We know that the normal electric field direction in a parallel plate capacitor normally goes from the positive plate to the negative plate, so going from a high potential to low potential. Thus, if we were to place a positive charged object into the plates, the positive object would go from a higher potential to lower potential, as the object would start with potential energy and end with kinetic energy. However, if we place a electron in the capacitor, the electron has a high potential energy in the negative plate... and accelerate towards the positive plate. Would we say that it is going from a lower potential to a higher potential in terms of the parallel plate, or would the field follow the energy flow of the electron, going from high potential to low potential?
3 Answers
Since potential does not depend on the "test charge", then yes, we say that the negative charge is going from a lower potential to a higher potential. i.e. the negative charge just moving in the field will experience a positive potential change $\Delta V>0$
However, since the change in electric potential energy is given by $\Delta U=q\Delta V$, if $q<0$ and $\Delta V>0$ then $\Delta U<0$. So the negative charge still loses potential energy, which is needed since it is gaining kinetic energy, and the electrostatic interaction is conservative.
No, you can't say that the electron increases it's potential energy. It may be true regarding the parallel plate capacitor to say that the electron moves from a region of lower potential to a region of higher one (considering the convention).
Any charged particle will tend to move from a region of higher potential energy to a region of lower potential energy.
It's just a matter of convention that the electric field lines are drawn in accordance with the behavior of a unit positive test charge. If you want your test charge to be an electron, just reverse the direction of the electric field lines.
Potential energy is given by qV. Q for an electron is negative and by moving from the negative to the positive plate, it will have negative potential energy gain or will lose potential energy.
