Electrical potential energy: 3 charges in a line

[alicemunro]

Homework Statement

A positive charge, q1 is at (-20cm,0). An equal and opposite charge q2 is at (20cm, 0). A positive charge q3, equal to q1, is placed at (60cm,0). What is the potential energy of the three charges??

Homework Equations

EPE= kq / r

The Attempt at a Solution

EPE = k( (5 X 10^-9)/.2) + (5 X 10^-6)/.6) - (5 X 10^-6)/.2))
= wrong answer

 

[dark adonis]

Homework Statement

A positive charge, q1 is at (-20cm,0). An equal and opposite charge q2 is at (20cm, 0). A positive charge q3, equal to q1, is placed at (60cm,0). What is the potential energy of the three charges??

Homework Equations

EPE= kq / r

The Attempt at a Solution

EPE = k( (5 X 10^-9)/.2) + (5 X 10^-6)/.6) - (5 X 10^-6)/.2))
= wrong answer

It looks like you have additional information to provide such as the charges of q1,q2, q3 also your equation for electric potential energy is incorrect should be:
$$U = k \frac{q_a q_b}{r_{ab}}$$
Finally I need to note that r is not the position but rather the relative distance between charges a and b. With these adjustments the answer should clear up a bit