The electric potential energy of a system of two point charges \( 7\,\mu\mathrm{C} \) and \( -2\,\mu\mathrm{C} \) placed at a distance of \( 18 \) cm apart is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \( -0.7 \) J
The electric potential energy of two point charges is the work done in bringing them from infinity to their separation r; the charges are put in with their signs.
\( r = 18\,\mathrm{cm} = 0.18\,\mathrm{m} \)
\( \displaystyle U = \frac{1}{4\pi\varepsilon_0}\,\frac{q_1 q_2}{r} \)
\( \displaystyle U = 9\times10^{9}\,\mathrm{N\,m^{2}\,C^{-2}} \times \frac{(7\times10^{-6}\,\mathrm{C})(-2\times10^{-6}\,\mathrm{C})}{0.18\,\mathrm{m}} \)
\( \displaystyle U = \frac{-0.126}{0.18}\,\mathrm{J} \)
\( \displaystyle U = -0.7\,\mathrm{J} \)
The energy is negative because the charges are unlike: they attract, and work must be done to pull them apart.
Note: This question is from NCERT (2026-27). NTA sets numerical questions from NCERT too, so do not ignore NCERT while practising numericals.
NCERT (2026-27):
Potential Energy of a System of Two Charges in an External Field – …
