity, don't you?

(also, once you've moved the first one, you can forget about it when you move the second one)

 

[ayreia]

Oh, right! That makes sense, thanks a lot! :D