Question

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

• A. 0.7 J
• B. -0.7 J
• C. 70 J
• D. -70 J

Answer 1

Answer: (B)

The electrostatic potential energy due to system of two charges is given as $U=\frac{K q_{1} q_{2}}{r_{12}}$
$U=\frac{1}{4 \pi \varepsilon_{0}} \frac{q_{1} q_{2}}{r}$
$=9 \times 10^{9} \times \frac{7 \times(-2) \times 10^{-12}}{0.18}=-0.7\, J$