NEET Practice Questions (MCQs) with Answers & Solutions

According to the NCERT text, 'Magnetic monopoles do not exist. If you slice a magnet in half, you get two smaller magnets.' Also, 'Unlike electric charges, isolated magnetic north and south poles known as magnetic monopoles do not exist.' This clearly states that magnetic monopoles are not observed in nature.

The NCERT text states: 'Substances, which at room temperature, retain their ferromagnetic property for a long period of time are called permanent magnets.' This differentiates them from other magnetic materials whose magnetic properties are temporary.

According to the NCERT summary: 'When a bar magnet of dipole moment $\vec{m}$ is placed in a uniform magnetic field $\vec{B}$, (a) the force on it is zero, (b) the torque on it is $\vec{m} \times \vec{B}$.' In a uniform magnetic field, the net force on a magnetic dipole (like a bar magnet) is zero, but it experiences a torque aiming to align it with the field.

The NCERT text states: 'The magnetic potential energy $U_m$ is given by $U = -\vec{m} \cdot \vec{B}$, where we choose the zero of energy at the orientation when $\vec{m}$ is perpendicular to $\vec{B}$.'

From the 'POINTS TO PONDER' table under 'Physical quantity Symbol Nature Dimensions Units Remarks', the magnetic moment $\vec{m}$ has dimensions $\text{[L}^{2}\text{A]}$. Its unit is A m$^2$.

The NCERT 'POINTS TO PONDER' states: 'A consequence of the fact that magnetic monopoles do not exist is that the magnetic field lines are continuous and form closed loops. In contrast, the electrostatic lines of force begin on a positive charge and terminate on the negative charge (or fade out at infinity).'

The NCERT text states: 'For diamagnetic materials $\chi$ is negative and small and for paramagnetic materials it is positive and small.' Also, under 'Points to Ponder', it specifies 'For diamagnetic materials $\chi = -10^{-5}$ whereas $\chi = +10^{-5}$ for paramagnetic materials.'

The NCERT 'POINTS TO PONDER' states: 'There exists a perfect diamagnet, namely, a superconductor. This is a metal at very low temperatures. In this case $\chi = -1$, $\mu_r = 0$, $\mu = 0$. The external magnetic field is totally expelled.'

According to the NCERT summary: 'The three quantities, $\chi$, the relative magnetic permeability $\mu_r$, and the magnetic permeability $\mu$ are related as follows: $\mu = \mu_0 \mu_r$, $\mu_r = 1 + \chi$. Therefore, $\mu = \mu_0 (1 + \chi)$.

The NCERT text states: 'The resemblance of magnetic field lines for a bar magnet and a solenoid suggest that a bar magnet may be thought of as a large number of circulating currents in analogy with a solenoid.' It further notes that a bar magnet and a solenoid produce similar magnetic fields, and one can verify this by observing compass needle deflections.