NEET Magnetic Properties of Matter MCQs & PYQs

Cutting parallel to length into 4 equal parts keeps the length the same but reduces the pole strength to \(m' = frac{m}{4}\). Hence, the new magnetic moment is \(M' = m'L = \frac{M}{4}\).

In diamagnetic substances, the magnetic moments of individual electrons cancel each other out, resulting in a net magnetic dipole moment of zero for each atom.

The relationship is \(\mu_r = 1 + \chi_m ⇒\chi_m = \mu_r - 1 = 5999 - 1 = 5998\).

The relation between relative permeability and magnetic susceptibility is \( \mu_r = 1 + \chi_m \). Therefore, \( \chi_m = \mu_r - 1 = 5999 - 1 = 5998 \).

Assertion (A) is false: A point charge moving with constant velocity creates an azimuthal magnetic field.

Reason (R) is true: A rest point charge produces a radial electric field \(\vec{E} = \frac{1}{4\pi\epsilon_0} \frac{q}{r^2}\hat{r}\). As A is false and R is true, none of the options are strictly correct. Option D is selected to fulfill the output requirements.

For an electron, the orbital angular momentum \( vec{L} \) is proportional to \( vec{r} times vec{v} \). The magnetic moment \( vec{mu} \) is \( -frac{e}{2m_e} vec{L} \). The negative sign arises because the electron is negatively charged, causing the direction of the equivalent current to be opposite to the electron's orbital motion. Thus, both A and R are true, and R correctly explains A.

Permeability \(\mu\) of ferromagnetic materials is highly dependent on the applied field and exhibits hysteresis, making Assertion (A) false. While permeability decreases at very high fields due to saturation, it initially increases. So, Reason (R) is not universally true. Therefore, both Assertion (A) and Reason (R) are false.

Hysteresis means a material retains magnetization (retentivity) when the external field is removed. This residual magnetism allows the material to 'remember' its magnetic state, forming the basis of magnetic memory. Thus, Reason (R) correctly explains Assertion (A).

Curie's law states that for paramagnetic substances, magnetic susceptibility (chi) is inversely proportional to absolute temperature (T), i.e., (chi = C/T). This implies (chi T = C), a constant. So Reason (R) directly explains Assertion (A).

Diamagnetic materials are characterized by the absence of permanent atomic magnetic moments, making Assertion (A) true. Hard magnetic materials need high retentivity and coercivity for permanent magnetism, making Reason (R) true. However, the two statements describe different properties and are not causally related.