1. The problem statement, all variables and given/known data We have two vertical plates with the surface area of A. The charge of the left one is -(Q-q), while the charge of the one on the right is +Q. The left plate is held stationary. The question is, what is the force pulling the plate on the right side? The plates are made of metal (conductor) 2. Relevant equations Gauss's equation (E=Q/e0) E=F/Q Laws of superposition +the fact that the charge in a conductor, when in an electric field, is positioned on the surface of the conductor. 3. The attempt at a solution I actually know the solution, but I don't understand it. In the attachment I tried to draw down the two solutions I came up with, the first one (I) being the one that's supposed to be correct, and the second one (II) being the one I don't see why it isn't correct. The first method is simple: we find what electric field the left plate makes, and using F=EQ, we simply multiply that by the charge of the right plate, Q. The second method is more complicated. I calculated the eletric field made by both plates, added them together (purple arrows, summa E); and I also checked how the charges are positioned on the right plate. Then I also used F=EQ to see what is the summa force acting on the right side plate. If anyone can tell me why the second one is wrong, please do; it's been driving me crazy for a few days now. The second solution is exactly two times as much as the first one. Sorry if the images are hard to get, I'll explain anything in more detail if asked for.