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Cambridge IGCSE Physics · 0625

Chapter 4: Electricity and magnetism — Part 12

Topic 4.5.2 · The a.c. generator

A rotating coil (or rotating magnet)

A simple a.c. generator has a coil that rotates in a magnetic field (or a magnet that rotates next to a coil). The changing flux linkage induces an alternating e.m.f.

Slip rings turn with the coil and brushes press on them so the external circuit can take the current without the wires twisting. (A rotating magnet design may not need slip rings.)

Simple a.c. generator: coil rotating in a magnetic field, with slip rings and brushes taking the induced current to an external circuit.
Diagram 1: Simple a.c. generator: coil rotating in a magnetic field, with slip rings and brushes taking the induced current to an external circuit.

e.m.f. against time

The graph of e.m.f. against time is a wave that repeatedly reverses.

  • Peaks and troughs (maximum e.m.f.): the coil sides are moving straight across the field (coil plane parallel to the field) so field lines are cut at the greatest rate.
  • Zeros: the coil sides are moving along the field (coil plane perpendicular to the field) so the rate of cutting field lines is zero.

Faster rotation means a larger peak e.m.f. and a higher frequency.

e.m.f. against time for an a.c. generator: peaks when the coil is cutting field lines fastest (coil plane parallel to the field), zeros when the coil is not cutting field lines (coil plane perpendicular to the field).
Diagram 2: e.m.f. against time for an a.c. generator: peaks when the coil is cutting field lines fastest (coil plane parallel to the field), zeros when the coil is not cutting field lines (coil plane perpendicular to the field).

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