State the principle of an a.c. generator and explain its working with the help of a labelled diagram. Obtain the expression for the emf induced in a coil having N turns, each of cross-sectional area A, rotating with a constant angular speed ω in a magnetic field B directed perpendicular to the axis of rotation.
Principle: An a.c. generator works on electromagnetic induction — a changing flux through a rotating coil induces an emf. Working: The armature coil (N turns, area A) is rotated at angular speed ω in the field B. As it turns, the flux linkage changes continuously, and slip rings with brushes deliver the resulting alternating emf to the circuit. Derivation: With the coil normal at angle ωt to B,
