A.C. Generator (Alternator)

An a.c. generator changes spinning motion into alternating electrical energy. It works by electromagnetic induction: moving a wire through a magnetic field makes an e.m.f. (voltage).

Key parts

• Rotating coil between the poles of a magnet (or a rotating magnet inside a fixed coil).
• Slip rings: smooth metal rings attached to the coil ends; they spin with the coil.
• Carbon brushes: press lightly on the slip rings to carry current out to the circuit without twisting wires.
• Magnets (often with an iron core) provide a strong, steady magnetic field.

How it works

• As the coil turns, the number of magnetic field lines “cut” per second changes.
• This changing magnetic flux induces an e.m.f. that keeps reversing direction every half turn, so the output is a.c.
• The e.m.f. varies smoothly with time, like a sine wave:

$$E = E_{\text{max}}\,\sin(\omega t)$$

The size of the peak depends on coil design and speed: $$E_{\text{max}} \propto N B A \omega$$ where N = turns, B = field strength, A = coil area, $\omega$ = spin rate.

Linking coil position to the graph

• Zero e.m.f. when the coil’s face is perpendicular to the field (face-on): flux is maximum but not changing.
• Peak + when the coil’s plane is parallel to the field (edge-on) while turning forward: flux is changing fastest.
• Peak − half a turn later (still edge-on) as the direction has reversed.

One full turn gives one full sine wave. Spinning faster increases frequency and peak size.

Slip rings vs split ring

• A.C. generator: slip rings give an alternating output.
• D.C. motor/generator: a split-ring commutator flips connections to make one-direction output.

Real-world examples

• Power station alternators
• Bicycle “dynamos” (small alternators)

Common misconceptions

• Zero e.m.f. does not mean zero flux; it means flux is not changing at that instant.
• Rotating the magnet instead of the coil gives the same a.c. output.