Alternating Current

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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.

Topics:

• Amperage and Voltage
• Impedance
• Advantages of Alternating Current

Advantages of Alternating Current

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Alternating current has several characteristics that make it more attractive than direct current as a source of electric power, both for industrial installations and in the home. The most important of these characteristics is that the voltage or the current may be changed to almost any value desired by means of a simple electromagnetic device called a transformer. When an alternating current surges back and forth through a coil of wire, the magnetic field about the coil expands and collapses and then expands in a field of opposite polarity and again collapses. In a transformer, a coil of wire is placed in the magnetic field of the first coil, but not in direct electric connection with it. The movement of the magnetic field induces an alternating current in the second coil. If the second coil has more turns than the first, the voltage induced in the second coil will be larger than the voltage in the first, because the field is acting on a greater number of individual conductors. Conversely, if there are fewer turns in the second coil, the secondary, or induced, voltage will be smaller than the primary voltage.

The action of a transformer makes possible the economical transmission of electric power over long distances. If 200,000 watts of power is supplied to a power line, it may be equally well supplied by a potential of 200,000 volts and a current of 1 amp or by a potential of 2,000 volts and a current of 100 amp, because power is equal to the product of voltage and current. The power lost in the line through heating, however, is equal to the square of the current times the resistance. Thus, if the resistance of the line is 10 ohms, the loss on the 200,000-volt line will be 10 watts, whereas the loss on the 2,000-volt line will be 100,000 watts, or half the available power. Accordingly, power companies tend to favor high voltage lines for long distance transmission.