Generation of Electricity
Generation of electricity tests the practical sources of EMF and the magnetic induction principles behind aircraft generators. You must recognise friction, pressure, heat, light, chemical, and magnetic methods, and apply Faraday's and Lenz's laws.
Core rules
- Six common sources: friction (static), pressure (piezoelectric), heat (thermocouple), light (photovoltaic), chemical (cells), and magnetism (electromagnetic induction).
- Faraday's law: induced EMF equals the rate of change of magnetic flux linkage, e = N ΔΦ/Δt for a coil of N turns.
- Lenz's law: the induced EMF drives a current whose magnetic effect opposes the change producing it.
- A generator needs relative motion between a conductor and a magnetic field; no motion or no flux change means no generated EMF.
- Slip rings give AC output; a commutator gives DC output from a rotating armature.
- Generator frequency depends on poles and rotational speed; more pole pairs give higher frequency at the same speed.
Worked example
A coil of 200 turns experiences a flux change of 0.02 Wb in 0.10 s. Find the average induced EMF. e = N ΔΦ/Δt = 200 × 0.02 / 0.10 = 40 V. Reasoning: multiply the flux change by the number of turns to get flux linkage change, then divide by the time taken.
Traps
- Using flux instead of flux linkage; N must multiply ΔΦ.
- Thinking Lenz's law aids the change; it always opposes it.
- Claiming a stationary conductor in a steady field generates EMF.
- Confusing slip rings (AC) with commutators (DC).