Question

A bar magnet when placed at an angle of ${30}^o$ to the direction of magnetic field of induction of $5\times {10}^{-5}T$, experiences a moment of a couple $2.5\times {10}^{-6}N-m$. If the length of the magnet is $5cm$ its pole strength is:

• A. $2\times {10}^{-2}Am$
• B. $5\times {10}^{-2}Am$
• C. $2Am$
• D. $5Am$

Answer 1

The correct option is C $2Am$

Torque experienced by bar magnet is $\tau =MB\sin \theta$ where $M$magnetic moment, $B$ is magnetic field induction and $\theta$ is the angle between $MandB$

It is given that

$\tau =2.5\times {10}^{-6}N$

$B=5\times {10}^{-5}T$

$L=0.05m$

$2.5\times {10}^{-6}=M\times 5\times {10}^{-5}\times \frac{1}{2}$

$M={10}^{-1}$

$M=m\times L$

${10}^{-1}=m\times 0.05$

$m=2Am$