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Alternating Current

reactance, amperes, alternating current, IZ, Amperage

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An alternating current is an electric current that changes direction at regular intervals. When a conductor is moved back and forth in a magnetic field, the flow of current in the conductor will reverse direction as often as the physical motion of the conductor reverses direction. Most electric power stations supply electricity in the form of alternating currents. The current flows first in one direction, builds up to a maximum in that direction, and dies down to zero. It then immediately starts flowing in the opposite direction, builds up to a maximum in that direction, and again dies down to zero. Then it immediately starts in the first direction again. This surging back and forth can occur at a very rapid rate.

Two consecutive surges, one in each direction, are called a cycle. The number of cycles completed by an electric current in one second is called the frequency of the current. In the United States and Canada, most currents have a frequency of 60 cycles per second.

Although direct and alternating currents share some characteristics, some properties of alternating current are somewhat different from those of direct current. Alternating currents also produce phenomena that direct currents do not. Some of the unique traits of alternating current make it ideal for power generation, transmission, and use.

Amperage and Voltage

The strength, or amperage, of an alternating current varies continuously between zero and a maximum. Since it is inconvenient to take into account a whole range of amperage values, scientists simply deal with the effective amperage. Like a direct current, an alternating current produces heat as it passes through a conductor. The effective amperage of an alternating current is equal to the amperage of a direct current that produces heat at the same rate. In other words, 1 effective amp of alternating current through a conductor produces heat at the same rate as 1 amp of direct current flowing through the same conductor. Similarly, the voltage of an alternating current is considered in terms of the effective voltage.


Like direct current, alternating current is hindered by the resistance of the conductor through which it passes. In addition, however, various effects produced by the alternating current itself hinder the alternating current. These effects depend on the frequency of the current and on the design of the circuit, and together they are called reactance. The total hindering effect on an alternating current is called impedance. It is equal to the resistance plus the reactance.

The relationship of effective current, effective voltage, and impedance is expressed by V = IZ, where V is the effective voltage in volts, I is the effective current in amperes (amp), and Z is the impedance in ohms.

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