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Electromagnetic Induction – Class 12 Physics notes, important questions and MCQs

NCERT Class 12 Physics, chapter 6. Notes, key terms, important questions with answers and MCQs.

Chapter overview

A changing magnetic flux through a circuit induces an emf (Faraday's law), opposing the change (Lenz's law). This is the basis of generators and transformers.

Laws of induction

Magnetic flux Φ = BA cos θ (weber).
Faraday: e = −dΦ/dt (for N turns, e = −N dΦ/dt).
Lenz's law: induced current opposes the change causing it (energy conservation).
Motional emf e = Bvl.

Inductance

Self-inductance: e = −L dI/dt; solenoid L = µ₀n²Al.
Mutual inductance M between two coils.
Energy stored in an inductor U = ½LI²; eddy currents cause heating (used in induction cooktops, braking).

Key terms

Magnetic flux: Φ = BA cos θ
Self-inductance: property of a coil opposing change of current in itself
Eddy currents: induced currents in bulk conductors in changing magnetic flux; used in braking and induction furnaces

Formulas

e = −N dΦ/dt
e = Bvl
U = ½LI²

Important questions with answers

State Lenz's law and show it is consistent with conservation of energy.2 marks

The induced current flows in a direction that opposes the change in flux causing it. When a magnet is pushed towards a coil, the induced current makes the near face of the coil a like pole that repels the magnet, so work must be done; this work becomes electrical energy. If it attracted, energy would be created from nothing.

Derive the self-inductance of a long solenoid.3 marks

B = µ₀nI inside, flux through each turn = BA, total flux linkage NΦ = (nl)(µ₀nI)A = µ₀n²AlI. L = NΦ/I = µ₀n²Al (or µ₀N²A/l).

Explain the working of an AC generator and derive the expression for the induced emf. Find the peak emf for a coil of 100 turns, area 0.1 m², rotating at 50 rev/s in a field of 0.2 T.5 marks

A coil rotates with angular speed ω in a uniform field B; the flux through it changes as Φ = NBA cos ωt. By Faraday's law, e = −dΦ/dt = NBAω sin ωt = e₀ sin ωt, with e₀ = NBAω. Slip rings and brushes take out this alternating emf. Here ω = 2π × 50 = 100π rad/s, e₀ = 100 × 0.2 × 0.1 × 100π = 200π ≈ 628 V.

Quick revision

Changing flux induces emf.
Induced current opposes the change.
Unit of inductance: henry.

Exam tips

Lenz's law gives the direction; Faraday's law gives the magnitude.
Motional emf of a rod of length l moving with speed v ⟂ B is Blv.

Practice MCQs

SI unit of magnetic flux is
1. tesla
2. farad
3. henry
4. weber
Show answer
Answer: (d) weber. Weber.
Lenz's law follows from conservation of
1. charge
2. energy
3. mass
4. momentum
Show answer
Answer: (b) energy. Energy.
A 0.5 m rod moves at 4 m/s perpendicular to a 0.2 T field. Induced emf is
1. 4 V
2. 0.4 V
3. 0.1 V
4. 2 V
Show answer
Answer: (b) 0.4 V. Bvl = 0.2 × 4 × 0.5.
Energy stored in a 2 H inductor carrying 3 A is
1. 6 J
2. 3 J
3. 18 J
4. 9 J
Show answer
Answer: (d) 9 J. ½ × 2 × 9.
A coil of 100 turns has flux change from 0.05 Wb to 0.01 Wb per turn in 0.1 s. Induced emf is
1. 4 V
2. 400 V
3. 0.4 V
4. 40 V
Show answer
Answer: (d) 40 V. 100 × 0.04/0.1.
Flux through a 100-turn coil changes from 0.05 Wb to 0.01 Wb in 0.1 s. Average emf is
1. 4 V
2. 400 V
3. 0.4 V
4. 40 V
Show answer
Answer: (d) 40 V. 100 × 0.04/0.1.
Eddy currents are reduced in transformer cores by
1. laminating the core
2. using a solid core
3. increasing current
4. using copper
Show answer
Answer: (a) laminating the core. Laminations break current paths.
Mutual inductance of two coils is 2 H. Current in one changes from 0 to 5 A in 0.5 s. Emf in the other is
1. 5 V
2. 20 V
3. 10 V
4. 2.5 V
Show answer
Answer: (b) 20 V. M dI/dt = 2 × 10.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.