Magnetic Materials and its Properties – Para: NEET MCQs with Answers

15 free questions on Magnetic Materials and its Properties – Para for NEET (UG), starting with previous-year NEET and AIPMT questions. Try each one, then tap Show answer for the answer and a short explanation.

Syllabus covered: Para-, dia- and ferromagnetic substances with examples, effect of temperature on magnetic properties.

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Question 1

Previous-year question
An iron rod of susceptibility 599 is subjected to a magnetising field of 1200 A m⁻¹. The permeability of the material of the rod is (μ₀ = 4π × 10⁻⁷ T m A⁻¹) [NEET 2020]
  1. 8.0 × 10⁻⁵ T m A⁻¹
  2. 2.4π × 10⁻⁵ T m A⁻¹
  3. 2.4π × 10⁻⁷ T m A⁻¹
  4. 2.4π × 10⁻⁴ T m A⁻¹
Show answer
Answer: (D) 2.4π × 10⁻⁴ T m A⁻¹
Explanation: μ = μ₀(1 + χ) = 4π × 10⁻⁷ × 600 = 2.4π × 10⁻⁴ T m A⁻¹. The value of the magnetising field is not needed.

Question 2

Previous-year question
A thin diamagnetic rod is placed vertically between the poles of an electromagnet. When the current in the electromagnet is switched on, the diamagnetic rod is pushed up, out of the horizontal magnetic field. Hence the rod gains gravitational potential energy. The work required to do this comes from [NEET 2018]
  1. the current source
  2. the magnetic field
  3. the induced electric field due to the changing magnetic field
  4. the lattice structure of the material of the rod
Show answer
Answer: (A) the current source
Explanation: A magnetic field does no work by itself. The energy is drawn from the source driving the electromagnet, through the induced emf that opposes the change in flux.

Question 3

Previous-year question
The magnetic susceptibility is negative for [NEET 2016]
  1. diamagnetic material only
  2. paramagnetic material only
  3. ferromagnetic material only
  4. paramagnetic and ferromagnetic materials
Show answer
Answer: (A) diamagnetic material only
Explanation: Only diamagnetic substances are magnetised opposite to the applied field, which is what a negative susceptibility means; para- and ferromagnets both have positive values.

Question 4

An unmagnetised piece of a ferromagnetic material is placed in an external magnetic field B₀. What happens inside the material as a result?
  1. Domains orient along B₀ and those along B₀ grow, until the aligned domains merge into a single giant domain
  2. The domains break up into single atoms, each of which then aligns independently of its neighbours
  3. Its atoms acquire magnetic moments for the first time, having had none before the field was applied
  4. The domains stay fixed in size, and the magnetisation of each domain goes on reversing at random
Show answer
Answer: (A) Domains orient along B₀ and those along B₀ grow, until the aligned domains merge into a single giant domain
Explanation: The atoms already possess dipole moments, and these align spontaneously over a macroscopic volume called a domain, typically about 1 mm across and containing about 10¹¹ atoms. Before the field is applied, the magnetisation varies randomly from domain to domain, so there is no bulk magnetisation. In the field B₀ the domains orient along it, and those already along it grow in size, finally amalgamating into one giant domain.

Question 5

Which one of these groups consists only of paramagnetic materials (gases taken at STP)?
  1. Copper, lead, nitrogen
  2. Aluminium, sodium, copper chloride
  3. Gadolinium, oxygen, silicon
  4. Bismuth, calcium, water
Show answer
Answer: (B) Aluminium, sodium, copper chloride
Explanation: Aluminium, sodium, calcium, oxygen at STP and copper chloride are paramagnetic. Bismuth, copper, lead, silicon, nitrogen at STP, water and sodium chloride are diamagnetic. Gadolinium is ferromagnetic, like iron, cobalt and nickel.

