Question

If the bar magnet is turned around by ${180}^o$, where will the new null points be located?

Answer 1

When a bar magnet is turned by ${180}^0$, then the null points are obtained on the equatorial line. So, magnetic field on the equatorial line at a distance $d^{\prime }$ is given by

$B^{\prime }=\frac{{\mu }_{0}M}{4\pi d^{\prime 3}}=H$-----------------------------(1)

As the magnetic field is equal to the horizontal component of the earth magnetic field.

As we know that, magnetic field

$B_1=\frac{{\mu }_{0}2M}{4\pi d^3}=H$-------------------------- (2)

From both (1) and (2) we have

$\frac{{\mu }_{0}M}{4\pi d^{\prime 3}}=\frac{{\mu }_{0}2M}{4\pi d^3}$

$d^{\prime 3}=\frac{d^3}{2}=\frac{{14}^3}{2}$

$d^{\prime }=11.1cm$

Thus, the null points are located on the equatorial line at a distance of $11.1cm$