Magnetism and Matter: Class 12 Important Questions with Answers

Practice set: test yourself

Try every question before looking at the answers below. 64 or more out of 80 means you are ready for the board.

C. Very short answer (1 mark each)

1. State Gauss's law for magnetism.
2. Give two examples of diamagnetic substances.
3. What is the susceptibility of a superconductor?
4. Write the relation between B, H and M.
5. Why do field lines never cross?
6. At what angle is the torque on a magnet maximum?
7. Name the material used for electromagnet cores.
8. What happens to a paramagnet's M when it is cooled?
9. Unit of magnetic moment?
10. Write one use of superconducting magnets.

D. Short answer I (2 marks each)

1. Write four properties of magnetic field lines.
2. Why is a bar magnet equivalent to a solenoid?
3. Derive U = −m·B for a magnetic dipole.
4. Distinguish between hard and soft ferromagnetic materials.
5. A solenoid of 1000 turns/m carries 2 A with a core of ${\mu }_r$ = 800. Find B.
6. Give two differences between diamagnetic and paramagnetic substances.
7. Why does a diamagnetic sample move from a strong to a weak field?
8. Find the work needed to turn a magnet (m = 1 A m²) from 0° to 180° in 0.4 T.

Rough work

E. Short answer II (3 marks each)

1. Explain ferromagnetism with the help of domains and diagrams.
2. Define M, H and χ and show that $B={\mu }_0{\mu }_{r}H$.
3. Explain diamagnetism using the orbital motion of electrons.
4. A bar magnet of m = 0.5 J/T is at 60° to B = 0.2 T. Find the torque and the energy; what is the work needed to make it antiparallel?
5. Compare dia-, para- and ferromagnetic substances under: χ, ${\mu }_r$, behaviour in a non-uniform field.
6. Find B at 20 cm on the axis and on the equator of a short magnet of m = 0.8 A m².

F. Long answer (5 marks each)

1. Show that a current-carrying solenoid behaves like a bar magnet. Write the electric–magnetic dipole analogy. Find the moment of a solenoid of 500 turns, area 5 cm², 2 A.
2. Classify magnetic materials. Explain each class with the behaviour of field lines and examples. Why is ferromagnetism lost on heating?
3. State Gauss's law for magnetism and compare it with Gauss's law for electrostatics. Explain why magnetic field lines are closed. Which drawings of field lines can be ruled out by it?
4. A solenoid of 2000 turns/m with 0.5 A has a core of relative permeability 1000. Find H, B, M and the magnetising current. What happens to B if the core is replaced by aluminium (χ = 2.3 × 10⁻⁵)?

G. HOTS (higher-order thinking)

1. Why is a toroid's field not that of a dipole, though it is made of current loops?
2. A paramagnetic liquid in a U-tube rises in the limb placed between the poles of a strong magnet. Explain. What would a diamagnetic liquid do?
3. Could the field lines of a magnet start at the N pole and end at the S pole? Use Gauss's law.
4. A superconductor floats above a magnet. Explain.

20 MCQs

1. Net magnetic flux through a closed surface is
   (a) $q/{\epsilon }_0$ (b) ${\mu }_{0}I$ (c) zero (d) infinite
2. Diamagnetic among these
   (a) Bi (b) Fe (c) Al (d) O₂
3. Paramagnetic among these
   (a) Cu (b) water (c) NaCl (d) Al
4. χ of a superconductor
   (a) 0 (b) −1 (c) 1 (d) ∞
5. Unit of H
   (a) T (b) A/m (c) A m² (d) Wb
6. Relation among B, H, M
   (a) $B={\mu }_0(H-M)$ (b) $B=H+M$ (c) $B={\mu }_{0}HM$ (d) $B={\mu }_0(H+M)$
7. Torque on a magnet in B is maximum at
   (a) 90° (b) 45° (c) 0° (d) 180°
8. Stable equilibrium of a magnet in B
   (a) m antiparallel (b) m ⊥ B (c) m ∥ B (d) at 45°
9. ${\mu }_r$ of a ferromagnet is
   (a) < 1 (b) ≈ 1 (c) 0 (d) ≫ 1
10. A diamagnet in a non-uniform field moves
    (a) to a stronger field (b) to a weaker field (c) does not move (d) in circles
11. Domains are found in
    (a) diamagnets (b) paramagnets (c) ferromagnets (d) all
12. Permanent magnets are made of
    (a) alnico (b) copper (c) soft iron (d) aluminium
13. Field lines inside a bar magnet go
    (a) N to S (b) S to N (c) nowhere (d) both ways
14. ${\mu }_r$ in terms of χ
    (a) 1 − χ (b) χ (c) 1 + χ (d) 1/χ
15. Magnetisation is
    (a) flux per area (b) moment per unit mass (c) field per unit current (d) moment per unit volume
16. On heating, a ferromagnet becomes
    (a) paramagnetic (b) diamagnetic (c) superconducting (d) stronger
17. Energy of a dipole m at 180° to B
    (a) mB (b) 0 (c) −mB (d) 2mB
18. Meissner effect is shown by
    (a) iron (b) oxygen (c) aluminium (d) superconductors
19. Axial field of a short magnet at distance r
    (a) $\frac{{\mu }_0}{4\pi }\frac{m}{r^3}$ (b) $\frac{{\mu }_0}{4\pi }\frac{2m}{r^3}$ (c) $\frac{{\mu }_0}{4\pi }\frac{m}{r^2}$ (d) $\frac{{\mu }_{0}m}{2r}$
20. Susceptibility of a paramagnet
    (a) large positive (b) small negative (c) small positive (d) −1

1 (c) · 2 (a) · 3 (d) · 4 (b) · 5 (b) · 6 (d) · 7 (a) · 8 (c) · 9 (d) · 10 (b) · 11 (c) · 12 (a) · 13 (b) · 14 (c) · 15 (d) · 16 (a) · 17 (a) · 18 (d) · 19 (b) · 20 (c)

Assertion–Reason (6)

(a) Both true, R explains A. (b) Both true, R does not explain A. (c) A true, R false. (d) A false, R true.

1. A: Magnetic field lines are closed loops. R: Magnetic monopoles do not exist.
2. A: A diamagnetic substance is repelled by a magnet. R: Its induced moment is opposite to the field.
3. A: Soft iron is used in electromagnets. R: Soft iron retains its magnetism for a long time.
4. A: The magnetisation of a paramagnet increases on cooling. R: Thermal agitation opposes the alignment of atomic moments.
5. A: Copper is paramagnetic. R: Copper atoms have no net magnetic moment.
6. A: A bar magnet in a uniform field feels a torque. R: It also feels a net force.

Key: 1 (a) · 2 (a) · 3 (c) · 4 (a) · 5 (d) · 6 (c)

Answers to the practice set