Question

A bar magnet of dipole moment M is initially parallel to a magnetic field of induction B. The angle through which it should be rotated so that the torque acting on it is half the maximum torque is _____.

• A. ${90}^0$
• B. ${60}^0$
• C. ${45}^0$
• D. ${30}^0$

Answer 1

The correct option is D ${30}^0$

$\tau =MXB=MBsinQ$

At maximum torque the Q is ${90}^o$

$\tau =MBsin\left({90}^o\right)=MB$----------------1

At the half of maximum torque,

$\frac{\tau }{2}=MXB=MBsinT$-----------------2

1 to 2

$\tau /2\tau =\frac{MBsinT}{MB}$

$sinT=\frac{1}{2}$

$T={30}^o$