Two equal charges of $3 \times 10^{-5}$ C each are brought from infinity to a separation of 30 cm. The electrostatic potential energy of the configuration is:
A0.27 J
B2.7 J
C270 J
D27 J
Answer & Solution
Correct answer: D. 27 J
$U = \dfrac{1}{4\pi\varepsilon_0}\dfrac{q_1 q_2}{r} = 9\times10^9 \times \dfrac{(3\times10^{-5})^2}{0.30} = 9\times10^9 \times 9\times10^{-10}/0.3 = 27$ J. Positive because like charges repel — work was done against repulsion.
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