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Devices using varying field


You can change the strength of an electromagnet's magnetic field by varying the electrical current that passes through the wires wrapped around it. If you change the direction of the electrical current the polarity of the magnetic field reverses.
These effects can be used to move a loudspeaker cone back and forth, creating sound according to the electrical current through the wire. They also can be used to create magnetic fields in a magnetic tape or computer hard drive, such that it stores information.
Loudspeaker
The loudspeakers in your radio, television or stereo system consists of a permanent magnet surrounding an electromagnet that is attached to the loudspeaker membrane or cone.
By varying the electric current through the wires around the electromagnet, the electromanget and the speaker cone can be made to back and forth. If the variation of the electric current is at the same frequencies of sound waves, the resulting vibration of the speaker cone will create sound waves, including that from voice and music.

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