Q. A bar magnet when placed at an angle of $30{^\circ}$ to the direction of magnetic field induction of $5 \times 10^{- 2}\ T,$ experiences a moment of couple $25 \times 10^{- 6}\ N - m.$ If the length of the magnet is 5 cm, its pole strength is
A) $2 \times 10^{- 2}$ A-m
B) $5 \times 10^{- 2}$ A-m
C) 2 A-m
D) 5 A-m
Explanation
(a) Torque,$\ \ \ \ \ \ \ \ \ \tau = MB\sin\theta$
$\Rightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \tau = (mL)B\sin\theta$
$\Rightarrow \ 25 \times 10^{- 6} = \left( m \times 5 \times 10^{- 2} \right) \times 510^{- 2}\ \times \sin{30{^\circ}})\ $
$\Rightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ m = 2 \times 10^{- 2}\ A - m$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.