JEE Main Physics EMI & Alternating Currents 2027: Faraday, Lenz & LCR Resonance

Every power station on Earth runs on one discovery: a changing magnetic flux creates an electric field. Electromagnetic Induction and AC bring that idea to circuits, and it pairs neatly with the transformer that brings electricity to your home.

Nature resists change in flux. That is Lenz's law in one line.

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Chapter at a Glance

Priority reflects past-paper trends, not an official NTA weightage.

What the NTA Syllabus Covers

• Faraday's law, induced emf and current, Lenz's law, eddy currents, self and mutual inductance
• Peak and RMS values of AC, reactance and impedance, LCR series circuit, resonance
• Power in AC circuits, wattless current, AC generator and transformer

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Master These Topics

1. Faraday's Law, Lenz's Law and Motional EMF

Induced emf is ε = −dΦ/dt, where the flux is Φ = BA cosθ. The minus sign is Lenz's law: the induced current opposes the change in flux that produced it.

For a rod of length l moving at speed v perpendicular to a field B: ε = Blv.

Worked example: A 0.5 m rod moves at 4 m/s in a 0.2 T field. Induced emf = 0.2 × 0.5 × 4 = 0.4 V.

Trap: Lenz's law gives direction, not magnitude. If flux increases, the induced current creates a field opposing the increase.

2. Inductance

Self inductance L relates flux to current, and the emf induced is ε = −L dI/dt. Energy stored in an inductor is U = ½LI². In an LR circuit, current grows as I = I₀(1 − e^(−Rt/L)), with time constant τ = L/R.

3. AC Circuits and Resonance

RMS values are V_rms = V₀ / √2 and I_rms = I₀ / √2. Reactances are X_L = ωL and X_C = 1/ωC. For a series LCR circuit:

Z = √(R² + (X_L − X_C)²)

At resonance, X_L = X_C, so impedance is minimum (equal to R) and current is maximum. Resonant angular frequency is ω₀ = 1/√(LC).

Worked example (resonance): L = 1 mH and C = 1 μF give ω₀ = 1/√(10⁻⁹) ≈ 31,623 rad/s, i.e. f ≈ 5 kHz.

Worked example (impedance): R = 30 Ω, X_L = 60 Ω, X_C = 20 Ω. Z = √(900 + 1600) = 50 Ω, and power factor = R/Z = 0.6.

Average power in an AC circuit is P = V_rms I_rms cosφ. An ideal inductor or capacitor draws wattless current, because the phase difference is 90° and average power is zero.

4. Transformer

An ideal transformer follows V_s / V_p = N_s / N_p. A step-down transformer converting 220 V to 11 V with 1000 primary turns needs 50 secondary turns. Power is conserved in the ideal case, so current scales inversely with voltage.

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Common Traps to Avoid

• Using peak values where RMS is required (or the reverse).
• Getting the direction of induced current wrong by ignoring Lenz's law.
• Forgetting that at resonance the impedance is R, not zero.
• Assuming a transformer changes power. It changes voltage and current, not power.

60-Second Revision Sheet

• ε = −dΦ/dt, motional ε = Blv
• U = ½LI², LR time constant L/R
• Z = √(R² + (X_L − X_C)²), ω₀ = 1/√(LC)
• P = V_rms I_rms cosφ, cosφ = R/Z; transformer V_s/V_p = N_s/N_p

Your Study Plan

1. Day 1: flux, Faraday's law and Lenz's law directions.
2. Day 2: motional emf and induced current in loops and rods.
3. Day 3: LCR circuits, phasors, resonance and power factor.
4. Day 4: transformers, generator and a timed mixed set.

Practice Electromagnetic Induction & Alternating Currents Questions Free → (opens in a new tab)

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Continue Your Physics Journey

• Previous chapter: Magnetic Effects of Current & Magnetism
• Next chapter: Electromagnetic Waves
• All 20 JEE Main Physics chapters
• Complete JEE Main Syllabus 2027 guide

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