howstuffworks:

From How Alternators Work:

An automotive charging system is made up of three major components: the battery, the voltage regulator and an alternator. The alternator works with the battery to generate power for the electrical components of a vehicle, like the interior and exterior lights and the instrument panel. An alternator gets its name from the term alternating current (AC).

For the most part, alternators are relatively small and lightweight. Roughly the size of a coconut, the alternators found in most passenger cars and light trucks are constructed using an aluminum outer housing, as the lightweight metal does not magnetize. This is important since aluminum dissipates the tremendous heat generated by producing the electrical power and since the rotor assembly produces a magnetic field.

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