Glossary definition of 'Impedance'
A measure of opposition of a circuit to alternating current (AC). Impedance is the sum of resistance and reactance. Resistance is analogous to friction in mechanical systems and introduces no phase difference between the voltage and current flowing through it. Reactance is caused by the build up of electric or magnetic fields due to the current flow. These cause an opposition voltage that is either proportional to the rate of change of current or the time integral of current through the circuit and causes a phase shift between voltage and current. The equivalent in a mechanical system would be a mass on a spring in which case the reactance comes from the inertial mass (inductance) and spring constant (1/capacitance). A pure reactance would not dissipate any power.