Reactance: resistance that depends on frequency
Inductors and capacitors oppose AC current, but unlike a resistor the opposition — reactance, in ohms — changes with frequency. An inductor fights faster changes harder (XL rises with f); a capacitor does the opposite (XC falls with f):
XL = 2πfL XC = 1 / (2πfC)
Unlike a resistor, an ideal reactance dissipates no power — it stores energy and hands it back each cycle, with current and voltage 90° out of phase.
Resonance
At one special frequency the inductive and capacitive reactances are equal and cancel — that's resonance:
f = 1 / (2π√(LC))
An LC circuit at resonance rings like a bell; it's the heart of radio tuning, oscillators, and filters. A 1 µH coil with a 100 pF capacitor resonates near 15.9 MHz.
Worked examples
Coupling cap: a 1 µF capacitor at 1 kHz has XC = 1/(2π·1000·1e-6) ≈ 159 Ω.
RF choke: a 10 µH inductor at 100 MHz presents XL = 2π·1e8·1e-5 ≈ 6.3 kΩ — nearly an open circuit to RF while passing DC.
Frequently asked questions
- Is reactance the same as impedance?
- Not quite. Impedance combines resistance and reactance as a complex quantity (Z = R + jX). For a pure L or C, the impedance is the reactance (with a ±90° phase); real parts have some series resistance too.
- Why doesn't reactance dissipate power?
- Because voltage and current are 90° out of phase, the average of their product over a cycle is zero — energy shuttles in and out of the field rather than turning to heat. That's why "reactive power" is measured in VAR, not watts.
- What sets the sharpness of resonance?
- The quality factor Q — higher Q means a narrower, taller resonance peak. It depends on the series resistance and the L/C ratio, which this calculator doesn't model; it gives the center frequency.
- Need the plain frequency-period relationship?
- See the frequency & period calculator, which also gives wavelength and angular frequency.