Let a pure resistor and a pure capacitor be connected in series to an alternating emf. Let E be the rms value of applied emf and I be the rms value of the current flowing in the circuit. Let VR and VC be the potential drop across R and C respectively.
The potential drop across capacitor is:
The potential drop across resistance is:
From the fig the resultant OH is given by:
This is the effective opposition offered by RC circuit for the flow of current.
This shows that the potential lags the current by π/2 in RC circuit.