While
Ohm's Law applies directly to resistors in
DC or in
AC circuits, the form of the current-voltage relationship in AC circuits in general is modified to the form:
where I and V are the
rms or "effective" values. The quantity Z is called impedance. For a pure resistor, Z = R. Because the
phase affects the impedance and because the contributions of
capacitors and
inductors differ in phase from resistive components by 90 degrees, a process like vector addition (
phasors) is used to develop expressions for impedance. More general is the
complex impedance method.