Electromagnetic Induction
Year 11 (IGCSE) ⚡ Electricity & Magnetism Explain Faraday's law and Lenz's law; describe generators.
🔄 Faraday's Law of Induction
When the magnetic flux through a conductor changes, an EMF is induced. This is the reverse of the motor effect — instead of electricity creating motion, motion creates electricity!
$\mathcal{E}$ = induced EMF (V) · $N$ = number of turns · $\Delta\Phi/\Delta t$ = rate of change of magnetic flux (Wb/s = V)
• Move the magnet faster (↑ ΔΦ/Δt)
• Use a stronger magnet (↑ B)
• Increase number of coil turns (↑ N)
• Use a larger coil area (↑ A)
⚡ Lenz's Law
Lenz's Law states that the induced current always flows in a direction to oppose the change that caused it — this is a consequence of energy conservation.
A conductor of length $L$ moving at velocity $v$ perpendicular to field $B$
🔌 AC Generators
An AC generator (alternator) converts kinetic energy into electrical energy using electromagnetic induction.
| 🔩 Component | 📝 Function |
|---|---|
| Rotating coil (armature) | Sweeps through magnetic field; flux changes continuously |
| Permanent magnets (stator) | Provide the external magnetic field |
| Slip rings | Maintain electrical contact as the coil rotates (unlike commutator — allows AC output) |
| Carbon brushes | Press against slip rings to collect current |