Question

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. $2A-m$
• D. $5A-m$

Answer 1

The correct option is A $2\times {10}^{-2}A-m$

$\tau =MBsin\theta \Rightarrow \tau =\left(mL\right)Bsin\theta$
$\Rightarrow 25\times {10}^{-6}=(m\times 5\times {10}^{-2})\times 5\times {10}^{-2}\times sin30$
$\Rightarrow m=2\times {10}^{-2}A-m$