Q.

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                            [2024]

1 16×10-2J  
2 zero  
3 8×10-2J  
4 4×10-2J  

Ans.

(3)     

         At stable equilibrium,U=-MBcos0°=-MB

          At unstable equilibrium,U=-MBcos180°=+MB

          U=2MB=2×0.5×8×10-2

         Work done,W=ΔU=2MB

         W=8×10-2J