Derive the expression for potential energy of a system of two charges in the absence of the external electric field.
Coulombic force acting on a test charge F=kQqr2 where k=14πϵo
Potential energy of the two charges U=−∫F.dr
∴ U=−∫kQqr2dr
Or U=−kQq∫1r2dr
Or U=−kQq×−1r
⟹ U=kQqr