68 free MCQs on Magnetism and Matter with worked answers and explanations. Bar magnets, Earth's magnetism, and how different materials respond to an external magnetic field - diamagnetic, paramagnetic, and ferromagnetic behaviour.
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Magnetic field lines form closed loops: outside the bar magnet they run from the N pole to the S pole; inside, they continue from S back to N, so the lines never start or end anywhere.
Q1.
A magnetic pole that does not exist alone is called:
Answer: D. Poles always come in pairs (no monopoles)
Why: Magnetic monopoles do not exist. Every magnet always has a north AND south pole. Cut a magnet and two smaller magnets form.
Q2.
Earth has a magnetic field. Geographic north corresponds to:
Answer: B. Magnetic south pole
Why: Earth's geographic north pole is near a magnetic south pole (as compass north points toward it). By convention, the pole near geographic north is sometimes called the magnetic north geographic pole.
Q3.
Which of these elements is ferromagnetic at room temperature?
Answer: B. Iron
Why: Iron (along with cobalt and nickel) is ferromagnetic, showing strong attraction to an external field and retaining magnetism once the field is removed.
Q4.
What happens to a diamagnetic material when placed in an external magnetic field?
Answer: C. It is weakly repelled
Why: Diamagnetic materials (e.g., bismuth, copper, water) are weakly repelled by an external magnetic field, having a small negative magnetic susceptibility.
Q5.
The SI unit of magnetic moment is:
Answer: C. Ampere metre squared (A·m²)
Why: Magnetic moment m = NIA has SI unit ampere metre squared (A·m²). Tesla is the unit of B, weber is the unit of magnetic flux, and henry is the unit of inductance.
Q6.
What is the angle that the Earth's magnetic field makes with the horizontal called?
Answer: B. Inclination (dip)
Why: Magnetic inclination, or dip, is the angle the Earth's magnetic field makes with the horizontal plane. It is 0° at the magnetic equator and 90° at the magnetic poles.
Q7.
Above the Curie temperature, a ferromagnetic material becomes:
Answer: B. Paramagnetic
Why: Above the Curie temperature, thermal agitation disrupts the alignment of magnetic domains, and the material loses its ferromagnetic ordering, behaving as a simple paramagnetic substance.
Q8.
When a bar magnet is suspended freely and allowed to rotate, it eventually comes to rest pointing approximately:
Answer: D. Along the north-south direction
Why: A freely suspended bar magnet aligns itself roughly along the geographic north-south direction due to Earth's magnetic field, which is how early compasses worked.
Q9.
A material that gets weakly magnetised in a direction opposite to the applied magnetic field is called:
Answer: A. Diamagnetic
Why: Diamagnetic materials develop a weak magnetisation opposite to the applied field, causing them to be feebly repelled by magnets.
Q10.
The imaginary lines used to represent a magnetic field, drawn so that the tangent at any point gives the field direction, are called:
Answer: B. Magnetic field lines
Why: Magnetic field lines are continuous closed curves whose tangent at any point gives the direction of the magnetic field there, and they do not intersect each other.
Q11.
A freely suspended bar magnet finally comes to rest pointing in the:
Answer: A. north–south direction
Why: A bar magnet aligns itself along the Earth’s magnetic field, i.e. north–south.
Q12.
Like magnetic poles placed near each other:
Answer: A. repel each other
Why: Like poles (N–N or S–S) repel; unlike poles attract.
Q13.
Unlike magnetic poles placed near each other:
Answer: A. attract each other
Why: A north pole and a south pole attract each other.
Q14.
An isolated single magnetic pole (a magnetic monopole):
Answer: A. does not exist
Why: Magnetic poles always occur in north–south pairs; isolated monopoles have never been observed.
Q15.
The region around a magnet where its influence is felt is called the:
Answer: A. magnetic field
Why: The magnetic field is the region where a magnetic force acts on other magnets or currents.
Q16.
The SI unit of magnetic pole strength is the:
Answer: A. ampere-metre (A·m)
Why: Pole strength is measured in ampere-metre (A·m).
Q17.
The Earth as a whole behaves approximately like a giant:
Answer: A. bar magnet
Why: The Earth acts like a huge bar magnet tilted slightly from its rotation axis.
Q18.
Magnetic field lines outside a magnet emerge from its ___ pole:
Answer: A. north
Why: Field lines leave the north pole and enter the south pole outside the magnet.
Q19.
Materials that are strongly attracted by a magnet are called:
Answer: A. ferromagnetic
Why: Ferromagnetic materials such as iron are strongly attracted by magnets.
