Question

A short magnet when placed at a distance of $15cm$ in $\tan A$ position produces a deflection of ${60}^o$. If the magnet is cut into $3$ equal parts and one of them is kept at the same distance in $\tan A$ position, the deflection is :

• A. ${20}^o$
• B. ${30}^o$
• C. ${45}^o$
• D. ${60}^o$

Answer 1

Answer: (B)

${30}^o$

The deflection of the magnetic needle in $\tan A$ position by a short magnet is given by
$\frac{{\mu }_o}{4\pi }\frac{2M}{d^3}=B_H\tan {\theta }_A$
$M_B=\frac{M}{3}$
$\frac{\tan {\theta }_A}{\tan {\theta }_B}=\frac{M_A}{M_B}$
$\tan {\theta }_B=\frac{M_B}{M_A}\times \sqrt{3}$
${\theta }_B={30}^o$