The a.c. generator
Section: Electricity & Magnetism | Syllabus: Cambridge IGCSE Physics (0625)
Structure of a Simple A.C. Generator
An a.c. generator converts kinetic energy to electrical energy using electromagnetic induction.
The coil rotates between the magnet poles while slip rings and brushes keep it connected to the external circuit
Key Components
- Coil: Rectangular wire coil that rotates between magnetic poles
- Permanent magnets: Provide the magnetic field
- Slip rings: Two separate rings attached to each end of the coil
- Brushes: Carbon contacts that maintain connection as coil rotates
How the Generator Works
- The coil is rotated (by an external force, e.g., turbine)
- Coil wires cut through magnetic field lines
- E.m.f. is induced in the coil (electromagnetic induction)
- Slip rings rotate with the coil
- Brushes transfer the current to the external circuit
- As coil rotates, direction of induced e.m.f. reverses every half turn
Why A.C.?
The coil sides alternately move up and down through the field as it rotates, so the direction of the induced e.m.f. reverses each half rotation, producing alternating current.
E.M.F. – Time Graph
The e.m.f. varies sinusoidally as the coil completes one full rotation
Coil Position and E.M.F.
| Coil Position | Cutting Field Lines | E.M.F. |
|---|---|---|
| Horizontal (0°, 180°, 360°) | Maximum rate | Maximum (peak) |
| Vertical (90°, 270°) | Parallel to field (zero rate) | Zero |
Frequency of the A.C. Output
The induced e.m.f. acts first in one direction then the other, driving an alternating current (a.c.) around the circuit. The frequency of an a.c. is the number of complete cycles it makes each second, measured in hertz (Hz): 1 cycle per second = 1 Hz.
- If the coil rotates twice per second, the a.c. has a frequency of 2 Hz
- The mains supply is a.c. of frequency 50 Hz
Increasing Generator Output
- Rotate coil faster (increases frequency and peak e.m.f.)
- Use more turns on coil (increases peak e.m.f.)
- Use stronger magnets (increases peak e.m.f.)
- Use larger coil area (more field lines cut)
Alternative Design: Rotating Magnet
Some generators have a stationary coil with a rotating magnet inside. This avoids the need for slip rings on the output coil.
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