LearnTronics
Impedance
Impedance — the AC version of opposition
In an AC circuit, resistance and reactance work together. Their combined opposition is called impedance.
The symbol Z
Z is the standard symbol for impedance. It is measured in ohms (Ω).
A useful first picture
For simple DC resistor circuits, we ask “how much resistance?” In AC circuits containing capacitors or inductors, we ask the broader question “how much impedance?”
What is really happening?
Resistance dissipates energy as heat. Ideal reactance stores energy temporarily in electric or magnetic fields and returns it. Because reactance also shifts the timing relationship between voltage and current, impedance must include both magnitude and phase.
Worked idea
In a simple series R-L circuit, resistance and inductive reactance do not simply add as ordinary numbers when finding the magnitude of impedance. They act like perpendicular components:
For example, R = 3 Ω and XL = 4 Ω gives |Z| = 5 Ω — the familiar 3-4-5 triangle.
Another place our little bit of trigonometry helps
The impedance triangle is a right triangle. Resistance is one side, reactance is the other, and the magnitude of impedance is the hypotenuse. That is why a 3 Ω resistance with 4 Ω of reactance gives 5 Ω of impedance.
Later, the angle φ in this same triangle will help us understand phase and power factor.
Try these yourself
A little deeper
Using complex-number notation, series impedance may be written as:
Electrical engineers commonly use j for √−1 because the letter i is already heavily used for current.