Q20.
When a bar magnet is cut into two pieces, each piece:
Answer: A. is a complete magnet
Why: Each fragment is again a complete magnet with its own north and south poles.
Q21.
How does the magnetic susceptibility of a paramagnetic substance change as temperature increases?
Answer: B. It decreases
Why: Paramagnetic susceptibility decreases as temperature increases, since greater thermal agitation disrupts the alignment of atomic magnetic dipoles with the external field. Diamagnetic susceptibility, by contrast, is essentially independent of temperature.
Q22.
A bar magnet of magnetic moment m is placed in a uniform external field B at angle theta to the field. What is the potential energy of the magnet?
Answer: C. U = -mB cos(theta)
Why: The potential energy of a magnetic dipole in a field is U = -m·B = -mB cos(theta), which is minimum (most stable) when the magnet is aligned with the field (theta = 0).
Q23.
Why does a superconductor behave differently from an ordinary diamagnetic material?
Answer: B. It completely expels magnetic field lines from its interior (Meissner effect), with susceptibility exactly -1
Why: Ordinary diamagnetism is weak (chi is about -10<sup>-5</sup>), but a superconductor exhibits perfect diamagnetism via the Meissner effect, completely expelling magnetic field lines with susceptibility exactly -1 - a qualitative jump, not just a stronger version of ordinary diamagnetism.
Q24.
Soft iron is preferred over steel for making transformer cores because soft iron has:
Answer: B. Low retentivity and a narrow hysteresis loop, minimising hysteresis energy loss per AC cycle
Why: Transformer cores undergo continuous magnetisation and demagnetisation as the AC current alternates, so a material with a narrow hysteresis loop (like soft iron) is used to minimise the energy lost as heat in each cycle. Materials with a wide hysteresis loop (like steel) are better suited for permanent magnets.
Q25.
A bar magnet is cut into two equal pieces along its axis (lengthwise, so each piece is thinner but the same length as the original). Compared to the original, the pole strength of each new piece is:
Answer: B. Roughly half of the original pole strength
Why: Cutting along the length divides the cross-sectional pole strength roughly in half while the length stays the same, so each piece ends up with about half the original pole strength.
Q26.
A bar magnet placed in a uniform magnetic field experiences zero net force but, in general, a non-zero torque. This happens because:
Answer: C. The two poles experience equal and opposite forces that form a couple unless the magnet is aligned with the field
Why: In a uniform field, the equal and opposite forces on the two poles cancel to give zero net force but, unless the dipole is aligned with the field, they act at different points and create a net torque.
Q27.
Curie's law states that the magnetic susceptibility of a paramagnetic material is:
Answer: D. Inversely proportional to the absolute temperature
Why: Curie's law gives χ = C/T, where C is the Curie constant, so paramagnetic susceptibility decreases as temperature rises.
Q28.
A magnetic compass shows zero dip at a certain location. This location is most likely close to:
Answer: A. The magnetic equator
Why: At the magnetic equator, Earth's field is purely horizontal, so a freely suspended needle shows zero angle of dip.
Q29.
A hysteresis loop for a magnetic material shows the area enclosed by the B-H curve during one complete cycle of magnetisation and demagnetisation. This enclosed area represents:
Answer: B. Energy dissipated as heat per unit volume of the material in one cycle
Why: The area of the hysteresis loop equals the energy lost as heat per unit volume of the material during one cycle of magnetisation, which is why transformer cores use materials with a small loop area.
Q30.
A bar magnet of magnetic moment m and a solenoid of the same size and magnetic moment are both suspended freely near each other. Compared to the bar magnet, the solenoid:
Answer: C. Produces essentially the same magnetic field pattern outside, since both behave as equivalent magnetic dipoles
Why: A current-carrying solenoid behaves as a magnetic dipole equivalent to a bar magnet of the same magnetic moment, producing essentially the same external field pattern, including a similar dipole field along its axis.
Q31.
The magnetic dipole moment of a bar magnet points from its:
Answer: A. south pole to north pole
Why: By convention the dipole moment points from the south pole to the north pole inside the magnet.
Q32.
Diamagnetic materials are ___ by a magnetic field:
Answer: A. weakly repelled
Why: Diamagnetic substances are weakly repelled by a magnetic field.
Q33.
Paramagnetic materials are ___ by a magnetic field:
Answer: A. weakly attracted
Why: Paramagnetic substances are weakly attracted toward a magnetic field.
Q34.
The magnetic susceptibility of a diamagnetic material is:
Answer: A. small and negative
Why: Diamagnets have a small negative susceptibility.
