The impedance of a load (a combination of components) in an AC current is a complex number where is the resistance of the load and is its reactance.
Impedance is the AC equivalent of resistance: it is used in the general AC versions of Ohm's Law: and , and it obeys the same series or parallel combination laws as resistance does.
Impedance depends on frequency (except for pure resistances).
Impedance is measured in ohms ().
Equations
For a load for which the voltage leads the current by a phase angle :
