Hi everybody, The situation is the classic one: a point charge q+ in a distance d above a conductor plane grounded: The conductor is grounded so V = 0, for z = 0. Also, far away from the system (x2 + y2 + z2 >> d) V --> 0 The argument to replace it for a q- charge seems perfect to me. What I can't understand why W (potential energy) of the two point charges is twice as much as the conductor plane + q+ charge system. I mean, why E = 0 for z < 0 in the conductor plane + q+. Thank you a lot!