CBSE Class 12 Physics: Electromagnetic Induction — Notes 2026

Faraday's Laws of EMI

• First law: any change in magnetic flux linked with a coil induces an EMF (electromotive force)
• Second law: magnitude of induced EMF = rate of change of flux; e = −dΦ/dt
• Magnetic flux: Φ = B·A cos θ; unit Weber (Wb); 1 Wb = 1 V·s

Lenz's Law

• Direction of induced current is such that it opposes the change causing it
• Example: north pole approached → induced current creates north pole in coil to oppose approaching magnet
• Conservation of energy basis: Lenz's law is a consequence of energy conservation

Motional EMF

• Conductor of length l moving with velocity v perpendicular to field B: EMF = Blv
• Free electrons in conductor experience force qvB; they separate creating potential difference
• Power delivered: P = F × v = BIlv = I²R; energy comes from work done in moving conductor

Self and Mutual Inductance

• Self inductance L: flux linked with coil = L × I; induced EMF = −L dI/dt; unit Henry (H)
• Solenoid: L = μ₀ n² V where n = turns/length, V = volume; typical values: mH for small coils
• Mutual inductance M: flux in coil 2 due to current in coil 1; e₂ = −M dI₁/dt; transformer uses mutual inductance

AC Generator

• Coil rotates in uniform magnetic field; EMF = NBAω sin(ωt) = E₀ sin(ωt)
• E₀ (peak EMF) = NBAω; A = area of coil; ω = angular frequency; N = number of turns
• Slip rings and brushes: maintain continuous electrical connection while coil rotates

Energy Stored in Inductor

• Energy U = ½LI²; stored in magnetic field (analogous to ½CV² in capacitor — electric field)
• Inductor opposes change in current; acts as inertia for current
• L-R circuit: time constant τ = L/R; current builds up as I = I₀(1 − e^(−t/τ))

CBSE Board Focus

• EMI: 6–8 marks; Lenz's law explanation with diagram, AC generator diagram and derivation
• Numerical: EMF = Blv, E₀ = NBAω — both frequently tested
• Self vs mutual inductance: definitions, units, formulae; comparison table