Direct current (DC) is the continuous, one-way flow of electrons through a conductor such as copper wire. This constant movement of electrons in one way is similar to the flow of water through a pipe. A closed DC circuit is necessary for the stream of electron to flow. This circuit consists of a source of electrical energy, such as a battery or solar cells, and a metal wire running from the positive end of the source to the negative terminal.
However, to produce and transport DC over long distance for massive industrial and home use is very expensive. A more practical and much less expensive way to provide the industry and house with energy is alternating current (AC). To produce this form of electricity, you do not need batteries but powerful generators, using electromagnets. Thus, direct current has been limited to all sort of batteries, such as the one employed in your car for the lights and starter motor.
Direct current is used in nearly all electronic systems as their power supply and also to charge batteries, which are the primary sources of DC. Some railway propulsion use direct current, too, specially in urban areas. High voltage direct current is used to interconnect alternating current power grids.
Direct current is commonly found in many low-voltage applications, specially where these are powered by batteries, which can produce only DC, or solar power systems, since solar cells can produce only DC. Most automotive applications use DC, although the alternator is an AC device which uses a rectifier to produce DC. Most electronic circuits require a DC power supply. Applications using fuel cells, mixing hydrogen and oxygen together with a catalyst to produce electricity and water as byproducts, also produce only DC.
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