Question

A bar magnet of magnetic moment 10⁴J/T is free to rotate in a horizontal plane. The work done in rotating the magnet slowly from a direction parallel to a horizontal magnetic field of 4×10^–5 T to a direction 60° from the field will be

• A. 0.2 J
• B. 2.0 J
• C. 4.18 J
• D. 2 × 10² J

Answer 1

Answer: (A)

0.2 J

Answer 2

Magnetic moment of bar

$M = 10 ^ { 4 } \mathrm {~J} / T$

$B = 4 \times 10 ^ { - 5 } T$

Hence work done $W = \vec { M } \cdot \vec { B }$