Question 6

A compass needle must keep its magnetisation for a long time after the magnetising field is removed, while a rod in a certain device must lose its magnetisation as soon as the field is removed. Which choice of materials suits the needle and the rod, in that order?
  1. Alnico for both, since every ferromagnet behaves alike
  2. Alnico for the needle; soft iron for the rod
  3. Soft iron for the needle; Alnico for the rod
  4. Aluminium for the needle; bismuth for the rod
Show answer
Answer: (B) Alnico for the needle; soft iron for the rod
Explanation: Ferromagnets in which the magnetisation persists after the field is removed are called hard ferromagnets; Alnico and lodestone are examples, and they form permanent magnets such as compass needles. In soft ferromagnets such as soft iron, the magnetisation disappears when the external field is removed. Aluminium and bismuth are only weakly magnetic and cannot serve either purpose.

Question 7

For a magnetic material, the magnetisation M and the magnetic intensity H are related by M = χH. Which statement about their units is correct?
  1. H is in tesla and M is in A m⁻¹, so χ is in A m⁻¹ T⁻¹
  2. H is in A m⁻¹ and M is in A m², so χ is in m³
  3. H and M are both in tesla, so χ is dimensionless
  4. H and M are both in A m⁻¹, so χ is dimensionless
Show answer
Answer: (D) H and M are both in A m⁻¹, so χ is dimensionless
Explanation: Magnetisation is net magnetic moment per unit volume, so its unit is A m²/m³ = A m⁻¹. Magnetic intensity H has the same unit and dimensions, since B = μ₀(H + M) requires the two to add. The ratio χ = M/H is therefore a pure number.
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Question 8

A ferromagnetic material has a Curie temperature of 1000 K. At 1100 K its magnetic susceptibility is 8 × 10⁻³. Its susceptibility at 1400 K is closest to:
  1. 4.0 × 10⁻³
  2. 2.0 × 10⁻³
  3. 3.2 × 10⁻²
  4. 6.3 × 10⁻³
Show answer
Answer: (B) 2.0 × 10⁻³
Explanation: Above the Curie temperature the material is paramagnetic and χ = C/(T − Tc). The ratio is χ₂/χ₁ = (1100 − 1000)/(1400 − 1000) = 1/4, so χ₂ = 2.0 × 10⁻³. Using the simple 1/T form would wrongly ignore the Curie temperature in the denominator.

Question 9

In an atom of a diamagnetic substance the orbital magnetic moments of the electrons cancel, giving zero net moment. How does the substance acquire a small magnetisation in an applied magnetic field?
  1. Electrons with orbital moments along the field slow down and those with moments against it speed up, leaving a net moment opposite to the field
  2. All orbiting electrons speed up equally, leaving a net moment along the field
  3. Permanent atomic moments, initially random, turn to point opposite to the field
  4. Domains whose moments oppose the field grow at the expense of the other domains
Show answer
Answer: (A) Electrons with orbital moments along the field slow down and those with moments against it speed up, leaving a net moment opposite to the field
Explanation: Switching on the field induces changes in the orbital motion in accordance with Lenz's law. Electrons whose orbital moments lie along the field slow down, while those with moments against it speed up. The balance is upset and a small net moment appears opposite to the field. No permanent atomic moments or domains are involved, which is why this effect does not depend on temperature.

Question 10

When a metal is cooled into its superconducting state and placed in a weak magnetic field, the field lines are completely expelled from its interior (the Meissner effect). What are the magnetic susceptibility χ and the relative permeability μᵣ of the material in this state?
  1. χ = −1, μᵣ = 1
  2. χ = −1, μᵣ = 0
  3. χ = 0, μᵣ = 1
  4. χ = +1, μᵣ = 2
Show answer
Answer: (B) χ = −1, μᵣ = 0
Explanation: Zero field inside means B = μ₀μᵣH = 0 for a non-zero H, so μᵣ = 0. Since μᵣ = 1 + χ, the susceptibility is χ = −1. This is perfect diamagnetism, the extreme case of the small negative χ of ordinary diamagnets, and it explains why a superconductor repels a magnet.

