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
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.