A bar magnet of magnetic moment 3 J/T lies aligned with the directions of a uniform magnetic field of 0.2 T. The amount of work, required so as to align its magnetic moment opposite to the field direction is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 1.2 J
A magnetic dipole in a uniform field has potential energy \( U = -mB\cos\theta \). The work an external agent must do to turn it slowly is the change in this energy.
\( \displaystyle W = U_f - U_i = (-mB\cos 180^\circ) - (-mB\cos 0^\circ) = 2mB \)
\( W = 2 \times 3\,\mathrm{J\,T^{-1}} \times 0.2\,\mathrm{T} = 1.2\,\mathrm{J} \)
Note: NTA picked this question from an NCERT (2026-27) exercise and changed the figures. Do not ignore NCERT while practising numericals.
NCERT (2026-27):
Exercises – …
