The setup is: Suppose that we have a point charge q held a distance d from an infinite, grounded, conducting plate. Let the plate lie in the xy-plane, and suppose that the point charge is located at coordinates (0, 0, d). What is the scalar potential above the plane? The solution to this problem is here: http://farside.ph.utexas.edu/teaching/em/lectures/node64.html In the example they conclude that "the total charge induced on the plate is equal and opposite to the point charge which induces it" - so it is -q. My question is: The plate is a grounded conductor, so the potential at z=0, and then the electric field, must be zero at z=0. How can the electric field be zero at z=0 when the plate is negatively charged? I hope you can help me. I've spent most of the time in the bus home worrying about this.