Question

Potential energy of two charges $10 \, nC$ each separated by a distance of 0.09 m in air is

• A. $10 \mu J$
• B. $1 \mu J$
• C. $10 m J$
• D. $10 J$

Answer 1

Answer: (A)

$10 \mu J$

Answer 2

Potential energy of two charge system is given by
$U= \frac{1}{4 \pi e_{0} e_{r}} \times\frac{q_{1}q_{2}}{r}$
Here, $q_{1 } = q_{2} = 10 nC = 10\times 10^{-9}C$
$\frac{1}{4 \pi e_{0} } = 9 \times 10^{9} N m^{2} C^{-2} r = r = 0.09 m$
$\therefore \, U = 9 \times 10^{9} \times \frac{10 \times 10^{-9} \times 10 \times 10^{-9}}{0.09} = 10 \mu J$