Impedance is a property of electrical circuits that “impedes” current from flowing. Fundamentally, there are two types of impedance: **Resistance** and **Reactance**.

The magnitude of impedance (represented by <math>Z\,</math>) of a circuit or component can be computed by taking the sum of the squares of the resistance (represented by <math>R\,</math>) and reactance (represented by <math>X\,</math>).

<math>Z = \sqrt{R^2 + X^2}\,</math>

However, it is more useful to represent impedance as a complex number and use it in phasor analysis.

Impedance can be used in the normal Ohm’s Law equation:

<math>V = IZ\,</math>

where V is voltage and I is current. For working with alternating current, this is more correct than the usual direct current equation <math>V = IR\,</math>.