Generators and Microphones (Higher)

This lesson covers Higher Tier content. Questions on generators, alternators, dynamos, loudspeakers and microphones may appear only on Higher Tier papers in the Edexcel GCSE Combined Science specification (1SC0).

This lesson builds on electromagnetic induction to explain how AC generators (alternators), DC generators (dynamos), loudspeakers and microphones work.

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The AC Generator (Alternator)

An AC generator (alternator) converts kinetic energy into electrical energy by rotating a coil in a magnetic field.

Structure

Component	Role
Rectangular coil	Rotates in the magnetic field; a voltage is induced in it
Permanent magnets	Provide the external magnetic field
Slip rings	Two complete rings attached to the coil; rotate with it
Brushes	Stationary contacts that press against the slip rings and connect to the external circuit

How It Works

1. The coil rotates in the magnetic field.
2. As the coil cuts through field lines, an alternating voltage is induced (electromagnetic induction).
3. The slip rings rotate with the coil and maintain continuous electrical contact via the brushes.
4. Because the coil repeatedly passes through the field in alternating directions, the output is alternating current (AC).

Output Voltage Trace

The voltage produced by an AC generator is a sine wave:

Coil Position	Voltage
Coil parallel to field (sides cutting max field lines)	Maximum (peak) voltage
Coil perpendicular to field (sides cutting no field lines)	Zero voltage

The voltage alternates between positive and negative peaks, passing through zero twice per revolution.

Exam Tip: The voltage is at a maximum when the coil plane is parallel to the field (sides moving perpendicular to the field lines, cutting through them fastest). It is zero when the coil plane is perpendicular to the field (sides moving along the field lines, not cutting any).

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Increasing the Generator Output

Factor	Effect
Increase the speed of rotation	Increases the frequency and the peak voltage
Use a stronger magnet	Increases the peak voltage
Increase the number of turns	Increases the peak voltage

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AC Generator vs DC Generator (Dynamo)

Feature	AC Generator (Alternator)	DC Generator (Dynamo)
Output	Alternating current (AC)	Direct current (DC) — always in one direction
Contact rings	Slip rings (two complete rings)	Split-ring commutator (two half-rings)
Output waveform	Sine wave	Positive-only bumpy waveform

The split-ring commutator in a dynamo reverses the connections every half turn, so the current in the external circuit always flows in the same direction (though it still varies in magnitude).

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Loudspeakers

A loudspeaker converts an electrical signal (AC) into sound waves. It uses the motor effect.

How a Loudspeaker Works

1. An alternating current passes through a coil of wire (the voice coil) attached to a cone (diaphragm).
2. The coil sits inside the field of a permanent magnet.
3. The AC causes the coil to experience alternating forces (motor effect), making it vibrate back and forth.
4. The cone vibrates with the coil, pushing and pulling the surrounding air to create sound waves (compressions and rarefactions).
5. The frequency of the AC determines the pitch of the sound; the amplitude determines the loudness.

graph LR
    subgraph "Loudspeaker"
    direction LR
    AC["AC signal"] --> Coil["Voice coil vibrates (motor effect)"]
    Coil --> Cone["Cone vibrates"]
    Cone --> Sound["Sound waves produced"]
    end

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Microphones

A microphone converts sound waves into an electrical signal. It uses electromagnetic induction.

How a Microphone Works

1. Sound waves hit a diaphragm, causing it to vibrate.
2. Attached to the diaphragm is a coil of wire that sits inside the field of a permanent magnet.
3. As the coil vibrates back and forth, it experiences a changing magnetic field → a voltage is induced (electromagnetic induction).
4. The induced voltage varies with the frequency and amplitude of the sound wave, producing an electrical signal that represents the original sound.

Comparing Loudspeakers and Microphones

Feature	Loudspeaker	Microphone
Energy conversion	Electrical → kinetic → sound	Sound → kinetic → electrical
Principle	Motor effect	Electromagnetic induction
Input	AC electrical signal	Sound waves
Output	Sound waves	AC electrical signal

Exam Tip: A loudspeaker and a microphone are essentially the reverse of each other. In fact, you can use a loudspeaker as a crude microphone and vice versa. Knowing this helps you remember how each works.

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Worked Example

A student connects a small loudspeaker to a CRO (cathode ray oscilloscope) and speaks into the loudspeaker. A waveform appears on the CRO screen. Explain why.

When the student speaks, sound waves cause the loudspeaker cone (diaphragm) to vibrate. The attached coil moves back and forth inside the magnetic field of the permanent magnet. This changing magnetic field induces an alternating voltage in the coil (electromagnetic induction). The CRO displays this varying voltage as a waveform. The loudspeaker is acting as a microphone.

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Common Misconceptions

Misconception	Correction
An AC generator uses a split-ring commutator	No — an AC generator uses slip rings; the split-ring commutator is used in a DC generator (dynamo)
The voltage is maximum when the coil is parallel to the field	The voltage is zero when parallel; it is maximum when perpendicular
A loudspeaker works by electromagnetic induction	A loudspeaker uses the motor effect; a microphone uses electromagnetic induction

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Summary

• An AC generator (alternator) rotates a coil in a magnetic field using slip rings to produce AC.
• A DC generator (dynamo) uses a split-ring commutator to produce a unidirectional (DC) output.
• The output voltage is increased by rotating faster, using a stronger magnet or more turns on the coil.
• A loudspeaker uses the motor effect: AC through a coil in a magnetic field vibrates a cone to produce sound.
• A microphone uses electromagnetic induction: sound vibrates a coil in a magnetic field, inducing an AC signal.

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Deeper Dive: The Sine Wave Output

Why does a rotating coil produce a sine wave voltage? Consider the rate at which the coil cuts magnetic field lines as it spins:

Coil Position	Motion of Sides	Rate of Cutting Field Lines	Induced EMF
Plane parallel to field (horizontal)	Sides moving perpendicular to field	Maximum	Maximum (peak)
Plane at 45°	Intermediate	Intermediate	~70% of peak
Plane perpendicular to field (vertical)	Sides moving along field lines	Zero	Zero
Past 90°	Sides moving perpendicular but opposite direction	Maximum	Peak (negative)

The EMF is therefore proportional to $\sin(\omega t)$sin(ωt), where $\omega$ω is the angular velocity of the coil. Because the slip rings simply maintain contact, the external circuit sees this sinusoidal voltage as an alternating current.