Ac Waves and the hertz
This chapter, and this whole first section, is concerned with direct current, that is, current that always flows in the same direction, and that does not change in intensity with time. But the current used in household is not of this kind. It reverses direction periodically, exactly once every 1/120 sec. It goes for a complete cycle every 1/60 second. Every repetition is identical to another. This is alternating current. In some countries, the direction reverses every 1/100 second, and the cycle is completed every 1/50 sec.
Figure shows the characteristic wave of alternating current, as a graph of volt- age 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 the basic reason for this difference. The effective voltage for an alternating current wave is never the same as the instantaneous maximum, or peak, voltage. Actually for the common waveshape shown in the Figure drawn below, the effective value is 0.707 times the peak value. On the other hand, the peak value is 1.414 times effective value.
Figure-- One cycle of utility alternating current. The peak voltage is 165 V.
As the whole cycle repeats itself every 1/60 second, frequency of utility alternating current wave is said to be 60 Hertz, abbreviated 60 Hz. The word Hertz translates literally to cycles per second. In the United States this is the standard frequency for ac. In some places it is 50 Hz.
In radio practice, higher frequencies are common, and you'll hear about kilohertz, megahertz and gigahertz. You should know right away the size of these units, but in case you are still not sure about how the prefixes work, the relationships are as follows:
1 kHz = 1000 Hz
1 MHz = 1000 kHz = 1,000,000 Hz
1 GHz =1000 MHz = 1,000,000 kHz
Usually, the waveshapes are of the type shown in Figure given to us. This wave- form is called as a sine wave or a sinusoidal waveform.