Potential energy of an arrangement of point charges

[acdurbin953]

Homework Statement

What is the potential energy of the system composed of the three charges q1, q3, and q4, when q1 is at point R? Define the potential energy to be zero at infinity.

Charges are arranged in a triangle, with q3 and q4 located up and down 2.2 cm from the origin, and q1 on the x-axis at a distance R = 3.5 cm from the origin.

q1 = 1.9uC
q3 = q4 = -1.8 uC
distance from q1 to each of the other 2 charges is 4.1 cm

Homework Equations

U-kqq/r

The Attempt at a Solution

U1 = [(9E9)(-1.8E-6)(1.9E6)]/0.041 = -0.751

Since there is symmetry I doubled U1 to find U = -1.501, however that answer is wrong. Where am I messing up?

 

[acdurbin953]

I just figured out that you have to also considered the two point charges potential energy on each other. Case closed! Sorry for the (unnecessary) post.