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Reactance is the opposition a capacitor or inductor gives to alternating current — like resistance, but frequency-dependent and storing energy instead of dissipating it. A capacitor's reactance falls with frequency (it passes high frequencies), while an inductor's reactance rises with frequency (it blocks high frequencies). Both are measured in ohms.
| Quantity | Formula |
|---|---|
| Capacitive reactance | XC = 1 / (2πfC) |
| Inductive reactance | XL = 2πfL |
| Angular frequency | ω = 2πf |
| At resonance | XC = XL |
The current through a capacitor leads the voltage by 90°, and through an inductor it lags by 90° — which is why reactance is "imaginary" and combines with resistance to form impedance.
Reactance is the opposition a capacitor or inductor offers to alternating current. Unlike resistance it depends on frequency and stores energy rather than dissipating it, and it is measured in ohms.
Resistance dissipates energy as heat and is frequency-independent; reactance stores and returns energy each cycle and changes with frequency. Together they form impedance.
It decreases as frequency rises (XC = 1/2πfC), so a capacitor blocks DC and low frequencies but passes high frequencies.
It increases with frequency (XL = 2πfL), so an inductor passes DC and low frequencies but blocks high frequencies.
Because, like resistance, it relates voltage to current (V = I×X). It just does so with a 90° phase shift and a frequency dependence.
In a capacitor the current leads the voltage by 90°; in an inductor the current lags by 90°. This quadrature is why reactance is treated as imaginary in impedance.
When XC = XL the two reactances cancel, leaving only resistance. The circuit is at its resonant frequency, where current (series) or impedance (parallel) peaks.
As impedance: Z = √(R² + X²), with X = XL − XC. The phase angle is arctan(X/R).
No — ideal reactance stores energy and returns it each cycle, so it consumes no real power (only reactive power). Real components have some resistance that does dissipate.
Infinite — at 0 Hz the formula gives XC = ∞, meaning a capacitor blocks DC once charged.
Zero — at 0 Hz XL = 0, so an inductor looks like a plain wire (just its winding resistance) to DC.
Pick a value whose reactance is much smaller than the surrounding resistance at the frequency you want to pass, so it acts like a short at that frequency.
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