The AC GeneratorCambridge IGCSE Physics: Revision notes
Section 1
Electromagnetic Induction — the Basis of Generators
Generators work because of electromagnetic induction: a conductor moving across a magnetic field, or a changing magnetic field linking with a conductor, induces an e.m.f. in that conductor.
This can be demonstrated experimentally by moving a magnet in and out of a coil connected to a sensitive meter (galvanometer) and observing a deflection, showing an induced e.m.f. and current.
There must be relative motion, or a changing field — a stationary conductor in a steady magnetic field will not have an e.m.f. induced in it.
Section 2
What Affects the Size of an Induced e.m.f.?
Several factors affect the magnitude of an induced e.m.f.:
- The speed of relative motion between the conductor and the field — faster motion induces a larger e.m.f.
- The strength of the magnetic field — a stronger field induces a larger e.m.f.
- The number of turns on a coil — more turns induce a larger e.m.f.
The direction of the induced e.m.f. always opposes the change that causes it.
Moving a magnet faster into a coil produces a bigger deflection on a galvanometer than moving it slowly, because a larger e.m.f. is induced.
Section 3
Relative Directions of Force, Field and Current
The direction of the induced current depends on the relative directions of motion, field and force. These three directions are mutually related, and this relationship can be used to predict which way current flows for a given motion and field direction.
Section 4
A Simple A.C. Generator
A simple a.c. generator consists of a coil rotating in a magnetic field (or a magnet rotating inside a fixed coil).
- As the coil rotates, it continually changes its orientation relative to the field, inducing a changing e.m.f.
- Slip rings and brushes are used to connect the rotating coil to the external circuit without the wires becoming twisted.
- Because the coil's orientation reverses every half-turn, the induced e.m.f. also reverses direction every half-turn, producing alternating current.
Don't confuse slip rings (used in a.c. generators, allowing continuous reversal of connection) with the split-ring commutator used in d.c. motors, which reverses connections to keep current flowing one way.
Section 5
The e.m.f.–Time Graph of an A.C. Generator
A graph of e.m.f. against time for a simple a.c. generator is a smooth wave.
- The e.m.f. is zero when the coil is parallel to the field (moving parallel to the field lines, cutting no field lines).
- The e.m.f. is at a peak (maximum, positive or negative) when the coil is perpendicular to the field (cutting field lines fastest).
- One full rotation of the coil corresponds to one complete cycle of the e.m.f.–time graph.
Must Know
- A changing magnetic field, or relative motion between a conductor and a field, induces an e.m.f. (electromagnetic induction).
- Induced e.m.f. increases with speed of motion, field strength, and number of coil turns; its direction always opposes the change causing it.
- An a.c. generator uses a rotating coil (or magnet) with slip rings and brushes to produce an alternating e.m.f.
- The e.m.f.–time graph is zero when the coil is parallel to the field and peaks when the coil is perpendicular to the field.
- One full coil rotation = one complete cycle of the a.c. output.
That's the notes covered.
Carry on to the next subtopic.