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Electricity - Alternating Current And Direct Current

electrical power voltage unit

Direct current, or DC, results from an electric charge that moves in only one direction. A car's battery, for example, provides a direct current when it forces electrical charge through the starter motor or through the car's headlights. The direction of this current does not change.

Current that changes direction periodically is called alternating current, or AC. Our homes are supplied with alternating current rather than direct current because the use of AC makes it possible to step voltage up or down, using an electromagnetic device called a transformer. Without transformers to change voltage as needed, it would be necessary to distribute electrical power at a safer low voltage but at a much higher current. The higher current would increase the transmission loss in the powerlines. Without the ability to use high voltages, it would be necessary to locate generators near locations where electric power is needed.

Southern California receives much of its electrical power from hydroelectric generators in the state of Washington by a connection through an unusually long DC transmission line that operates at approximately one million volts. Electrical power is first generated as alternating current, transformed to a high voltage, then converted to direct current for the long journey south. The direct-current power is changed back into AC for final distribution at a lower voltage. The use of direct current more than compensates for the added complexity of the AC to DC and DC to AC conversions.



Asimov, Isaac. Understanding Physics: Light, Magnetism, and Electricity. Vol. 2. Signet Science Series. New York: NAL, 1969.

Giancoli, Douglas C. Physics: Principles With Applications. 3rd ed. Upper Saddle River, NJ: Prentice Hall, 1991.

Hewitt, Paul. Conceptual Physics. Englewood Cliffs, NJ: Prentice Hall, 2001.

Donald Beaty


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—A standard unit for measuring electric current.


—Materials that permit electrons to move freely.

Coulomb force

—Another name for the electric force.

Electric field

—The concept used to describe how one electric charge exerts force on another, distant electric charge.


—A device for converting kinetic energy (the energy of movement) into electrical energy.


—An object or material that does not conduct heat or electricity well.


—The unit of energy in the mks system of measurements.


—The unit of electrical resistance.

Semiconductor devices

—Electronic devices made from a material that is neither a good conductor or a good insulator.


—A standard unit of electric potential and electromotive force


—The basic unit of electrical power equal to 1 joule per second.

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