How Everything Works
How Everything Works How Everything Works

Question 508

How does an electric guitar amplify the sound from the strings?
As the steel strings of an electric guitar vibrate, they move back and forth across electromagnetic pickups on the guitar's surface. Each of these pickups consists of a coil of wire with a permanent magnet passing through its center. This permanent magnet has a north magnetic pole at one end and a south magnetic pole at the other end. Surrounding the permanent magnet are lines of magnetic flux that arc gracefully through space from the magnet's north pole to its south pole. These magnetic flux lines are associated with the forces that magnets exert on one another. Some of these flux lines pass very near the permanent magnet on their way from the north pole to the south pole and thus pass inside the coil of wire around the magnet. Other flux lines arc far outward and pass outside the coil of wire around the magnet. And a few of the flux lines pass through the steel string that lies just above one pole of the permanent magnet. Steel is a ferromagnetic metal, meaning that it easily develops strong north and south poles of its own when exposed to another magnet. This ferromagnetism is the result of a remarkable ordering process that takes place among the electrons inside the steel. The steel string is magnetized by its proximity to the permanent magnet in the pickup and it interacts strongly with the magnetic flux lines that pass near it. Some flux lines leaving the north pole of the permanent magnet connect to the south pole of the magnetized string and an equal number of flux lines leaving the north pole of the magnetized string connect to the south pole of the permanent magnet. Thus when the steel string vibrates back and forth, it pulls some of the flux lines with it. The paths that these flux lines take shift back and forth rhythmically as the string vibrates.

Whenever magnetic flux lines move, they create electric fields. An electric field is a phenomenon that exerts forces on charged particles, such as the mobile electrons in the coil of wire around the permanent magnet. As the string vibrates and the magnetic flux lines shift back and forth with it, electric fields appear in the wire coil and begin to push electrons through that coil. These electrons flow back and forth in the wire as the string vibrates. Wires connecting the pickup's coil to an electronic audio amplifier carry these moving electrons (actually an electric current) to the amplifier, where they are detected and used to control a much larger electric current. When this amplified current is sent through a speaker, the speaker produces a very loud sound that's an amplified version of the sound that the string itself is making as it pushes weakly on the air.


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