Magnetism and Electromagnetic Induction
What this domain tests
This domain examines magnetic flux and flux density, magnetic circuits, and the induction laws that underpin transformers and generators. Numerical questions usually involve Faraday's law, Lenz's law and inductance.
Core rules
- Flux Φ is measured in webers; flux density B = Φ/A in tesla. A stronger field or larger area increases flux.
- Magnetomotive force mmf = NI (ampere-turns); reluctance opposes flux, analogous to resistance. Flux Φ = mmf/reluctance.
- Faraday's law: induced EMF e = −N dΦ/dt. The minus sign is Lenz's law: the induced current opposes the change producing it, which explains why transformers and generators load the driving source.
- Fleming's left-hand rule gives motor force direction; the right-hand rule gives generator EMF direction.
- Self-inductance L links flux to current: e = −L di/dt; energy stored = ½LI². Mutual inductance transfers energy between coupled coils.
Worked example
A coil of 200 turns links a flux that rises uniformly from 2 mWb to 6 mWb in 0.05 s. Change in flux = 4 × 10⁻³ Wb. Induced EMF = N ΔΦ/Δt = 200 × 4 × 10⁻³ / 0.05 = 16 V. If the flux were falling, the EMF would reverse direction, not simply change magnitude.
Traps that cost marks
Using flux density B where flux Φ is required, or forgetting area; dropping the factor N; treating the induced EMF as constant when the rate of change is not uniform; and misapplying Lenz's law so the polarity is reversed.