Question

Magnetic moment of bar magnet is \( M \). The work done to turn the magnet by 90° of magnet in direction of magnetic field \( B \) will be:

• A. zero
• B. \( \frac{1}{2} MB \)
• C. \( 2 MB \)
• D. \( MB \)

Hint:

The work done to rotate a magnetic moment in a magnetic field is given by \( W = -M B \cos \theta \).

Answer 1

The Correct Option is B

Step 1: Work done in rotating a magnetic moment.
The work done \( W \) to rotate a magnetic moment \( M \) through an angle \( \theta \) in a magnetic field \( B \) is given by: \[ W = -M B \cos \theta \] where \( \theta = 90^\circ \).

Step 2: Calculate the work done.
When \( \theta = 90^\circ \), the work done is \( W = -M B \left( \cos 90^\circ \right) = \frac{1}{2} M B \). Thus, the correct answer is (2).