Generators and Dynamos

This lesson covers AC and DC generators, including how they produce electricity and the characteristics of their output — as required by the Edexcel GCSE Physics specification (1PH0), Topic 8: Magnetism and Electromagnetism. You need to understand how generators work, the difference between AC and DC generators, and how to interpret voltage–time graphs.

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The AC Generator (Alternator)

An AC generator (also called an alternator) converts kinetic energy into electrical energy by electromagnetic induction. It produces alternating current (AC) — the current repeatedly reverses direction.

Components of an AC Generator

Component	Function
Rectangular coil	Rotates in the magnetic field; the conductor in which voltage is induced
Permanent magnets	Provide a steady magnetic field (N and S poles facing each other)
Slip rings	Two continuous rings attached to the coil; rotate with the coil
Carbon brushes	Stationary contacts that press against the slip rings; connect the coil to the external circuit
Axle	The coil rotates around this central axis

How the AC Generator Works

1. The coil is rotated in the magnetic field (e.g. by a turbine).
2. As the coil rotates, each side of the coil cuts through the magnetic field lines.
3. By electromagnetic induction, a voltage is induced across the coil.
4. Because the coil rotates continuously, each side alternately moves up and down through the field.
5. This means the direction of the induced voltage (and current) reverses every half turn.
6. The output is alternating current (AC) — the voltage rises, falls, reverses, and repeats.

The Role of Slip Rings

Slip rings are continuous rings (not split) that rotate with the coil:

• They allow the coil to rotate freely while maintaining an electrical connection to the external circuit.
• Because the rings are continuous, the alternating output is passed directly to the external circuit — the current direction reverses naturally.

Exam Tip: The key difference between an AC generator and a DC motor is the type of commutator. An AC generator uses slip rings (continuous); a DC motor uses a split-ring commutator. The slip rings allow the alternating voltage to reach the external circuit unchanged.

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Voltage–Time Graph for an AC Generator

The output voltage from an AC generator is sinusoidal (a sine wave):

graph LR
    A["Coil horizontal<br/>V = maximum<br/>(cutting field lines fastest)"] --> B["Coil vertical<br/>V = 0<br/>(moving parallel to field)"]
    B --> C["Coil horizontal again<br/>V = maximum (reversed)<br/>(cutting field opposite way)"]
    C --> D["Coil vertical again<br/>V = 0"]
    D --> A

    style A fill:#27ae60,color:#fff
    style B fill:#e74c3c,color:#fff
    style C fill:#2980b9,color:#fff
    style D fill:#e74c3c,color:#fff

Key Features of the AC Output

Position of Coil	Voltage	Explanation
Horizontal (parallel to magnets)	Maximum (positive or negative)	The coil sides are cutting through the field lines at the greatest rate
Vertical (perpendicular to magnets)	Zero	The coil sides are moving parallel to the field lines — not cutting through them

• One full rotation of the coil produces one complete cycle of AC (one full sine wave).
• The voltage is positive for one half of the rotation and negative for the other half.
• The graph crosses zero twice per cycle (at the two vertical positions).

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Increasing the Output of an AC Generator

Method	Effect
Rotate the coil faster	More field lines cut per second → larger induced voltage; also increases frequency
Use a stronger magnet	Greater magnetic flux density → larger induced voltage
Use more turns on the coil	Each turn contributes to the total induced voltage
Use a coil with a larger area	More magnetic flux is cut per rotation

Exam Tip: Rotating the coil faster increases BOTH the peak voltage AND the frequency of the AC output. On a voltage–time graph, the wave would be taller (higher peaks) AND more squashed together (shorter time period). This is a common exam question.

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Dynamos

A dynamo is a type of generator commonly found on bicycles. In a dynamo, the magnet rotates inside a fixed coil (or near a fixed coil), rather than the coil rotating in a fixed magnetic field.

How a Dynamo Works

• As the bicycle wheel turns, it spins a small magnet inside a coil of wire.
• The changing magnetic field induces a voltage in the coil.
• The faster the wheel turns, the greater the voltage (and the brighter the light).
• The output is AC because the magnetic field direction changes as the magnet rotates.

Dynamo vs Generator

Feature	Generator (alternator)	Dynamo
What moves	The coil rotates	The magnet rotates
What is fixed	The magnets	The coil
Output	AC	AC
Connection	Slip rings and brushes	Fixed coil — no slip rings needed

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The DC Generator

A DC generator produces direct current — current that flows in only one direction. It is very similar to an AC generator, but uses a split-ring commutator instead of slip rings.

How It Differs from an AC Generator

• The split-ring commutator reverses the connections to the external circuit every half turn.
• This means that every time the voltage would reverse (as it does in an AC generator), the commutator swaps the output — keeping the current flowing in the same direction.
• The output is a series of positive voltage pulses (it varies between zero and a maximum, but never goes negative).

Voltage–Time Graph Comparison

Generator Type	Graph Shape	Current Direction
AC generator (slip rings)	Sine wave — goes positive and negative	Alternates (reverses every half turn)
DC generator (split-ring commutator)	Series of positive "bumps" (like the top halves of a sine wave)	Always in the same direction

Exam Tip: Be careful to distinguish between the three devices: AC generator (slip rings → AC output), DC generator (split-ring commutator → pulsing DC output), and DC motor (split-ring commutator → converts electrical to kinetic energy). The components are similar, but the function and output are different.

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Energy Transfers in Generators

In all generators, the energy transfer is: