HSC Physics — Module 6
AC Generator — Flashcards & Quiz
An AC generator converts mechanical rotation into a sinusoidal EMF using Faraday's law, and HSC Physics Module 6 expects you to derive the output ε = NBAω sin ωt from first principles. Understand why EMF is zero when the coil is perpendicular to B (flux maximum but changing slowly) and maximum when parallel to B (flux zero but changing fastest). The slip-ring geometry is what makes the generator AC rather than DC.
Key Points
- An AC generator uses Faraday's law: rotating a coil in a magnetic field produces a sinusoidal EMF ε = NBA ω sin(ωt).
- Peak EMF: ε₀ = NBA ω — depends on number of turns N, field strength B, coil area A, and angular velocity ω.
- RMS values relate to peak: V_rms = V₀/√2, I_rms = I₀/√2. Used for power calculations because P_avg = V_rms × I_rms (for resistive loads).
- Slip rings (continuous ring contacts) make the generator AC; split-ring commutators make it DC.
- The EMF is zero when the coil is parallel to B (flux maximum but instantaneously constant) and maximum when perpendicular (flux zero but changing fastest).
- Real generators lose energy to friction, resistive heating, eddy currents, and hysteresis — account for these when calculating efficiency.
Common Mistakes to Avoid
- Forgetting that the generator produces SINUSOIDAL (not constant) EMF.
- Confusing slip rings (AC) with split-ring commutators (DC).
- Missing the peak EMF formula ε₀ = NBA ω.
- Mixing up rms and peak values — V_rms = V₀/√2 for sinusoidal.
- Claiming EMF is maximum when the coil is perpendicular to B — it's actually maximum when parallel (zero flux but fastest change).
Exam Strategy
HSC Module 6 generator questions ask you to (1) derive the sinusoidal EMF, (2) calculate peak values, or (3) explain why output is AC. Method: apply Faraday's law (ε = -N dΦ/dt) to a rotating coil, use Φ = BA cos θ = BA cos(ωt), differentiate to get ε = NBA ω sin(ωt), identify peak as NBA ω.
Sample Flashcards
Q1: How does an AC generator work?
A coil rotates in a magnetic field. As it rotates, the magnetic flux through the coil changes continuously, inducing an alternating EMF (Faraday's law). Slip rings and brushes maintain continuous contact. The EMF varies sinusoidally: ε = NBAω sinωt, maximum when the coil is parallel to B.
Q2: What is the difference between an AC generator and a DC generator?
AC generator uses slip rings — continuous rings that maintain contact, producing alternating current. DC generator uses a split-ring commutator — reverses connections every half turn, converting AC to pulsating DC. Both work by electromagnetic induction (rotating coil in a magnetic field).
Sample Quiz Questions
Q1: In an AC generator, the EMF is maximum when the coil is perpendicular to the magnetic field.
Answer: FALSE
EMF is maximum when the coil is PARALLEL to B (rate of change of flux is greatest). When the coil is perpendicular to B, flux is maximum but its rate of change is zero, so EMF = 0.
Q2: The output of an AC generator is a sinusoidal waveform.
Answer: TRUE
As the coil rotates at constant angular velocity, the flux changes sinusoidally: Φ = BAcosωt. The induced EMF ε = NBAωsinωt is a sine wave with frequency equal to the rotation frequency.
Revision Tip
The derivation from Faraday's law to sinusoidal output is a classic exam walkthrough — drill Revizi flashcards that ask you to derive it step by step.
Related Concepts
Last updated: 9 October 2026 · 2 sample flashcards · 2 sample quiz questions