Q35.
The magnetic susceptibility of a ferromagnetic material is:
Answer: A. large and positive
Why: Ferromagnets have a very large positive susceptibility.
Q36.
Which of the following is a ferromagnetic material?
Answer: A. iron
Why: Iron (also cobalt and nickel) is ferromagnetic; copper and bismuth are diamagnetic.
Q37.
The torque on a magnetic dipole of moment m in a uniform field B is:
Answer: A. mB sinθ
Why: τ = m × B = mB sinθ, where θ is the angle between the dipole and the field.
Q38.
The relative permeability of a ferromagnetic material is:
Answer: A. much greater than 1
Why: Ferromagnets have relative permeability far greater than 1 (hundreds to thousands).
Q39.
The angle of dip is 90° at the Earth’s:
Answer: A. magnetic poles
Why: At the magnetic poles the field is vertical, so the angle of dip is 90°.
Q40.
The angle of dip is 0° at the Earth’s:
Answer: A. magnetic equator
Why: At the magnetic equator the field is horizontal, so the dip is 0°.
Q41.
In MRI machines, strong magnetic fields are used. A proton's Larmor frequency in 3 T field:
Answer: C. 127.7 MHz
Why: Larmor frequency f = gamma x B/(2pi) where gamma_H = 2.675 x 10<sup>8</sup> rad/(T s). f = 2.675x10<sup>8</sup> x 3/(2pi) = 127.7 MHz.
Q42.
Magnetic susceptibility of diamagnetic materials is:
Answer: C. Small and negative
Why: Diamagnetic materials: small negative susceptibility. They are weakly repelled by magnets (e.g., gold, water, bismuth).
Q43.
A ferromagnetic material loses its magnetisation as soon as a small external field is removed, while another retains strong magnetisation. What do these two materials represent, and why does only one retain magnetism?
Answer: A. Soft ferromagnet (low retentivity, narrow hysteresis loop) vs hard ferromagnet (high retentivity, wide hysteresis loop) - the difference lies in how strongly domain alignment persists without an external field
Why: Within ferromagnetic materials, 'soft' ferromagnets (e.g., soft iron) have low retentivity and a narrow hysteresis loop, losing magnetisation quickly once the field is removed - ideal for transformer cores. 'Hard' ferromagnets (e.g., steel, alnico) have high retentivity and a wide hysteresis loop, retaining strong magnetisation - ideal for permanent magnets. The difference comes from how strongly the domain structure resists realignment once formed.
Q44.
A short bar magnet of magnetic moment 0.5 J/T is placed at 30° with a uniform external magnetic field of 0.2 T. The torque acting on the magnet is approximately:
Answer: A. 0.05 N·m
Why: Torque τ = mB sin(θ) = 0.5 × 0.2 × sin(30°) = 0.1 × 0.5 = 0.05 N·m.
Q45.
A bar magnet has magnetic moment m and length 2l. It is cut into two equal halves with a cut perpendicular to its length, so each piece is half as long. The magnetic moment of each new piece is:
Answer: B. m/2
Why: Cutting perpendicular to the axis halves the length while the pole strength stays the same, so the new magnetic moment of each piece is half the original moment.
Q46.
A magnetic needle free to rotate in a vertical plane oriented along the magnetic meridian dips at an angle of 60° at a place where the horizontal component of Earth's field is 0.3 × 10⁻⁴ T. The vertical component of Earth's magnetic field at that place is approximately:
Answer: C. 0.52 × 10⁻⁴ T
Why: tan(dip) = Bv/Bh, so Bv = Bh tan(60°) = 0.3 × 10⁻⁴ × 1.732 ≈ 0.52 × 10⁻⁴ T.
Q47.
A solenoid with a ferromagnetic core has a magnetising field H = 1500 A/m, producing a magnetic field B = 2.4 T inside the core. The relative permeability of the core material is approximately:
Answer: D. 1273
Why: Relative permeability μr = B/(μ0 H) = 2.4/(4π×10⁻⁷ × 1500) ≈ 2.4/(1.885×10⁻³) ≈ 1273, showing the material is strongly ferromagnetic.
Q48.
Two identical magnetic dipoles of moment m each are placed at a separation r, one in the end-on (axial) position and one in the broadside-on (equatorial) position relative to a reference point at the same distance r. The ratio of the magnetic field at that point due to the axial arrangement to the field due to the equatorial arrangement is:
Answer: A. 2:1
Why: For a short dipole, the axial field is B<sub>axial</sub> = (μ0/4π)(2m/r³) while the equatorial field is B<sub>equatorial</sub> = (μ0/4π)(m/r³), giving a ratio of 2:1.
