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AC Circuits

# Impedance and Admittance

**Impedance** is the general expression for opposition to current in
alternating current circuits. Impedance may be pure resistance
or pure reactance, but usually it is a combination of resistance and
reactance. The symbol * Z* is used for impedance, which
is expressed in ohms. Impedance takes the general phasor form

Impedance is shown on the complex plane in the igure below.

Impedance is the ratio of phasor voltage to phasor current in an AC circuit, as shown by the equation below:

Equation above is the general Ohm's-law relationship for AC circuits, and the equation can be handled algebraically in the usual way. It must be remembered that computations involving phasors must be done as outlined in the section about complex numbers.

**Example 1**: The impedance of the circuit of figure below is

**Example 2**: The impedance of the circuit of figure below is

**Admittance** has the symbol * Y* and is defined as the
reciprocal of impedance. Admittance is expressed in mhos or siemens (symbol S).

**Example 3**: Calculate the impedance and admittance of the circuit of
figure below at a frequency of 150 Hz.

Solution: