Answer the following questions:
(a) Why does a paramagnetic sample display greater magnetisation (for the same magnetising field) when cooled.
(b) Why is diamagnetism, in contrast, almost independent of temperature?
(c) If a toroid uses bismuth for its core, will the field in the core be (slightly) greater or (slightly) less than when the core is empty?
(d) Is the permeabililty of a ferromagnetic material independent of the magnetic field? If not, is it more for lower or higher fields?
(e) Magnetic field lines are always nearly normal to the surface of a ferromagnet at every point (This fact is analogous to the static electric field lines being normal to the surface of a conductor at every point). Why?
(f) Would the maximum possible magnetisation of a paramagnetic sample be of the same order of magnitude as the magnetisation of a ferromagnet?

Explanation

(a) Owing to therandom thermal motion of molecules, the alignments of dipoles get disrupted at high temperatures. On cooling, this disruption is reduced. Hence, a paramagnetic sample displays greater magnetisation when ccoled.
(b) The induced dipole moemnt in a diamagnetic substance is alwyas opposite to the magnetising field Hence, the internal motion of the atoms (which is related to the temperature) doe snot affect the diamagnetism of a material.
(c) Bismuthis a diamagnetic substance, Hence, a toroid with a bismuth core has a magnetic field slightly greater than a toroid whose core is empty.
(d) The permeability of ferromagnetic materials is not independent of the applied
magnetic field. it is greater for a lower field and vice versa.
(e) The permeabilty of a ferromagnetic material is not less than one. it is alwys greater thatn one, Hence magnetic field lines are alwyas nearly normal to the surface of such materials at every point.
(f) The maximum possible mangetisation of a pramagnetic sample can be of the same order of magnitude as the magnetisation of a ferromagnet this requires high magnetising fields for saturation.