Further physics - The operation of a speaker
Cheung Kai-chung (Translation by Janny Leung)  

A loudspeaker
Fig. 1  Loudspeaker
The structure of a loudspeaker
Fig. 2  The structure of a loudspeaker
A speaker uses electromagnets to transform electric current into sound (Fig. 1). Electric current and magnetic force have a close relationship. Try to wind a copper wire round a long iron nail and connect it with a small battery, you would discover that the iron nail attracts paperclips. When an electric current is passed through the wire, a magnetic field would be produced, and the direction of the magnetic field could be determined by the right hand thumb rule.

A speaker uses both an electromagnet and a permanent magnet (Fig. 2). Suppose we now have to play a C tone (the frequency being 256 Hz, that is 256 vibrations per second), the player would output an alternating current. That is, the direction of the current alternates 256 times in a second. Every time when the current changes its direction, the direction of the magnetic field produced by the wire round the iron nail also changes. Everyone knows that the "Like poles repel, unlike poles attract" principle applies to magnetic force. The magnetic pole of the coil keeps on alternating, sometimes attracting the permanent magnet and sometimes repelling it, producing 256 vibrations per second. Attach a diaphragm to the coil. When they vibrate together, the surrounding air would be mobilized. Isn't vibration of air sound? This is the operation of a speaker.

Actually there are a lot of electrical appliances that make use of the same mechanism! Can you list them?