Question

A short bar magnet is placed with its south pole towards geographical north. The neutral points are situated at a distance of $20cm$ from the centre of the magnet. If $B_H=0.3\times {10}^{-4}Wb/m^2$ then the magnetic moment of the magnet is :

• A. $9000ab-amp\times {cm}^2$
• B. $900ab-amp\times {cm}^2$
• C. $1200ab-amp\times {cm}^2$
• D. $225ab-amp\times {cm}^2$

Answer 1

The correct option is C $1200ab-amp\times {cm}^2$

At neutral point magnetic field due to magnet$=$ Horizontal component of the earth's magnetic field

$\Rightarrow \frac{{\mu }_0}{4\pi }.\frac{2M}{r^3}=B_H\Rightarrow \frac{{10}^{-7}\times 2\times M\times 1}{{\left(0.2\right)}^3}=0.3\times {15}^4$

$\Rightarrow M=1.2amp\times m^2=1200ab-amp\times c{m}^2$