You know the electric Potential Energy of two charges is given by
Notice that the Potential Energy is negative!
What does negative Potential Energy mean, anyway?
Two charges infinitely far from each other have zero Potential Energy -- by our definition! If they have negative Potential Energy, that means we must do work from the outside to get the charges to this reference state of zero Potential Energy. We must do work on this system to separate the charges.
(c) 2000, Doug Davis; all rights reserved.