Magnetism and Matter — Class 12 MCQs with Answers

Magnetism and Matter — important questions & MCQs with answers (Class 12 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Magnetic monopoles:

   AAre commonBHave not been observedCExist in ironDAre positive only
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   Answer: Have not been observed

   Explanation: Isolated N or S poles have never been observed — every magnet, however small, has both poles, which is why Gauss's law for magnetism gives ∮B·dA = 0, not μ₀q_m.

2. Q2 of 6Easy

   Iron, nickel, cobalt are:

   AFerromagneticBDiamagneticCParamagneticDNon-magnetic
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   Answer: Ferromagnetic

   Explanation: Fe, Ni, Co have unpaired electron spins that align into domains under a field, giving strong magnetisation (χ ~ 10³) — unlike paramagnets, this alignment survives as domains even without a field.

3. Q3 of 6Medium

   Earth's magnetic dipole has its 'N' approximately at:

   AGeographic North poleBGeographic South poleCEquatorDCentre
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   Answer: Geographic South pole

   Explanation: Since opposite poles attract, a compass's north tip pointing toward geographic north means Earth's own dipole south pole sits near geographic north — so the dipole's north pole is near geographic south.

4. Q4 of 6Medium

   Assertion (A): A ferromagnetic material loses its ferromagnetic property when it is heated to a high enough temperature. Reason (R): On being heated to a high enough temperature, a ferromagnetic material becomes diamagnetic.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: A is true but R is false

   Explanation: A is true: as the temperature rises, the domain structure breaks up and the magnetisation gradually disappears. R is false: at high enough temperature a ferromagnet becomes paramagnetic, not diamagnetic.

5. Q5 of 6Hard

   Torque on magnetic dipole m in field B:

   AmB²Bm·BCm/BDm × B
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   Answer: m × B

   Explanation: Torque on a magnetic dipole is the vector cross product τ = m × B, magnitude mB sinθ, directed to align m with B — not the dot product m·B, which instead gives potential energy.

6. Q6 of 6Hard

   Assertion (A): Filling the inside of a current-carrying solenoid with a paramagnetic material makes the magnetic field inside slightly stronger. Reason (R): In a paramagnetic material, the magnetisation M is in the same direction as the magnetic intensity H.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: A is true and R is true. Since B = μ₀(H + M) and M is along H for a paramagnet (χ small and positive), the field inside becomes slightly larger than μ₀H. R is the reason for A.

Magnetism and Matter notes: key concepts (Class 12 Physics)

Magnetism as a property of matter: the bar magnet and its equivalence to a solenoid, the absence of magnetic monopoles, torque and energy of a magnetic dipole, the elements of Earth's magnetic field including declination and dip, and the three classes of magnetic material with the hysteresis loop of a ferromagnet.

Bar magnet and field lines

Magnetic field lines form closed loops running from north to south outside the magnet and from south to north INSIDE it — unlike electric field lines, which begin and end on charges.

No magnetic monopoles

Isolated poles have never been observed; breaking a magnet in half yields two complete magnets. This is why the net magnetic flux through any closed surface is zero.

Magnetic dipole moment

The strength of a magnet or current loop, NIA for a loop, measured in A m². It points from the south pole to the north pole of the magnet.

Bar magnet as a solenoid

A bar magnet produces the same external field pattern as a current-carrying solenoid, which is how the magnetic moment of a magnet is defined at all.

Field of a bar magnet

The axial field is twice the equatorial field at equal distance, and both fall off as the cube of distance — exactly parallel to the electric dipole.

Torque on a magnetic dipole

A uniform field exerts a torque mB sin θ that aligns the dipole with the field, which is why a compass needle swings to point north.

Energy of a magnetic dipole

Equal to −m·B, minimum when the dipole is aligned with the field and maximum when it is anti-parallel.

Earth's magnetism

Earth behaves as a giant dipole whose magnetic south pole lies near the geographic north — which is why a compass needle's north end points north.

Magnetic declination

The angle between the geographic and magnetic meridians at a place, i.e. the correction between true north and compass north.

Angle of dip

The angle the total field makes with the horizontal. It is 0° at the magnetic equator and 90° at the magnetic poles.

Components of Earth's field

The total field resolves into horizontal and vertical components, whose ratio gives the tangent of the dip angle.

Magnetisation and susceptibility

Magnetisation is the net dipole moment per unit volume, and susceptibility is the magnetisation produced per unit magnetising field.

Magnetic permeability

Measures how readily a material carries magnetic flux. Relative permeability is one plus the susceptibility.

Diamagnetic materials

Weakly REPELLED by a magnetic field, with small negative susceptibility. Bismuth, copper and water are examples; superconductors are perfect diamagnets.

Paramagnetic materials

Weakly attracted, with small positive susceptibility that falls as temperature rises according to Curie's law. Aluminium and oxygen are examples.

Ferromagnetic materials

Strongly attracted, with very large susceptibility and permanent magnetisation from aligned domains. They show hysteresis and lose ferromagnetism above the Curie temperature.