Electromagnetic Induction and Alternating Current

Unit 4. Covers Faraday’s and Lenz’s laws; Self and mutual induction; Alternating current; Resonance and power in AC circuits; AC generator and transformer.

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About this unit

This unit explains how a changing magnetic field produces an emf (electromagnetic induction) and how this is used to generate and control alternating current. It covers Faraday’s and Lenz’s laws, self and mutual inductance, AC circuits with resistors, inductors and capacitors, resonance and power, and finally the AC generator and transformer. Power stations, chargers and induction cooktops all depend on these ideas.

How to prepare

  • Memorise ε = –N dΦ/dt, ε = Blv, ε = –L dI/dt, U = ½LI².
  • Learn X_L = ωL, X_C = 1/(ωC), Z = √[R² + (X_L – X_C)²], ω₀ = 1/√(LC).
  • Practise RMS conversions: I_rms = I₀/√2.
  • Understand phase: in an inductor, current lags voltage by 90°; in a capacitor, current leads by 90°.
  • Learn the transformer ratio V_s/V_p = N_s/N_p and the causes of energy loss.

Topics in this chapter

Faraday’s and Lenz’s laws

Induced emf and current.

5 questions

Self and mutual induction

Inductance.

5 questions

Alternating current

Peak and RMS values, reactance, impedance and LCR circuits.

5 questions

Resonance and power in AC circuits

Power factor and wattless current.

5 questions

AC generator and transformer

Working principles.

5 questions

Practice questions

The magnetic flux through a 100-turn coil falls from 0.05 Wb to 0.01 Wb in 0.2 s. The average induced emf is:Easy
A rod 0.2 m long moves at 10 m/s perpendicular to a uniform field of 0.5 T. The emf across its ends is:Easy
Lenz’s law is a consequence of the law of conservation of:Medium
The SI unit of magnetic flux is:Medium
The cores of transformers are laminated mainly to:Hard

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