An ALTERNATOR is an AC generator — produces alternating current.
CONSTRUCTION:
Coil of wire rotating in a magnetic field.
SLIP RINGS and BRUSHES connect the rotating coil to the external circuit.
Because slip rings rotate with the coil, they don't reverse the connections — AC produced.
OUTPUT:
Sinusoidal AC — voltage varies as sine wave.
Frequency of AC = frequency of coil rotation.
UK mains: 50 Hz — the alternator must rotate at 50 revolutions per second (or 3000 rpm).
USES:
Power stations: large alternators driven by steam turbines or gas turbines.
Car alternators: charge the car battery while the engine runs.
Wind turbines: blades drive an alternator (or permanent magnet generator).
Dynamos
A DYNAMO produces direct current (DC) using a split-ring commutator.
CONSTRUCTION:
Same as alternator but with a SPLIT-RING COMMUTATOR instead of slip rings.
The commutator is a ring split into two halves.
Brushes make contact with the two halves.
Every half-rotation, the connections swap → current in external circuit always flows in the same direction.
OUTPUT:
Pulsing DC — voltage varies from 0 to maximum, never reverses.
Not perfectly smooth — usually smoothed with capacitors for practical use.
USES:
Bicycle dynamos: wheel drives a small permanent magnet generator → powers lights.
Small DC generators in portable equipment.
ALTERNATOR vs DYNAMO:
Alternator: slip rings → AC output.
Dynamo: split-ring commutator → pulsing DC output.
Both: rotating coil in magnetic field (or rotating magnet in fixed coil).
Electromagnetic Induction in Practice
ROTATING MAGNET GENERATORS (modern design):
Many modern generators rotate the MAGNET inside a fixed coil of wire.
Advantage: no slip rings needed for the high-current output — simpler, more reliable.
The changing flux through the fixed coil still induces an emf.
WIND TURBINES:
Blades rotate by wind → drive a gearbox → speed up rotation → drive a generator.
Modern turbines often use permanent magnet generators without a gearbox.
Output: AC electricity fed to the grid.
HYDROELECTRIC POWER:
Falling water drives turbines → alternators → electricity.
Very efficient (~90%): gravitational PE → kinetic → electrical.
NUCLEAR AND GAS POWER STATIONS:
Heat from nuclear fission or burning gas → steam → turbine → alternator.
Electrical output: typically 50 Hz AC.
BACK-EMF in MOTORS:
A running electric motor generates a back-emf that opposes the supply voltage.
At startup: back-emf = 0, current is high → motor may overheat.
At full speed: back-emf nearly equals supply voltage → current is low, motor runs efficiently.
⚠️ Common Mistake
ALTERNATORS use SLIP RINGS (give AC). DYNAMOS use SPLIT-RING COMMUTATORS (give DC). It's the type of connection, not the mechanical principle, that determines AC vs DC output. In modern generators, the magnet often rotates inside a fixed coil — but electromagnetic induction still occurs.
📌 Key Note
Alternator: slip rings → AC output (50 Hz in UK). Dynamo: split-ring commutator → pulsing DC. Uses: alternators in power stations (steam turbines), car engines (charges battery), wind turbines. Dynamos: bicycle lights. Back-emf in motors: opposes supply, limits current at full speed.
🎯 Matching Activity — Alternators and Dynamos
Match each generator type to its construction and output. — drag the symbols on the right to match the component names on the left.
Opposing voltage generated by rotating motor — limits current at full speed
⭐ Higher Tier Only
HT only — describe the construction and output of alternators and dynamos. Explain the difference between slip rings (AC) and split-ring commutators (DC). Describe back-emf in electric motors.
🔬 Triple Science Only
Uses of the generator effect — alternators and dynamos (HT only, physics only) — not in Combined Science.
🎯 Test Yourself
Question 1 of 2
1. Why does an alternator produce AC while a dynamo produces DC?
2. What is 'back-emf' in an electric motor, and why is it important?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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🤖 Ask Mr Badmus AI
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