With a labelled diagram, explain the principle and working of an AC generator. Derive the expression for the emf induced in its coil.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · State the principle. Electromagnetic induction: a coil turning in a magnetic field has a changing flux, so an emf is induced.
2 · Draw the generator. [Pole pieces N and S; coil ABCD on a shaft with a rotation arrow; slip rings R₁ and R₂; brushes B₁ and B₂; a load; all labelled]
3 · Name the parts. An armature coil on a soft iron drum, a field magnet, two slip rings and two brushes.
4 · Derive the emf. θ = ωt, so φ = AB cos ωt per turn; ε = −N dφ/dt = NABω sin ωt = ε₀ sin ωt, with ε₀ = NABω.
5 · Explain why it alternates. Each side of the coil moves up and then down through the field, so the current reverses every half turn.
6 · Link to life. Power stations use the same idea, with water or steam turbines turning the shaft.
7 · Close in one line. So, a coil turning steadily in a uniform field gives a sine-shaped AC whose peak is NABω.
Scoring tip: Mark where the emf is zero and where it is largest on a small sine sketch. It shows that you understand the derivation.
