Question

What is electrostatic potential energy.

Answer 1

The electrostatic potential energy $U_E$ of one point charge $q$ at position $r$ in the presence of an electric field $E$ is defined as the negative of the work $W$ done by the electrostatic force to bring it from the reference position $r_{ref}$ to that position $r$:

$U_E(r)=-W_{r_{ref}\to r}=-\int q E(r')dr'$ .