Question

The ratio of magnetic moments of two bar magnets is $5:2$. If the deflection produced by the first magnet in the D.M. in $\tan A$ position is ${60}^o$ , the deflection due to the second magnet kept at the same distance in tan A position is :

• A. greater than ${45}^o$
• B. less than ${45}^o$
• C. less than ${30}^o$
• D. greater than ${90}^o$

Answer 1

The correct option is B less than ${45}^o$

The deflection off the magnetic needle in tan A position by a short magnet is given by
$\frac{{\mu }_o}{4\pi }\frac{2M_A}{d^3}=B_H\tan {\theta }_A$
$\frac{\tan {\theta }_A}{\tan {\theta }_B}=\frac{M_A}{M_B}$
$\tan {\theta }_B=\frac{M_B}{M_A}\times \sqrt{3}$
$\tan {\theta }_B\approx 0.7$
$\theta$ less than ${45}^o$