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# Determine the electrostatic potential energy of a system consisting of two charges $7 \mu C$ and $–2 \mu C$ (and with no external field) placed at (–9 cm, 0, 0) and (9 cm, 0, 0) respectively.

$U= \large\frac{1}{4 \pi \in_0 } \frac{q_1q_2}{r}$
$\quad= 9 \times 106{9} \times \large\frac{7 \times -2 \times 10^{-12}}{0.18}$$=-0.7 J$