When determining potential energy we have the relationship: PE = V*q = E*q*d But depending on the scenario we can't use them interchangeable right? If you want to calculate the work needed to be done by a particle going from a distance Y to right next to another particle of the same charge, then you couldn't use PE = E*q*d since the electric field wouldnt be the same as the particle got closer and closer. But you could use the voltage relationship here if you calculated the voltage difference between the two points. In what situation would you be able to use the PE= E*q*d? Where there is a constant electric field between two particles? (so not between a parallel plate capacitor).