Question

A short bar magnet is kept in the earth's magnetic field with its north pole pointing geographic north. The magnetic moment of the short bar magnet is $0.4{\text{Am}}^2$. If earth's horizontal magnetic field is $4\times {10}^{-5}\text{T}$, then the distance between the null points is

Answer 1

At null point

$B_H=Beq.$

$B_H=\frac{{\mu }_{0}M}{4\pi d^3}$

$4\times {10}^{-5}={10}^{-7}\times \frac{0.4}{d^3}$

$\Rightarrow d^3=1\times {10}^{-3}{m}^3$

$\Rightarrow d={10}^{-1}\text{m}=10\text{cm}$.

The distance between the null points is , $D=2d=2\times 10=20\text{cm}$

Accepted answer : $20$