The electric potential energy for a system of two point charges of \( 6\,\mu\mathrm{C} \) and \( -3\,\mu\mathrm{C} \) placed at points \( x = -9\,\mathrm{cm} \) and \( x = +9\,\mathrm{cm} \), respectively is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( -0.9\,\mathrm{J} \)
The electric potential energy of two point charges is \( U = k q_1 q_2 / r \), where the signs of the charges are kept, so unlike charges give a negative value.
\( \displaystyle r = 9 - (-9) = 18\,\mathrm{cm} = 0.18\,\mathrm{m} \)
\( \displaystyle U = \frac{9\times10^{9} \times (6\times10^{-6}) \times (-3\times10^{-6})}{0.18} \)
\( \displaystyle U = \frac{-0.162}{0.18} = -0.9\,\mathrm{J} \)
Note: NTA picked this question from an NCERT (2026-27) example and changed the figures. Do not ignore NCERT while practising numericals.
NCERT (2026-27):
Potential Energy of a System of Two Charges in an External Field – …
