Definition of alternating current
Recall that direct current has a polarity, or direction, that stays the same over a long period
of time. Although the magnitude might vary—the number of amperes, volts, or
watts can fluctuate—the charge carriers always flow in the same direction through the
circuit.
In alternating current, the polarity reverses again and again at regular intervals.
The magnitude usually changes because of this constant reversal of polarity, although
there are certain cases where the magnitude doesn’t change even though the polarity
does.
The rate of change of polarity is the third variable that makes ac so much different
from dc. The behavior of an ac wave depends largely on this rate: the frequency.
Period and frequency
In a periodic ac wave, the kind that is discussed in this chapter (and throughout the rest
of this book), the function of magnitude versus time repeats itself over and over, so that
the same pattern recurs countless times. The length of time between one repetition of the
pattern, or one cycle, and the next is called the period of the wave. This is illustrated in
Fig. 9-1 for a simple ac wave. The period of a wave can, in theory, be anywhere from a
minuscule fraction of a second to many centuries. Radio-frequency currents reverse polarity
millions or billions of times a second. The charged particles held captive by the magnetic
field of the sun, and perhaps also by the much larger magnetic fields around
galaxies, might reverse their direction over periods measured in thousands or millions of
years. Period, when measured in seconds, is denoted by T.
The frequency, denoted f, of a wave is the reciprocal of the period. That is, f = 1/T
and T = 1/f .Originally, frequency was specified in cycles per second, abbreviated cps.
High frequencies were sometimes given in kilocycles, megacycles, or gigacycles, representing
thousands, millions, or billions of cycles per second. But nowadays, the unit is
known as the hertz, abbreviated Hz. Thus, 1 Hz = 1 cps, 10 Hz = 10 cps, and so on.
Higher frequencies are given in kilohertz (kHz), megahertz (MHz), or gigahertz
(GHz). The relationships are:
1 kHz = 1000 Hz
1 MHz = 1000 kHz = 1,000,000 Hz
1 GHz = 1000 MHz = 1,000,000,000 Hz
Sometimes an even bigger unit, the terahertz (THz), is needed. This is a trillion
(1,000,000,000,000) hertz. Electrical currents generally do not attain such frequencies,
although electromagnetic radiation can.
Some ac waves have only one frequency. These waves are called pure. But often,
there are components at multiples of the main, or fundamental, frequency. There
might also be components at “odd” frequencies. Some ac waves are extremely complex,
consisting of hundreds, thousands, or even infinitely many different component,
frequencies. In this book, most of the attention will be given to ac waves that have just
one frequency
