Class 12 Physics Chapter 6: Electromagnetic Induction

CBSE 2026–27 | NCERT-aligned free notes, diagrams, examples and practice

Electromagnetic induction explains how a changing magnetic environment produces an induced emf. This chapter connects magnetic flux, Faraday’s laws, Lenz’s law, motional emf, inductance and energy stored in an inductor.

1. Magnetic Flux

Magnetic flux through a plane surface is Φ_B = B A cosθ, where θ is the angle between B and the area vector. Flux is measured in weber (Wb).

2. Faraday’s Laws

Whenever magnetic flux linked with a circuit changes, an emf is induced. For N turns, ε = −N dΦ_B/dt. The negative sign expresses Lenz’s law.

3. Lenz’s Law

The induced current flows in a direction that opposes the change in magnetic flux that produces it. This is a statement of energy conservation, not simply a rule for choosing current direction.

4. Motional EMF

A conductor of length l moving with velocity v perpendicular to a magnetic field B can develop motional emf ε = Blv.

5. Eddy Currents

Changing magnetic flux in bulk conductors can create circulating currents called eddy currents. They are used in electromagnetic braking, induction heating and damping, while laminated cores reduce unwanted losses in transformers and machines.

6. Self-Inductance

A changing current in a coil changes its own magnetic flux and induces an emf opposing the change: ε = −L dI/dt. L is measured in henry.

7. Mutual Inductance

A changing current in one coil can induce emf in a nearby coil. The phenomenon is mutual induction and forms the operating principle of transformers.

8. Energy in an Inductor

The energy stored in an inductor carrying current I is U = ½LI².

Worked Example

A 200-turn coil experiences a flux change of 4×10⁻³ Wb to 1×10⁻³ Wb in 0.10 s. Magnitude of average induced emf = N|ΔΦ|/Δt = 200×3×10⁻³/0.10 = 6 V.

Common Exam Traps

• Flux depends on the area-vector angle, not simply the visual angle of the coil.
• The minus sign in Faraday’s law represents Lenz’s law.
• Induced emf requires changing flux, not necessarily a changing magnetic field alone.
• Separate self-inductance from mutual inductance.

Practice Questions

1. Define magnetic flux and state its SI unit.
2. Explain Faraday’s laws and the meaning of the negative sign.
3. Use Lenz’s law to determine the direction of induced current when a magnet approaches a coil.
4. Derive motional emf Blv.
5. Explain eddy currents and two applications.
6. Calculate energy stored in an inductor for given L and I.

📌 Concept Diagram: Faraday’s Law

More Application-Based Practice

1. A coil of N turns has its flux reduced uniformly to zero. Explain how the average induced emf depends on N and the time taken.
2. A bar magnet is moved toward a coil and then away from it. Compare the direction of induced current in the two cases using Lenz’s law.
3. Why are transformer cores laminated? Relate the answer to eddy currents.
4. A conductor moves parallel to a uniform magnetic field. Explain why the simple motional-emf expression Blv does not give a non-zero value for v parallel to B.

Essential Diagrams & Problem Solving

Faraday’s Law

The negative sign represents Lenz’s law: the induced current opposes the change in magnetic flux producing it.

Motional EMF

For a conductor of length l moving with speed v perpendicular to a magnetic field B, the magnitude of motional emf is ε=Blv.

Worked Numerical

If B=0.5 T, l=0.4 m and v=3 m/s, then ε=Blv=0.5×0.4×3=0.6 V.

Self and Mutual Induction

Self-inductance opposes changes in current in the same circuit. Mutual induction occurs when changing current in one circuit induces emf in another.

CBSE-Style Practice

1. Calculate induced emf from changing flux.
2. Determine direction of induced current using Lenz’s law.
3. Solve a motional-emf problem.
4. Explain energy conservation in electromagnetic induction.
5. Distinguish self-induction and mutual induction.

Common Mistakes to Avoid

• Ignoring the minus sign in Faraday’s law when determining direction.
• Confusing magnetic flux with magnetic field.
• Using motional-emf formula when the geometry is not perpendicular.