A short bar magnet of magnetic moment of $0.15 \mathrm{~J} / \mathrm{T}$ is placed in a uniform magnetic field of 0.4 T . The potential energy of the magnet in unstable equilibrium is

Susceptibility of ferromagnetic substance is

The susceptibility of a magnetism at $$300 \mathrm{~K}$$ is $$1.5 \times 10^{-5}$$. The temperature at which the susceptibility increases to $$4.5 \times 10^{-5}$$ is

A bar magnet suspended by a horse's hair lies in the magnetic meridian where there is no twist in the hair, on turning the upper end of the hair through $$150^{\circ}$$, the magnet is deflected through $$30^{\circ}$$ from the meridian. Then the angle through which upper end of the hair has to be twisted to deflect the magnet through $$90^{\circ}$$ from the meridian is