Magnetism and Matter

The magnetic moment of a bar magnet is 0.5 Am2. It is suspended in a uniform magnetic field of 8 10-2 T. The work done in rotating it from its most stable to most unstable position is:

1. 4 10-2 J
2. 16 10-2 J
3. 8 10-2 J
4. Zero

Answer

(3) 8 10-2 J

Solution

U = -mB : most stable = 0 , most unstable = 180

W = U180 - U0 = mB - (-mB) = 2mB

= 2 0.5 8 10-2 = 8 10-2 J

Answer: (3)