Question

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

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Updated on : 2022-09-05
Solution
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## (a) This is because at lower temperatures, Â the tendency to disrupt the alignment of dipoles(due to magnetizing field) decreases on account of reduced random thermal motion. (b) In a diamagnetic sample, each molecule is not a magnetic dipole in itself.Therefore random thermal motion of molecules does not affect the magnetism of the specimen. That is why diamagnetism is independent of temperature. (c) As bismuth is diamagnetic, therefore the field in the core would be slightly less than when the core is empty. (d) No, permeability of ferromagnetic material is not independent of magnetic field. As is clear from hysteresis curve, is greater for lower fields. (e) Magnetic field lines are always nearly normal to the surface of a ferromagnet at every point. The proof of this important fact is based on the boundary conditions of magnetic Â fields (B and H) at the interface of two media. (f) The maximum possible magnetisation of a paramagnetic sample can be of the same order of magnitude as the magnetisation of a ferromagnet. For this, a high magnetising fields for saturation will be required.

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