Reactance is how a capacitor or inductor opposes alternating current. It is measured in ohms, but unlike resistance it does not turn power into heat: the part stores energy and gives it back each cycle. Impedance is the total opposition to AC, combining resistance and reactance.
Reactance changes with frequency
Log axes. Inductive reactance climbs with frequency, capacitive reactance falls, and where they cross the two cancel.
Inductive reactance XL445 Ω
Capacitive reactance XC225 Ω
Series total X = XL − XC+220 Ω (inductive)
An inductor resists changes in current. The faster the AC reverses, the harder it pushes back, so inductive reactance rises in proportion to frequency. A capacitor passes current in proportion to how fast its voltage changes (see Capacitance). Faster changes mean more current for the same voltage, so capacitive reactance falls as frequency rises.
The two are opposites. At DC a capacitor is an open circuit and an inductor is a short; at very high frequency the reverse is true. For example:
Arrows turn counter-clockwise. The arrow that is ahead of the other is leading.
Through a resistor, voltage and current rise and fall together. A reactive part breaks that:
Inductor: voltage leads current by 90°. The voltage is greatest when the current is changing fastest, which is at the current's zero crossing. ELI: voltage (E) before current (I) in an inductor (L).
Capacitor: current leads voltage by 90°. The current is greatest when the voltage is changing fastest. ICE: current (I) before voltage (E) in a capacitor (C).
Because voltage and current peak at different moments, a pure reactance absorbs no net power. Energy flows in during part of the cycle and out during the next.
Adding up to impedance
In a series circuit XL and XC point opposite ways and partly cancel, leaving a net X = XL − XC. Resistance and reactance are 90° apart, so they combine like the sides of a right triangle, not by simple addition:
Z = √(R² + X²), and the current is shifted by arctan(X ÷ R).
With R = 50 Ω and XL = 50 Ω: Z = 70.7 Ω, and the current lags the voltage by 45°. With R = 30 Ω and X = 40 Ω you get the familiar 3-4-5 triangle: Z = 50 Ω at 53°.
When XL and XC are equal they cancel completely, leaving only R. That is resonance.
In practice
An antenna analyzer reports impedance as R + jX: a feed point of 50 + j25 Ω has 50 Ω of resistance and 25 Ω of inductive reactance. The j is explained in Phasors and complex numbers.
Reactance in the antenna or line is a mismatch you can cancel with an opposite reactance. This is what a matching network and an antenna tuner do.
A small capacitor has low reactance at RF but very high reactance at DC and audio. Bypass capacitors use this to shunt RF to ground, and coupling capacitors to pass a signal while blocking DC.