Electricity

D

Electric Effects of Magnetism

If a wire is moved through a magnetic field in such a way that it cuts the magnetic lines of force, a voltage is created across the wire. An electric current will flow through the wire if the two ends of the wire are connected by a conductor to form a circuit. This current is called an induced current, and the induction of a current in this manner is called electromagnetic induction.

It does not matter whether the wire moves or the magnetic field moves, provided that the wire cuts through lines of force. If a magnet is moved near a stationary wire, the lines of magnetic force are cut by the wire and an electric current is induced in the wire.

Like any electric current, an induced current generates a magnetic field around it. Lenz’s law expresses an important fact concerning this magnetic field: The motion of an induced current is always in such a direction that its magnetic field opposes the magnetic field that is causing the current.

X

Alternating Current

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.

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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.

A

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.

B

Impedance

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.

C

Advantages of Alternating Current

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.

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