Question 11

Certain metals cooled to very low temperatures show perfect conductivity and also expel magnetic field lines completely from their interior. For such a superconductor, what are the magnetic susceptibility χ and relative permeability μᵣ, and what is the expulsion of the field called?
  1. χ = 0, μᵣ = 1; Meissner effect
  2. χ = −1, μᵣ = 0; Meissner effect
  3. χ = −1, μᵣ = 0; Curie effect
  4. χ = +1, μᵣ = 2; Meissner effect
Show answer
Answer: (B) χ = −1, μᵣ = 0; Meissner effect
Explanation: Inside the material the field B = μ₀(1 + χ)H must be zero, which requires χ = −1. Since μᵣ = 1 + χ, the relative permeability is 0. Such a material is a perfect diamagnet, and the complete expulsion of the field is called the Meissner effect.

Question 12

In the simple picture of a diamagnetic atom, two electrons revolve in opposite senses at the same speed, so their orbital magnetic moments cancel. What happens when an external magnetic field B is applied?
  1. Both electrons slow down equally, so the net moment of the atom stays zero
  2. Both orbits turn over so that both moments lie along B, giving a large net moment
  3. The electron whose moment is along B slows down and the other speeds up, giving a small net moment opposite to B
  4. The electron whose moment is along B speeds up and the other slows down, giving a small net moment along B
Show answer
Answer: (C) The electron whose moment is along B slows down and the other speeds up, giving a small net moment opposite to B
Explanation: The applied field acts on the orbiting electrons as Lenz's law requires, so the induced change opposes the field. The electron with its moment along B slows down and the one with its moment against B speeds up. The two moments no longer cancel, and a small net moment appears opposite to B. This is why a diamagnetic sample is weakly repelled from a magnet.

Question 13

A long solenoid carries a fixed current and its core is completely filled, in turn, with iron, aluminium and water. It is also operated with vacuum inside. In a weak field at room temperature, what is the correct increasing order of the magnetic field inside the solenoid?
  1. water < vacuum < aluminium < iron
  2. water < aluminium < vacuum < iron
  3. aluminium < vacuum < water < iron
  4. vacuum < water < aluminium < iron
Show answer
Answer: (A) water < vacuum < aluminium < iron
Explanation: The field inside is B = μ₀μᵣnI, so it follows the relative permeability. Water is diamagnetic with μᵣ slightly below 1, while aluminium is paramagnetic with μᵣ slightly above 1. Vacuum has μᵣ = 1, and iron is ferromagnetic with μᵣ far greater than 1.

Question 14

In a magnetising field H = 2000 A/m, sample X acquires a magnetisation M = +0.6 A/m and sample Y acquires M = −0.04 A/m. For both samples M varies linearly with H. Which statement is correct?
  1. X tends to move towards the weaker part of a non-uniform field, and the relative permeability of Y is slightly more than 1.
  2. X tends to move towards the stronger part of a non-uniform field, and the relative permeability of Y is slightly less than 1.
  3. Both samples tend to move towards the stronger part of a non-uniform field.
  4. Both samples tend to move towards the weaker part of a non-uniform field.
Show answer
Answer: (B) X tends to move towards the stronger part of a non-uniform field, and the relative permeability of Y is slightly less than 1.
Explanation: Susceptibility is χ = M/H, which gives +3 × 10⁻⁴ for X and −2 × 10⁻⁵ for Y. Small positive χ marks X as paramagnetic, so it drifts towards the stronger field. Small negative χ marks Y as diamagnetic, so it drifts towards the weaker field. Since μ_r = 1 + χ, the relative permeability of Y is just below 1.

Question 15

A long solenoid has 1000 turns per metre and carries a current of 2 A. Its core is completely filled with a material of magnetic susceptibility 799. The magnetic field inside the core is close to: (μ₀ = 4π × 10⁻⁷ T m A⁻¹)
  1. 2.5 × 10⁻³ T
  2. 1.0 T
  3. 4.0 T
  4. 2.0 T
Show answer
Answer: (D) 2.0 T
Explanation: The relative permeability is μᵣ = 1 + χ = 800. The field is B = μ₀μᵣnI = 4π × 10⁻⁷ × 800 × 1000 × 2. This gives about 2.0 T, which is 800 times the value for an empty solenoid.

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