Amplitude of alternating current
Amplitude is sometimes called magnitude, level, or intensity. Depending on the quantity
being measured, the magnitude of an ac wave might be given in amperes (for current),
volts (for voltage), or watts (for power).
Instantaneous amplitude
The instantaneous amplitude of an ac wave is the amplitude at some precise moment
in time. This constantly changes. The manner in which it varies depends on the waveform.
You have already seen renditions of common ac waveforms in this chapter. Instantaneous
amplitudes are represented by individual points on the wave curves.
Peak amplitude
The peak amplitude of an ac wave is the maximum extent, either positive or negative,
that the instantaneous amplitude attains.
In many waves, the positive and negative peak amplitudes are the same. But sometimes
they differ. Figure 9-9 is an example of a wave in which the positive peak amplitude
is the same as the negative peak amplitude. Figure 9-10 is an illustration of a wave
that has different positive and negative peak amplitudes.
Peak-to-peak amplitude
The peak-to-peak (pk-pk) amplitude of a wave is the net difference between the positive
peak amplitude and the negative peak amplitude (Fig. 9-11). Another way of saying
this is that the pk-pk amplitude is equal to the positive peak amplitude plus the
negative peak amplitude. Peak-to-peak is a way of expressing how much the wave level
“swings” during the cycle.
In many waves, the pk-pk amplitude is just twice the peak amplitude. This is the
case when the positive and negative peak amplitudes are the same.
Root-mean-square amplitude
Often, it is necessary to express the effective level of an ac wave. This is the voltage, current
or power that a dc source would have to produce, in order to have the same general
effect. When you say a wall outlet has 117 V, you mean 117 effective volts. The most common
figure for effective ac levels is called the root-mean-square, or rms, value.
For a perfect sine wave, the rms value is equal to 0.707 times the peak value, or
0.354 times the pk-pk value. Conversely, the peak value is 1.414 times the rms value, and
the pk-pk value is 2.828 times the rms value. The rms figures are most often used with
perfect sine waves, such as the utility voltage, or the effective voltage of a radio signal.
For a perfect square wave, the rms value is the same as the peak value. The pk-pk
value is twice the rms value or the peak value.
For sawtooth and irregular waves, the relationship between the rms value and the
peak value depends on the shape of the wave. But the rms value is never more than the
peak value for any waveshape.
The name “root mean square” was not chosen just because it sounds interesting.
It literally means that the value of a wave is mathematically operated on, by taking
the square root of the mean of the square of all its values. You don’t really have to be
concerned with this process, but it’s a good idea to remember the above numbers for
the relationships between peak, pk-pk, and rms values for sine waves and square
waves.
For 117 V rms at a utility outlet, the peak voltage is considerably greater. The pk-pk
voltage is far greater.
