A plane sheet has a uniformly distributed charge. Origin is selected at the middle of the sheet (in yz plane). A charge +Q is moved from point A(10a,2a,0) to point B(10a,0,2a) along a circle and then to point C(10a,0,0). If the initial electrostatic  potential energy is U0, What will be final electrostatic potential energy?

The electric field will be as shown. A flat surface of cylinder of radius 2a and length 10a will be an equipotential surface. Points ,4 and B are located on a circle forming flat surface of the cylinder. The electric field is perpendicular at all the points of this plane surface. Thus, there will be no change in electrostatic PE in moving the charge +Q on this plane surface.