350 Chapter 12 Electric Current and Related Parameters
to transform because of the high voltages needed for long-distance transmission. However,
developments in the 1960s have led to techniques that now allow the transmission of direct cur-
rent over long distances.
Alternating current (ac)is the flow of electric charge that periodically reverses. As shown
in Figure 12.4 (b), the magnitude of the current starts from zero, increases to a maximum value,
and then decreases to zero; the flow of electric charge reverses direction, reaches a maximum
value, and returns to zero again. This flow pattern is repeated in a cyclic manner. The time
interval between the peak value of the current on two successive cycles is called theperiod, and
the number of cycles per second is called thefrequency. The peak ( maximum) value of the alter-
nating current in either direction is called theamplitude. Alternating current is created by gen-
erators at power plants. The current drawn by various electrical devices at your home is
alternating current. The alternating current in domestic and commercial power use is 60 cycles
per second (hertz) in the United States.Kirchhoff’s Current Law
Kirchhoff ’s current lawis one of the basic laws in electricity that allows for the analysis of
currents in electrical circuits. The law states that at any given time, the sum of the currents
entering a node must be equal to the sum of the current leaving the node. This statement isPeriod
Current
Time
(a) Direct current
Current
Time
(b) Alternating current
Amplitude
■Figure 12.4 The direct and alternating currents.
2500
2000
1500
1000
500
0
Billion killowatt-hours
Coal Petroleum Natural gas Nuclear Renewable/Other
Fuel type
■Figure 12.3(b) U.S. electricity generation by fuel type (2005).
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