The circuit shown in Figure 1 contains both resistance and capacitance, and therefore both the
component and the frequency of the supply voltage affect the current in the circuit.

The a.c. resistance of such a circuit is known as the impedance of the circuit and is denoted by
the symbol Z. Impedance is measured in ohms.
We will now deduce the impedance of the circuit using the vector treatment.
Consider the voltages round the circuit. The supply voltage will be denoted by Vo and the
voltages across the resistor and capacitor by V_{R0} and V_{C0} respectively.
We know that for a resistor the current and voltage are In phase, while for a capacitor the
current leads the voltage by 90^{o}; v_{R0} therefore leads v_{C0} by 90^{o}, as shown in the vector
diagram in Figure 2.

The resultant voltage v

i

The angle θ that the resultant vector makes with V is known as the

You can see from Figure 3 that tanφ = v