A system consisting of two point charges 7μC and -4μC are placed at (- 9, 0, 0) cm and (9, 0, 0) cm respectively. The electrostatic potential energy of the system is:

-0.7 J -1.4 J -3.6 J -6.8 J

-1.4 J

$ U = \frac{1}{4\pi\epsilon_0} \frac{q_1q_2}{r} = \frac{9\times 10^9 \times -28\times 10^{-12}}{18\times 10^{-2}} = -1.4J$