Year+13+Impedance+and+Reactance

Reactance and Impedance

What are impedance and reactance? Circuits in which current is proportional to voltage are called linear circuits. (As soon as one inserts diodes and transistors, circuits cease to be linear, but that's another story.) The ratio of voltage to current in a resistor is its resistance. Resistance does not depend on frequency, and in resistors the two are in phase, as we have seen in the animation. However, circuits with only resistors are not very interesting.

In general, the ratio of voltage to current does depend on frequency and in general there is a phase difference. So impedance is the general name we give to the ratio of voltage to current. It has the symbol Z. Resistance is a special case of impedance. Another special case is that in which the voltage and current are out of phase by 90°: this is an important case because when this happens, no power is lost in the circuit. In this case where the voltage and current are out of phase by 90°, the ratio of voltage to current is called the reactance, and it has the symbol X. We return to summarise these terms and give expressions for them below in the section Impedance of components, but first let us see why there are frequency dependence and phase shifts for capacitors and for inductors.

Reactance is the opposition of a component to a change of electric current or voltage. Capacitance and inductance are inherent properties of an element, just like resistance.

Reactance of a capacitor differs with the size of the capacitor and the frequency of the AC.

Resistance and reactance both share the same SI unit, the ohm.

An ideal resistor has zero reactance, while ideal inductors and capacitors consist entirely of reactance.

Resistance and reactance combine to form impedance.