The loudspeaker
A magnetic field exerting a force on a conductor carrying a current is a very useful effect in everyday life. The electric motor, the loudspeaker, the galvanometer, the voltmeter and the ammeter (analogue) are some of the devices built on it.
Figure 13.9 shows the outside of a loudspeaker and how it is made. A loudspeaker produces a sound when an electric current that varies with the wave shape of that sound is made to flow through its coil.
Figure to be added
| Part | What it does |
|---|---|
| Ring magnet | Makes a strong field across the gap where the coil sits |
| Coil | Carries the varying current and is pushed by the field |
| Light cardboard cone | Moves with the coil and pushes the air to make sound waves |
| Metal frame | Holds the magnet and the wide end of the cone |
The coil is fixed to the narrow end of the cone so that it can vibrate freely backwards and forwards in the region between the magnet's poles. When a varying current flows through the coil, the force the magnet exerts on the conductor makes the coil vibrate back and forth in step with the changes in the current, and the cone vibrates with it and produces sound waves.
Why the cone copies the sound
By the left-hand rule, current one way round the coil pushes it forward, and current the other way pulls it back. A bigger current gives a bigger push. So when the current rises and falls hundreds of times a second in the pattern of a voice, the coil and cone move in exactly that pattern, pushing and pulling the air. That makes compressions and rarefactions — the sound wave you studied in chapter 4. The electrical energy of the current becomes the kinetic energy of the cone, and then sound energy in the air.
A loudspeaker is a motor effect
A varying current in the coil, in the magnet's field, makes the coil and cone vibrate in step with it: electrical energy becomes sound.