NEET Magnetism and Matter Previous Year Questions with Solutions

The magnetic needle performs 20 complete oscillations in 5 seconds. The time period T for one complete cycle is:

In a uniform horizontal magnetic field B, a magnetic needle of dipole moment M and moment of inertia I executes angular simple harmonic motion with time period:

Isolating M from the time period equation:

Substitute I = 9.8 × 10⁻⁶ kg m², B = 0.049 T, and T = 1/4 s:

Simplifying the numerical coefficients:

Comparing with M = x × 10⁻⁵ A m², we get x = 1280π², which corresponds to Option (2).

Magnetic susceptibility χ is a dimensionless proportionality constant that indicates the degree of magnetization M of a material in response to an applied magnetic field H:

Diamagnetic materials develop magnetization opposite to the applied field. Their susceptibility is negative and small, with superconductor limit χ = -1 (ideal diamagnetism):

Ferromagnetic materials possess spontaneous domain alignment and magnetize strongly in the direction of the applied field. Their susceptibility is positive and very large (typically 1000 to 100,000):

Paramagnetic materials possess permanent atomic dipole moments that weakly align with the applied field. Their susceptibility is positive, small, and temperature-dependent:

A completely non-magnetic substance (or vacuum) cannot be magnetized by an applied magnetic field, so its magnetization M = 0 and susceptibility is identically zero:

A magnetic sheet (such as iron or ferromagnetic alloy) in the non-uniform magnetic field of a strong magnetic pole experiences a strong net attractive force directed toward the pole. To hold the sheet at rest, an external restraining force must be applied in the opposite direction to balance this magnetic attraction:

A non-magnetic sheet experiences no static magnetic force in the field. In the absence of friction on the horizontal surface, no external force is needed to keep it stationary:

When a conducting sheet is moved away from the magnetic pole, the magnetic flux penetrating the sheet decreases with time (dΦ_B/dt ≠ 0). By Faraday’s law of induction and Lenz’s law, circulating eddy currents are induced in the metallic sheet. These eddy currents interact with the magnetic field to produce an opposing magnetic Lorentz drag force (electromagnetic damping) opposing the motion. To maintain a uniform velocity (zero net acceleration), an external force must be continuously exerted to overcome this eddy current drag:

In a non-conducting and non-polar sheet, free charge carriers are absent, so no eddy currents can flow. Furthermore, absence of permanent or induced dipole moments means no electromagnetic retarding force acts. By Newton’s first law, the sheet moves at uniform velocity without any external force:

Statements A and C are correct, which corresponds directly to Option (4).

For a straight iron bar magnet of length L with pole strengths -m (South pole) and +m (North pole), the magnetic dipole moment is defined as the product of pole strength and magnetic length:

When the bar is bent at its midpoint, it forms two straight segments of equal length l_1 = l_2 = L/2 meeting at an angle θ = 60°. Bending the magnet does not alter the intrinsic pole strength m at the ends.

The effective magnetic dipole moment depends on the direct straight-line distance L’ between the South pole and the North pole at the ends of the bent arms. Applying the law of cosines or isosceles geometry:

The new magnetic dipole moment M’ is the product of pole strength m and effective separation L’:

Gauss's law for magnetism is one of the four fundamental Maxwell equations governing classical electrodynamics. It states that the net magnetic flux Φ_B through any closed surface S is identically zero:

In electrostatics, Gauss's law allows net flux because isolated positive and negative charges exist (∮ E · dA = q_enclosed / ε₀). In magnetism, magnetic poles always occur in equal and opposite dipoles (North and South). The net magnetic charge enclosed by any closed volume is always zero:

Magnetic field lines form continuous closed loops without beginning or end. Every field line that enters a closed surface must necessarily emerge from it, ensuring total incoming flux equals total outgoing flux.

Magnetic susceptibility $$\chi_m$$ indicates how easily a substance magnetizes in an applied magnetic field. Relative magnetic permeability $$\mu_r$$ is related to susceptibility by: $$\mu_r = 1 + \chi_m$$.

Given $$\chi_m = 599$$: $$\mu_r = 1 + 599 = 600$$.

Absolute permeability $$\mu$$ is given by: $$\mu = \mu_r \mu_0$$ Substitute $$\mu_r = 600$$ and $$\mu_0 = 4\pi \times 10^{-7}\text{ T m A}^{-1}$$ $$\mu = 600 \times (4\pi \times 10^{-7}) = 2400\pi \times 10^{-7} = 2.4\pi \times 10^{-4}\text{ T m A}^{-1}$$.

The problem statement specifies $$H = 1200\text{ A m}^{-1}$$. Because permeability is an intrinsic material property in the linear regime defined by $$\mu = \mu_0(1+\chi)$$, $$H$$ is superfluous data. Matches Option 1.