WACE Physics — Unit 3
Faraday's Law — Flashcards & Quiz
Faraday's law quantifies electromagnetic induction: the induced EMF in a coil is proportional to the rate of change of magnetic flux. WACE Physics Year 12 Unit 3 expects you to apply ε = –N dΦ/dt to rotating coils, moving conductors and changing field strengths, and to use Lenz's law to determine the direction of the induced current.
Key Points
- Magnetic flux: Φ = BA cosθ, where θ is the angle between B and the normal to the loop.
- Faraday's Law: ε = –N dΦ/dt, where N is the number of turns. The minus sign encodes Lenz's law.
- Lenz's Law: the direction of induced current opposes the change in flux that caused it — energy conservation in action.
- Flux change sources: changing B (moving magnet, changing current), changing A (stretching loop), changing θ (rotating coil — the AC generator).
- For a rotating coil: ε = NBAω sin(ωt), giving sinusoidal AC output with peak EMF ε₀ = NBAω.
- Eddy currents: induced currents in bulk conductors; reduced by lamination in transformer cores.
Common Mistakes to Avoid
- Forgetting the minus sign in Faraday's law — it encodes the direction (Lenz's law).
- Confusing flux (Φ) with flux density (B).
- Missing the N factor when calculating EMF for a multi-turn coil.
- Ignoring Lenz's law when predicting current direction.
- Applying Faraday's law to DC current situations without noting that steady DC gives no induced EMF (constant flux).
Exam Strategy
SCSA Unit 3 Faraday's law questions ask you to calculate induced EMF for a changing flux scenario. Method: (1) write Φ = BA cosθ, (2) differentiate with respect to time based on what's changing, (3) apply ε = –N dΦ/dt, (4) use Lenz's law for direction. Draw a clear diagram showing the field, coil and induced current.
Revision Tip
Faraday's law applications are equation-heavy — drill a Revizi deck covering rotating coil, moving conductor, and collapsing field scenarios.
Last updated: March 2026