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

Answer 1

Answer: (A)

$\tau=M B \sin \theta \Rightarrow \tau=(m L) B \sin \theta$
$\Rightarrow 25 \times 10^{-6}=\left(m \times 5 \times 10^{-2}\right) \times 5 \times 10^{-2} \times \sin 30^{\circ}$
$\Rightarrow m=2 \times 10^{-2} Am$