A bar magnet of magnetic moment $M$ placed at right angles to a magnetic induction $B$. If a force $F$ is experienced by each pole of the magnet. The length of the magnet will be:

• Correct answer
  A

  $M B / F$
• B

  $B F / M$
• C

  $M F / B$
• D

  $F / M B$

Solution

Here $$ \begin{aligned} \tau & =\vec{M} \times \vec{B} \\ |\tau| & =M B \sin 90^{\circ} \end{aligned} $$ and we know that $\tau=F l$ $$ \therefore F l=\mathrm{MB} \quad \Rightarrow l=\frac{M B}{F} $$

Hence, the correct answer is option: 'A'.