Alternating Current and the Hertz

These topics, and this whole first section, is mostly concerned with direct current (dc). That’s electric current that always flows in the same direction and that does not change in intensity (at least not too rapidly) with time. But household utility current is not of this kind. It reverses direction periodically, exactly once every 1/120 second. It goes through a complete cycle every 1/60 second. Every repetition is identical to every other. This is alternating current (ac).

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One cycle of utility alternating current (ac). The instantaneous voltage is the voltage at any particular instant in time. The peak voltages are approximately plus and minus 165 V.

Above figure shows the characteristic wave of ac, as a graph of voltage versus time. Notice that the maximum positive and negative voltages are not 117 V, as you’ve heard about household electricity, but close to 165 V. There is a reason for this difference. The effective voltage for an ac wave is never the same as the instantaneous maximum, or peak, voltage. In fact, for the common waveform shown in following figure, the effective value is 0.707 times the peak value. Conversely, the peak value is 1.414 times the effective value.

Because the whole cycle repeats itself every 1/60 second, the frequency of the utility ac wave is said to be 60 hertz, abbreviated 60 Hz. The German word hertz literally translates to “cycles per second.” In the United States, this is the standard frequency for ac. In some places it is 50 Hz. In wireless communications, higher frequencies are common, and you’ll hear about kilohertz (kHz), megahertz (MHz), and gigahertz (GHz). The relationships among these units are as follows:

1 kHz = 1000 Hz
1 MHz = 1000 kHz = 1,000,000 Hz
1 GHz = 1000 MHz = 1,000,000 kHz = 1,000,000,000 Hz

Usually, but not always, the waveshapes are of the type shown in above figure. This waveform is known as a sine wave or a sinusoidal waveform.