CBSE Class 12 Physics Chapter 6 — Electromagnetic Induction (EMI) is the conceptual bridge between Magnetism (Chapter 4–5) and Alternating Current (Chapter 7). It carries an average of 5 marks in the CBSE board exam and is the third highest-yielding chapter in the Electromagnetism unit (worth 17 marks total). This NCERT-aligned guide covers Faraday’s Laws, Lenz’s Law, Motional EMF, Self & Mutual Induction, Eddy Currents and Energy Stored in an Inductor — with derivations, board-pattern numericals, common pitfalls and a 10-MCQ practice quiz at the end.
Why Electromagnetic Induction Matters in CBSE Class 12 Boards 2027
Faraday’s discovery (1831) is the foundation of every electrical generator, transformer, induction cooker, RFID card and wireless charger. CBSE board examiners almost always ask one derivation (Faraday’s law, motional EMF, or self-inductance of solenoid) PLUS one numerical from this chapter. Our Class 12 Physics chapter-wise hub tracks every PYQ since 2018.
Chapter 6 Weightage and Question Pattern (CBSE 2027)
| Topic | Typical Marks | Question Type |
|---|---|---|
| Faraday’s Laws + Lenz’s Law (statement & derivation) | 2–3 | VSA + SA-I |
| Motional EMF (derivation + numerical) | 2–3 | SA-II / Long answer |
| Self-Inductance of Solenoid | 2 | Derivation |
| Mutual Inductance between two coils | 2 | Derivation + numerical |
| Energy Stored in Inductor / Eddy Currents | 1–2 | SA-I / VSA |
| Total Chapter 6 Weightage | 5 marks | ~7% of paper |
Faraday’s Laws — Statement and Mathematical Form
First Law: Whenever the magnetic flux linked with a closed circuit changes, an EMF is induced in the circuit. The induced current lasts only as long as the change in flux continues.
Second Law: The magnitude of induced EMF is directly proportional to the rate of change of magnetic flux linked with the circuit.
Mathematically, EMF e = –N (dΦ/dt) where N is the number of turns and Φ = B.A.cos(theta) is the flux. The negative sign is the contribution of Lenz’s Law.
Lenz’s Law and Energy Conservation
Statement: The direction of induced current is such that it always opposes the cause that produced it. This is essentially the law of conservation of energy applied to electromagnetic phenomena. If induced current aided the change, we would have created perpetual motion — a violation of the first law of thermodynamics.
Motional EMF — Derivation
Consider a conducting rod PQ of length L moving with velocity v perpendicular to a uniform magnetic field B. Free electrons experience a Lorentz force F = qv × B that pushes them along the rod. Charge accumulates until the electric field developed balances the magnetic force. At equilibrium:
EMF e = B.v.L (when v, B and L are mutually perpendicular).
Self-Inductance — Solenoid Derivation
For a solenoid of length l, area A and N turns, the magnetic field inside is B = mu0 . n . I, where n = N/l. Total flux linked = N.B.A = mu0 . N^2 . A . I / l. Comparing with Φ = L.I:
L = mu0 . N^2 . A / l
Self-inductance depends only on geometry and the medium — NOT on current or voltage.
Mutual Inductance and Coupling
For two coaxial solenoids, primary with N1 turns and secondary with N2 turns, both of length l and area A:
M12 = M21 = mu0 . N1 . N2 . A / l
Coefficient of coupling k = M / sqrt(L1.L2). For perfect coupling k = 1; for no coupling k = 0.
Energy Stored in an Inductor
U = (1/2) . L . I^2 joules. This is analogous to U = (1/2)CV^2 stored in a capacitor’s electric field. The energy is stored in the magnetic field around the inductor.
Eddy Currents — Friend and Foe
| Application (Useful) | Power Loss (Harmful) |
|---|---|
| Electromagnetic braking (trains) | Transformer iron core |
| Induction furnace (melting metals) | Electric motor armature |
| Energy meters (galvanometer damping) | Generator field cores |
| Induction cooker plates | Choke coils |
Reduction technique: Lamination of cores with thin insulated sheets of soft iron — breaks the closed loops in which eddy currents would flow.
