In electrical and electronic systems, reactance is the opposition of a circuit element to a change of electric current or voltage, due to that element's inductance or capacitance. A built-up electric field resists the change of voltage on the element, while a magnetic field resists the change of current. The notion of reactance is similar to electrical resistance, but they differ in several respects.
- is the impedance, measured in ohms.
- is the resistance, measured in ohms.
- is the reactance, measured in ohms.
Both capacitive reactance and inductive reactance contribute to the total reactance .
Although and are both positive by convention, the capacitive reactance makes a negative contribution to total reactance.
- If , the reactance is said to be inductive.
- If , then the impedance is purely resistive.
- If , the reactance is said to be capacitive.
Capacitive reactance 
Capacitive reactance is an opposition to the change of voltage across an element. Capacitive reactance is inversely proportional to the signal frequency (or angular frequency ω) and the capacitance .
At low frequencies a capacitor is open circuit, as no current flows in the dielectric. A DC voltage applied across a capacitor causes positive charge to accumulate on one side and negative charge to accumulate on the other side; the electric field due to the accumulated charge is the source of the opposition to the current. When the potential associated with the charge exactly balances the applied voltage, the current goes to zero.
Driven by an AC supply, a capacitor will only accumulate a limited amount of charge before the potential difference changes polarity and the charge dissipates. The higher the frequency, the less charge will accumulate and the smaller the opposition to the current.
Inductive reactance 
- making it appear as if the inductive reactance to a square wave was
An inductor consists of a coiled conductor. Faraday's law of electromagnetic induction gives the counter-emf (voltage opposing current) due to a rate-of-change of magnetic flux density through a current loop.
For an inductor consisting of a coil with loops this gives.
The counter-emf is the source of the opposition to current flow. A constant direct current has a zero rate-of-change, and sees an inductor as a short-circuit (it is typically made from a material with a low resistivity). An alternating current has a time-averaged rate-of-change that is proportional to frequency, this causes the increase in inductive reactance with frequency.
Phase relationship 
The phase of the voltage across a purely reactive device (a device with a resistance of zero) lags the current by radians for a capacitive reactance and leads the current by radians for an inductive reactance. Note that without knowledge of both the resistance and reactance the relationship between voltage and current cannot be determined.
The origin of the different signs for capacitive and inductive reactance is the phase factor in the impedance.
For a reactive component the sinusoidal voltage across the component is in quadrature (a phase difference) with the sinusoidal current through the component. The component alternately absorbs energy from the circuit and then returns energy to the circuit, thus a pure reactance does not dissipate power.
See also 
- Pohl R. W. Elektrizitätslehre. – Berlin-Göttingen-Heidelberg: Springer-Verlag, 1960.
- Popov V. P. The Principles of Theory of Circuits. – M.: Higher School, 1985, 496 p. (In Russian).
- Küpfmüller K. Einführung in die theoretische Elektrotechnik, Springer-Verlag, 1959.
- Young, Hugh D.; Roger A. Freedman and A. Lewis Ford (2004) . Sears and Zemansky's University Physics (11 ed ed.). San Francisco: Addison Wesley. ISBN 0-8053-9179-7.
- Irwin, D. (2002). Basic Engineering Circuit Analysis, page 274. New York: John Wiley & Sons, Inc.