The average value of a waveform, which swings symmetrically above and below zero, will be obviously zero, when measured over a long period of time. Hence, average values of currents and voltages are invariably taken over one complete half-cycle (either positive or negative) rather than over one complete full-cycle (which would result in an average value of zero).
The amplitude (or peak value) of waveform is a measure of the extent of its voltage or current excursion from the resting value (usually zero). The peak-to-peak value for a wave which is symmetrical about its resting value is twice its peak value as shown here.
The R.M.S. (or effective) value of an alternating voltage or current (AC) is the value which would produce the same amount of heat energy in a resistor as a direct voltage or current (DC) of the same magnitude. The R.M.S. (root mean square) value of a waveform is very much dependent upon its wave shape. The R.M.S. values are, therefore, only meaningful when dealing with a waveform of known shape. When it is an irregular waveform, the R.M.S. value is normally assumed to refer to sinusoidal (or sine) waveform.
Calculations of R.M.S. value of AC voltage: Suppose an AC voltage has peak voltage (Vp) of 10V. Then its peak–to–peak voltage (Vpp) will be 20V. But due to changing nature of AC voltage, what is its effective value? The AC voltage is always measured in effective value, which is called ROOT MEAN SQUARE (R.M.S.) value. Its value is given by –
Note: the R.M.S. value of sine wave is calculated at 45° which is 70.7% of its peak voltage.
Peak, R.M.S. & Average AC values of E and I: