Work required to move particles

[RKOwens4]

Homework Statement

"Three particles with charges q1 = +10 µC, q2 = -20 µC, and q3 = +35 µC are positioned at the vertices of an isosceles triangle as shown in the figure. a = 12 cm and b = 6.0 cm.

(a) How much work must an external agent do to exchange the positions of q1 and q3?

(b) How much work must an external agent do to exchange the positions of q1 and q2, instead?"

Figure: http://imageshack.us/photo/my-images/641/webassign.jpg/

Homework Equations

W=(kq1q2)/r

The Attempt at a Solution

I know (or think) that you have to sum up the work required. I know how to do this when the problem says that the particles are taken from infinity to some given position, but I don't know how to calculate the work required to move them from the given position in the figure to the new arrangement.

 

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Calculate the change of the potential energy of the system: it is equal to the work done by the external agent.

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