In our electrostatics course we were talking about how if you increase the separation of the plates by a tiny amount, then as the field is still constant, the potential difference increases. This energy comes from the work you do to separate the plates. As the surface has a charge Q, and this charge is in a field E, the overall net force upwards on the bottom plate here:
Should just be EQ. Now I can follow the math and show that if you increase the seperation by dx, the potential increases by X and this is equal to the work done or the force applied times dx, but I cant see why the force we apply ends up being less than the net force on the bottom plate.