As in the case with DC power, the instantaneous electric power in an AC circuit is given by P = VI, but these quantities are continuously varying. So the average power is given by
Pavg = VI cosφ
where φ is the phase angle between the current and the voltage and where V and I are understood to be the effective or rms values of the voltage and current.
Now RMS value of I is =
So, RMS value of V=
Therefore, Power will be
P= (because cosφ= 1 for circuit containing resistance only)
In Series LCR Circuit, current is same across all elements. Since for a resistor both current and voltage are in same phase, Vc is along the same direction as current in the phasor diagram.
In capacitor voltage lags current by 900 so it is at right angle with VR and in an inductor voltage leads current by 900 so it is at right angle with VR and directly opposite to Vc