Common Mistakes to Avoid
- Forgetting N (number of turns): EMF formula needs N — easy mark loss in derivations.
- Sign of EMF: CBSE accepts magnitude in numericals, but always mention “negative sign represents Lenz’s Law” in derivation.
- Units of inductance: Henry (H) = Volt-second/Ampere = Weber/Ampere — don’t confuse with Tesla.
- cos(theta) in flux: Φ = BA only when B is perpendicular to area; otherwise Φ = BA.cos(theta).
- Self-inductance formula: Don’t forget the squared N — students often write mu0.N.A/l.
Board-Pattern Numerical (5-mark)
Q. A 100-turn coil of area 50 cm² is rotated about an axis perpendicular to a uniform magnetic field of 0.5 T at 10 revolutions per second. Find (a) maximum EMF induced, and (b) the EMF at the instant the plane of the coil makes 60° with the field.
Solution.
- Angular frequency w = 2.pi.f = 2 × 3.14 × 10 = 62.8 rad/s.
- (a) e_max = N.B.A.w = 100 × 0.5 × 50 × 10^-4 × 62.8 = 0.5 × 62.8 / 2 = 15.7 V.
- (b) When plane makes 60° with field, normal makes 30°: e = e_max × sin(30°) = 15.7 × 0.5 = 7.85 V.
Recommended Books and Reference Material
| Resource | Use | Time Required |
|---|---|---|
| NCERT Class 12 Physics Part-I (Chapter 6) | Primary text + every exercise | 5 hours |
| H.C. Verma — Concepts of Physics Vol-2 | Conceptual depth + advanced problems | 4 hours |
| D.C. Pandey — Electricity & Magnetism | JEE/NEET-style numericals | 3 hours |
| CBSE Sample Paper 2026-27 | Latest board pattern questions | 1 hour |
| Ready For Boards EMI Formula Sheet PDF | Quick revision aid | 30 min |
Frequently Asked Questions
Q1. What is the difference between Faraday’s First and Second Laws?
The First Law is qualitative — it tells us EMF exists when flux changes. The Second Law is quantitative — it gives the magnitude as the rate of change of flux. Together they describe the EMI phenomenon completely.
Q2. Why is Lenz’s Law a consequence of energy conservation?
If induced current flowed in the direction that aids the change, we could obtain energy without doing work — perpetual motion. Lenz’s law (current opposes the cause) ensures we must do work against the induced magnetic field to keep the flux changing — that work becomes the electrical energy.
Q3. Does self-inductance depend on the current flowing through the inductor?
No — self-inductance L is purely geometric (depends on N, A, l and medium). The induced EMF depends on dI/dt, not on I itself.
Q4. Why are transformer cores laminated?
Lamination splits the cross-section into thin insulated sheets. Eddy currents that would flow in large circular paths through a solid core are now restricted to tiny loops within each thin sheet — drastically reducing power loss as I^2.R heating.
Q5. How much time should I devote to Chapter 6 for boards?
15–18 hours is sufficient (5 hrs theory + 5 hrs derivations + 5 hrs numericals + 3 hrs revision). Yields a guaranteed 5 marks.
Practice Quiz — 10 MCQs on Electromagnetic Induction
Quiz data missing.
Next Steps for Class 12 Physics Mastery
Chapter 6 EMI is the gateway to Chapter 7 (Alternating Current) and Chapter 8 (Electromagnetic Waves) — together they account for ~17 marks. Download our Class 12 Physics All Formula Sheet PDF and enrol in the Class 12 Physics Live Mentor Programme for board-pattern derivation practice + weekly numerical drills + 1:1 doubt clearing. Master these three chapters and a 17/35 in Electromagnetism is locked.
