1. The problem statement, all variables and given/known data (see picture) I understand one approach to this homework problem. I don't understand why an alternative approach fails. 2. Relevant equations V = E*d 3. The attempt at a solution The correct answer is (d), zero. An acceptable approach: 1) Realize that V is the same at both corners 2) Thus, delta V = 0 3) delta U = q*V 4) If potential energy doesn't change, then no work has been done (or been "lost") from the system. An alternative: 1) The particle experiences a rightwards force in the direction of the 2+ charge 2) To offset the right-wards pulling force and maintain a straight path, a new force, Fn, needs to be applied leftwards. 3) The force (Fn) is applied over a distance (Not the vertical distance between both charges. Instead: The horizontal distance that would otherwise exist were Fn not exerted) 4) This implies that energy has been exerted.