In bringing an electron towards another electron, the electrostatic potential energy of the system: [AIPMT 1999]

[b] The electron has negative charge. When an electron is bringing towards another electron, then DUE to same negative charge repulsive force is produced between them. So, to bring them closer a work is done against this repulsive force. This work is stored in the FORM of electrostatic POTENTIAL energy. Thus, electrostatic potential energy of system increases. ALTERNATIVE: Electrostatic potential energy of system of two electrons \[U=\frac{1}{4\pi {{\varepsilon }_{0}}}\frac{(-e)(-e)}{r}=\frac{1}{4\pi {{\varepsilon }_{0}}}\frac{{{e}^{2}}}{r}\] Thus, as \[r\] decreases, potential energy U increases.