Conservation of Electric Energy

A charge moving through a field converts potential energy into kinetic energy, $\Delta K = q(V_{\text{start}} - V_{\text{end}})$, with the signs of the charge and both potentials kept. Because the hill a charge experiences is $U = qV$ rather than $V$ itself, a positive charge speeds up moving toward lower potential and a negative charge speeds up moving toward higher potential. The energy relation holds for any path in any field, while constant-acceleration kinematics holds only where the force is constant, which in this unit means the uniform field between parallel plates.

Two errors dominate. Releasing every charge from high potential toward low regardless of sign, so an electron beside the $100$ V plate is predicted to gain kinetic energy crossing to the $0$ V plate when that trip costs it energy. And running constant-acceleration relations on a charge moving near a point source, where the force changes as the separation changes, by computing $a = qE/m$ at the release point and carrying that value through the whole trip.