The Moving Coil Loudspeaker
The moving coil loudspeaker used in radios and stereo systems works by varying the force exerted on a current carrying coil in a magnetic field.
Electric fields are generated by the electronic circuitry inside the system corresponding to the pattern of dots on a compact disc or the signal received by a radio. These are passed through a short cylindrical voice coil free to move in the radial magnetic field generated by a system of magnets. The magnetic field cuts the turns of the voice coil at right angles, and consequently, as the current through the coil varies, the voice coil will move up and down in accordance with Fleming's left hand rule.