Describe briefly, with the help of a labelled diagram, the basic elements of an a.c. generator. State its underlying principle. Show diagrammatically how an alternating emf is generated by a loop of wire rotating in a magnetic field. Write the expression for the instantaneous value of the emf induced in the rotating loop.
Solution
Basic elements: a field magnet (provides B), an armature coil of N turns and area A, slip rings and carbon brushes to tap the output. Principle: electromagnetic induction — the emf induced in a coil whose flux linkage changes as it rotates. Working: As the loop rotates with angular speed ω, the normal to the coil makes angle θ=ωt with B, so the flux is
ϕ=NBAcosωt
As the loop turns, one side moves up and the other down through the field; each half-rotation reverses the sense of the induced current, giving an alternating emf:
