Question

When a short bar magnet is kept at a distance of 20 cm from the centre of D.M., in Tan A position, the deflection is 45${}^0$ . If $H=30$ A/m, the moment of the magnet is :

• A. 1.5 $\times$ 10${}^{-2}$ Am${}^2$
• B. 1.51Am${}^2$
• C. 3.01Am${}^2$
• D. 1.31Am${}^2$

Answer 1

The correct option is B 1.51Am${}^2$

The deflection off the magnetic needle in tan A position by a short magnet is given by
$\frac{{\mu }_o}{4\pi }\frac{2M_A}{d^3}=B_{H}tan{\theta }_A$
$\theta ={45}^o$
$B={\mu }_o\times H$
$4\pi \times {10}^{-7}\times 30={10}^{-7}\times \frac{2M}{d^3}$
$M=1.51A{m}^2$