GCSE Physics revision

Loudspeakers (physics only) (HT only)

The motor effect

AQA 4.7.2.4

Your specification

AQA student objectives

Learning pathway

ALL 🎯

Explain how moving-coil loudspeakers and headphones use the motor effect to convert variations in electrical current into sound-pressure waves.

MOST 🎯🎯

Apply the scientific explanation of loudspeakers to a relevant example.

SOME 🎯🎯🎯

Analyse a new situation involving loudspeakers and explain the scientific reasoning.

Revision summary

Read on screen, then print for Cornell-style active revision.

The motor effect describes the force experienced by a current-carrying wire when it is placed inside an external magnetic field, caused by the interaction of the two magnetic fields.
Any current-carrying wire produces its own magnetic field around it, which interacts with the field of a permanent magnet to produce a force on the wire.

1. The Motor EffectAQA

The motor effect describes the force experienced by a current-carrying wire when it is placed inside an external magnetic field, caused by the interaction of the two magnetic fields.
Any current-carrying wire produces its own magnetic field around it, which interacts with the field of a permanent magnet to produce a force on the wire.

A loudspeaker consists of a cone attached to a coil of wire, which is positioned inside the magnetic field of a permanent magnet.
The cone is responsible for pushing air back and forth to create the pressure variations we detect as sound.
The coil carries an alternating current (AC), which is essential to the operation of the loudspeaker.

2. Structure of a LoudspeakerAQA

A loudspeaker consists of a cone attached to a coil of wire, which is positioned inside the magnetic field of a permanent magnet.
The cone is responsible for pushing air back and forth to create the pressure variations we detect as sound.
The coil carries an alternating current (AC), which is essential to the operation of the loudspeaker.

As the alternating current flows through the coil, it induces a changing magnetic field that continuously interacts with the permanent magnet's field.
The direction of the force on the coil alternates between attraction and repulsion, causing the cone to vibrate back and forth.
These vibrations of the cone create compressions and rarefactions in the surrounding air, producing sound waves.

3. How Sound is ProducedAQA

As the alternating current flows through the coil, it induces a changing magnetic field that continuously interacts with the permanent magnet's field.
The direction of the force on the coil alternates between attraction and repulsion, causing the cone to vibrate back and forth.
These vibrations of the cone create compressions and rarefactions in the surrounding air, producing sound waves.

The pitch of the sound produced depends on the frequency of the alternating current supplied to the coil.
Higher frequency AC produces higher-pitched sounds, whilst lower frequency AC produces lower-pitched sounds.

The volume of the sound produced is controlled by the size (amplitude) of the current flowing through the coil.
A greater current induces a stronger magnetic field, resulting in a larger force on the cone and greater vibrations, producing a louder sound.