Electric potential of 3 point charges

[benchwarmer08]

Homework Statement

Three point charges, which initially are infinitely far apart, are placed at the corners of an equilateral triangle with sides "b". Two of the point charges are identical and have charge "q".

If zero net work is required to place the three charges at the corners of the triangle, what must the value of the third charge be?

Homework Equations

The Attempt at a Solution

I know the electric potential at infinity is zero, but I'm not sure how to start this

 

[vorcil]

Don't forget about conservation of energy, The final potential energy between each of the charges is the some of the potential energy between them,

You are given this information:
1:Three charges are set up in a triangle
2:Two of the charges have charge q
3:The net potential is ZERO

what does super position say?
the sum of the charge is,

q1+q2+q3=0

what happens to that equation when q1 and q2 are the same?

solve that for q3, the charge on the other charge

 

[nickjer]

What vorcil originally said is correct. The final potential energy is the sum of the potential energy between each pair of charges. But that isn't the same as the sum of the charges (not sure where that came from).

I suggest first writing out the potential energy equation and showing some work benchwarmer. And we can help you from there.

 

[benchwarmer08]

I figured it out guys, thanks