Question

A magnet of magnetic moment M is situated with its axis along the direction of a magnetic field of strength B. The work done in rotating it by an angle of 180^o will be

• A. $- M B$
• B. $+ M B$
• C. 0
• D. $+ 2 M B$

Answer 1

Answer: (D)

$+ 2 M B$

Answer 2

Work done $M B \left( \cos \theta _ { 1 } - \cos \theta _ { 2 } \right)$

and $\theta _ { 2 } = 180 ^ { \circ }$

$\Rightarrow W = M B ( \cos 0 - \cos 180 ) = 2 M B$