NEET Magnetic Properties of Matter MCQs & PYQs

According to Curie's Law, the magnetic susceptibility \(\chi\) of a paramagnetic material is inversely proportional to its absolute temperature \(T\) (i.e., \(\chi \propto \frac{1}{T})\). Thus, cooling (decreasing \(T\)) increases magnetisation. Both assertion and reason are true, and the reason correctly explains the assertion.

For a permanent magnet, the material must have high retentivity and high coercivity to retain magnetism. Ferromagnetic materials do exhibit positive intensity of magnetisation, which is true, but this general property is not the direct reason for choosing high retentivity for permanent magnets. Both A and R are true, but R is not the correct explanation of A.

For a perfectly diamagnetic substance (e.g., superconductor), its relative magnetic permeability \(mu_r\) is 0, not 1. Diamagnetic substances are weakly repelled by magnetic fields, not attracted. Therefore, both Assertion (A) and Reason (R) are false.

The magnetic properties of paramagnetic and ferromagnetic substances are strongly affected by temperature. Paramagnetic susceptibility is temperature-dependent (Curie's Law), and ferromagnets become paramagnets above their Curie temperature. Thermal agitation due to heating disorients molecular magnets. Both Assertion (A) and Reason (R) are false.

Soft iron is preferred for transformer cores because its narrow hysteresis loop indicates low energy loss (heat) during repeated cycles of magnetisation and demagnetisation. This property is crucial for efficient transformer operation. Both Assertion and Reason are true, and Reason is the correct explanation of the Assertion.

Ferromagnetic and paramagnetic materials indeed lose their specific magnetic properties upon strong heating (e.g., ferromagnets become paramagnetic above Curie temperature). So, Assertion (A) is true. However, Reason (R) is false; while diamagnetism is a universal property, most substances exhibit paramagnetism (due to unpaired electrons) or ferromagnetism, which mask diamagnetism.

Intensity of magnetisation \(I = \frac{M}{V}\). Volume \(V = 0.10 \times 0.05 \times 0.04 = 2 \times 10^{-4} \text{m}^3\). So, \(I = \frac{2.5}{2 \times 10^{-4}} = 1.25 \times 10^4 \text{A/m}\).

An electromagnet should become strongly magnetized when current is on, and lose its magnetization quickly when current is off. Therefore, it requires high retentivity and low coercivity.

Statement (A) is incorrect because elementary particles like electrons have intrinsic spin magnetic moments not due to actual circulating currents. Statement (B) is correct as \(\mu_l = 1 \mu_B\) for ground state.

In stable equilibrium, the magnetic moment vector aligns parallel to the magnetic field direction. Consequently, the plane of the coil becomes perpendicular to the magnetic field.