QCE Physics — Unit 3
Electromagnetic Induction — Flashcards & Quiz
Electromagnetic induction is the generation of EMF from a changing magnetic flux, described quantitatively by Faraday's law and qualitatively by Lenz's law. QCE Physics Unit 3 expects you to calculate induced EMF for rotating coils, moving conductors and changing fields, and to explain how the law underpins generators and transformers. The minus sign in Faraday's law encodes energy conservation.
Key Points
- Magnetic flux Φ = BA cosθ, where θ is the angle between B and the normal to the loop.
- Faraday's law: induced EMF ε = –N dΦ/dt; the minus sign reflects Lenz's law.
- Lenz's law: the induced current flows in a direction that opposes the change in flux that caused it — energy conservation in disguise.
- Three ways to change flux: change B (moving magnet), change A (stretching/closing loop), change θ (rotating coil — the basis of generators).
- Transformers use Faraday's law with mutual induction: Vₛ/Vₚ = Nₛ/Nₚ; they only work with AC because DC gives a constant flux and zero dB/dt.
- Eddy currents are induced currents in bulk conductors — reduced by lamination; used in induction braking, metal detectors.
Common Mistakes to Avoid
- Confusing Faraday's law (magnitude of EMF) with Lenz's law (direction of induced current).
- Forgetting the minus sign in ε = –N dΦ/dt.
- Claiming transformers work with DC — constant flux means dB/dt = 0 and no induced EMF.
- Mixing up Φ (flux, through an area) with B (flux density, field strength).
- Applying Lenz's law inconsistently when the direction of B or motion changes.
Exam Strategy
QCAA Unit 3 induction questions ask you to calculate induced EMF or determine the direction of the induced current. Method: (1) identify what is changing (B, A or θ), (2) write Φ = BA cosθ, (3) differentiate to get dΦ/dt, (4) apply ε = –N dΦ/dt for magnitude, (5) use Lenz's law to determine direction. Draw a clear diagram with field, motion and induced current labelled.
Sample Flashcards
Q1: State Faraday's law of electromagnetic induction.
The induced EMF equals the negative rate of change of magnetic flux: ε = −NΔΦ/Δt, where N = number of turns, Φ = BA cos θ is the magnetic flux (Wb). Faster flux change = larger EMF.
Q2: State Lenz's law and explain its physical basis.
The direction of induced current opposes the change in flux that produced it. This is a consequence of conservation of energy — if it aided the change, energy would be created from nothing.
Q3: Define magnetic flux and state its formula.
Magnetic flux Φ = BA cos θ (Wb), where B = field strength (T), A = area (m²), θ = angle between field and area normal. Maximum when field is perpendicular to the surface (θ = 0°).
Q4: List three ways to induce an EMF in a coil.
1) Move a magnet into/out of the coil. 2) Move the coil into/out of a magnetic field. 3) Change the magnetic field strength (e.g. vary current in a nearby coil). All change the magnetic flux through the coil.
Sample Quiz Questions
Q1: A stationary magnet inside a coil induces an EMF.
Answer: FALSE
No change in flux = no induced EMF. The magnet or coil must move to change flux.
Q2: Increasing the rate of flux change increases the induced EMF.
Answer: TRUE
ε = −NΔΦ/Δt — faster change produces larger EMF.
Q3: Lenz's law states the induced current aids the change in flux.
Answer: FALSE
The induced current OPPOSES the change — this conserves energy.
Revision Tip
Flux calculations are the base skill — drill a Revizi deck of dΦ/dt scenarios (moving bar, rotating coil, collapsing field) until the method feels automatic.
Last updated: March 2026 · 4 flashcards · 4 quiz questions