7.2.4 Loudspeakers and headphones (HT)
Converting an electrical signal into sound
Definition
Motor effect
The motor effect is the force experienced by a current-carrying conductor when it is placed in a magnetic field.
- Loudspeakers and headphones use the motor effect to convert a varying current into sound.
- The current passes through a coil in the field of a permanent magnet, so a force acts on the coil.
- As the current varies, the force varies, so the coil moves backwards and forwards.
Key Idea
A loudspeaker converts variations in current into variations in force, then into movement of a cone or diaphragm, and finally into pressure variations in the air.
How a moving-coil loudspeaker works
- A permanent magnet produces a magnetic field.
- A coil of wire sits within this field, with a cone attached to it.
- A varying current passes through the coil, and the motor effect produces a force on it.
- When the size of the current changes, the size of the force changes, and when the direction of the current reverses, the direction of the force reverses.
- The changing force makes the coil, and the attached cone, move backwards and forwards.
- The moving cone pushes the surrounding air, producing pressure variations that travel as a sound wave.
- Faster variations in current give a higher-frequency sound, and larger variations give larger cone movements and a louder sound.
How moving-coil headphones work
- Headphones work in the same way as a loudspeaker, but use a much smaller coil and a small diaphragm instead of a large cone.
- A varying current in the coil, which is in the field of a permanent magnet, produces a changing force through the motor effect.
- The coil and diaphragm move backwards and forwards, creating pressure variations in the air near the ear that form the sound wave the listener hears.
Common Mistake
- Do not say only that the current makes the cone vibrate; explain that a current-carrying coil in a magnetic field feels a motor-effect force.
- Sound is not an electrical current travelling through the air; the current moves the cone or diaphragm, and that movement produces pressure variations.
Exam technique
- Give a complete chain: current in the coil, coil in a magnetic field, motor-effect force, varying current gives a varying force, coil and cone move, pressure variations, sound wave.
- Use the phrase pressure variations, rather than stopping at the cone vibrating.
Self review
- What force acts on a current-carrying coil placed in a magnetic field?
- Why does the force on the coil vary?
- What happens to the force when the current reverses direction?
- How does the movement of the coil make the loudspeaker cone move?
- How does a moving cone or diaphragm produce a sound wave?
- How is the operation of moving-coil headphones similar to that of a loudspeaker?
