Reactance

Reactance is the opposition presented to alternating current by inductance or capacitance. In ECE140, I didn’t worry about reactance at all, just purely work with Impedance.

• Reactance depends on the angular frequency $\omega$, but Resistance doesn’t

Reactance is given by $X=X_L-X_C\text{Units in Ohms }(\text{ohm})$

Reactance can be of two types:

1. Capacitive Reactance $X_C$ $X_C=\frac{1}{\omega C}$ $X_C=\frac{1}{2\pi fC}\text{(alternative notation)}$

2. Inductive Reactance $X_L$ $X_L=\omega L$ $X_L=2\pi fL\text{(alternative notation)}$

The main difference is that capacitive reactance increases with increasing frequency $\omega$, while inductive reactance decreases.

Notation

I learned the $\omega$ notation through ECE140, but it seems that the notation using $2\pi f$ is much more common, presumably because one actually measures $f$ directly, and not $\omega$ direction.

Related

• Susceptance (its reciprocal)
• Impedance (closely related)