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 $(-9cm,0,0)$ and $(9cm,0,0)$, 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 $U=\frac{K{q}_1{q}_2}{r_{12}}$
$U=\frac{1}{4\pi {\epsilon }_0}\frac{q_1{q}_2}{r}$
$=9\times 10^9\times \frac{7\times (-2)\times 10^{-12}}{0.18}=-0.7J$