Electric potential energy and point charges problem

[abeltyukov]

Hi,

Homework Statement

Two point charges 2.0 cm apart have an electric potential energy -149.9 µJ. The total charge is 28 nC. What are the two charges?

Homework Equations

U = (kqQ) / r

The Attempt at a Solution

U = -149.9 x 10^-6 = [(9 x 10^9)(28 x 10^-9 - q)(q)] / (0.02). Is that correct? I seem to get the wrong answer.


Thanks!

 

[Chi Meson]

This appears to be a simulateous equation problem. You don't know what either charge is, but you do know that the sum of both charges is 28 nC. (If I understand the problem correctly).

Since the PE is negative, that means one charge is + and the other is -. Therefore, how much more is the + charge's magnitude greater than that of the negative?