Q49.
A toroid with a ferromagnetic core has 1000 turns per metre and carries a current of 2 A. If the core has a magnetic susceptibility of 599, the magnetisation of the core material is approximately:
Answer: B. 1.198 × 10⁶ A/m
Why: H = nI = 1000 × 2 = 2000 A/m, and magnetisation M = χH = 599 × 2000 ≈ 1.198 × 10⁶ A/m, illustrating the very large magnetisation typical of ferromagnetic cores.
Q50.
A compass needle is placed at a point where the resultant magnetic field has equal horizontal and vertical components. The angle of dip at that point is closest to:
Answer: C. 45°
Why: Since tan(dip) = Bv/Bh and the two components are equal here, tan(dip) = 1, giving a dip angle of 45 degrees.
Q51.
Above its Curie temperature, a ferromagnetic material becomes:
Answer: A. paramagnetic
Why: Thermal agitation destroys the aligned domains, so the material turns paramagnetic above the Curie point.
Q52.
The potential energy of a magnetic dipole in a field is minimum when the dipole is aligned ___ the field:
Answer: A. parallel to
Why: U = −mB cosθ is minimum (most negative) at θ = 0°, i.e. parallel alignment.
Q53.
Curie’s law states that the susceptibility of a paramagnet is ___ the absolute temperature:
Answer: A. inversely proportional to
Why: χ ∝ 1/T, so paramagnetic susceptibility falls as temperature rises.
Q54.
The three elements of the Earth’s magnetism are declination, the angle of dip and the ___ component of the field:
Answer: A. horizontal
Why: The horizontal component, along with declination and dip, fully specifies the Earth’s field at a place.
Q55.
A material that retains its magnetism after the external field is removed is:
Answer: A. hard magnetic, like steel
Why: Hard magnetic materials (e.g. steel) retain magnetism and are used for permanent magnets.
Q56.
Soft iron is preferred for electromagnet cores because it is:
Answer: A. easily magnetised and demagnetised
Why: Soft iron magnetises and demagnetises readily, ideal for temporary electromagnets.
Q57.
The magnetic field on the axis of a short bar magnet falls off with distance r as:
Answer: A. 1/r³
Why: For a magnetic dipole the axial field varies as 1/r³.
Q58.
The angle between the geographic meridian and the magnetic meridian is called the angle of:
Answer: A. declination
Why: Declination is the horizontal angle between true (geographic) north and magnetic north.
Q59.
Hysteresis is the lagging of the ___ behind the magnetising field:
Answer: A. magnetisation
Why: In a hysteresis loop, the magnetisation lags the applied field, and the loop area represents energy lost per cycle.
Q60.
The magnetic moment of both a current loop and a bar magnet is measured in:
Answer: A. A·m²
Why: Magnetic dipole moment has units of ampere·metre² (A·m²).
Q61.
A bar magnet of magnetic moment m is cut into two equal pieces along its length (perpendicular to its axis). The magnetic moment of each piece is:
Answer: B. m/2
Why: Cutting perpendicular to the axis halves the length (and pole strength stays), so each moment is m/2.
Q62.
Diamagnetic materials are characterized by a magnetic susceptibility that is:
Answer: A. small and negative
Why: Diamagnets have small negative susceptibility and are weakly repelled by magnetic fields.
Q63.
When a ferromagnetic material is heated above its Curie temperature, it becomes:
Answer: A. paramagnetic
Why: Above the Curie temperature thermal agitation destroys domain alignment, and the material turns paramagnetic.
Q64.
The angle of dip at the magnetic equator is:
Answer: A. 0°
Why: At the magnetic equator the Earth field is horizontal, so the dip angle is 0°.
Q65.
A material has magnetic susceptibility 0.5. Its relative permeability is:
Answer: C. 1.5
Why: μ_r = 1 + χ = 1 + 0.5 = 1.5.
Q66.
A bar magnet in a uniform magnetic field experiences maximum torque when the angle between its moment and the field is:
Answer: C. 90°
Why: Torque = mB sinθ is maximum at θ = 90°.
Q67.
In a vibration magnetometer, the time period is T = 2π√(I/mB). If the magnetic field is made four times stronger, the period:
Answer: A. halves
Why: T ∝ 1/√B, so 4× field gives T/2 - it halves.
Q68.
The magnetic moment of a flat current-carrying coil of N turns, current I, and area A is:
Answer: A. NIA
Why: Magnetic moment m = NIA, directed normal to the coil plane.