## Quick Reference

Symbol *Z.* The quantity that measures the opposition of a circuit to the passage of a current and therefore determines the amplitude of the current. In a d.c. circuit this is the resistance (*R*) alone. In an a.c. circuit, however, the reactance (*X*) also has to be taken into account, according to the equation: *Z*^{2} = *R*^{2} + *X*^{2}, where *Z* is the impedance. The **complex impedance** is given by *Z* = *R* + i*X*, where i = √−1. The real part of the complex impedance, the resistance, represents the loss of power according to Joule's law. The ratio of the imaginary part, the reactance, to the real part is an indication of the difference in phase between the voltage and the current.

**From:**
impedance
in
A Dictionary of Physics »

*Subjects:*
Physics — Chemistry.