Define electrostatic potential energy.

(N/A) The electrostatic potential energy of a system of point charges is defined as the total amount of work done by an external agent in bringing the charges from infinity to their respective positions in the configuration,without any acceleration.
Mathematically,for a system of two charges $q_1$ and $q_2$ separated by a distance $r$,the electrostatic potential energy $U$ is given by:
$U = \frac{1}{4\pi\epsilon_0} \frac{q_1 q_2}{r}$
where $\epsilon_0$ is the permittivity of free space.