RHYS LEWIS, AHS, DECD, UNISA / EQUINOX GRAPHICS / SCIENCE PHOTO LIBRARY RHYS LEWIS, AHS, DECD, UNISA / EQUINOX GRAPHICS / SCIENCE PHOTO LIBRARY
Animation of the inside of a loudspeaker, showing how the sound is generated by electricity and magnets. Inside the loudspeaker is a magnet, with the south pole surrounded by a coil of wire attached to a paper cone. The north pole of the magnet is outside the wire. When the wire carries a current, I, it induces a magnetic field around the wire. This interacts with the field of the magnet, producing a force that moves the coil, according to Faraday's Law of Induction. The direction of the movement can be predicted using a right-hand rule, demonstrated at bottom left. When the fingers point in the direction of the permanent magnetic field (from north to south), and the thumb finger points in the direction of the current, the palm, facing at right angles to the fingers and the thumb, faces in the direction of the force. This force moves the coil and its attached cone, which generates sound waves in the air. When the current is reversed, the force acts in the opposite direction. Controlling the varying current flowing in the wire therefore controls the vibration of the cone, and hence the sound it produces. See clip K004 3261 for this demonstration with the British left-hand rule.
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