In electrical and electronic systems, reactance is the opposition of a circuit element to a change in current or voltage, due to that element's inductance or capacitance. A builtup 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.
An ideal resistor has zero reactance, whereas ideal inductors and capacitors have zero resistance – that is, respond to current only by reactance. The magnitude of the reactance of an inductor rises in proportion to a rise in frequency, while the magnitude of the reactance of a capacitor decreases in proportion to a rise in frequency (or increases in proportion to wavelength). As frequency goes up, inductive reactance goes up and capacitive reactance goes down.
Analysis
In phasor analysis, reactance is used to compute amplitude and phase changes of sinusoidal alternating current going through the circuit element. It is denoted by the symbol \scriptstyle{X}.
Both reactance \scriptstyle{X} and resistance \scriptstyle{R} are components of impedance \scriptstyle{Z}.

Z = R + jX
where:

Z is the impedance, measured in Ohms;

R is the resistance, measured in Ohms. It is the real part of the impedance: {R=\Re{(Z)}}

X is the reactance, measured in Ohms. It is the imaginary part of the impedance: {X=\Im{(Z)}}

j is the square root of minus one, usually represented by i in nonelectrical formulas. j is used so as not to confuse the imaginary unit with current, commonly represented by i.
Both capacitive reactance \scriptstyle{X_C} and inductive reactance \scriptstyle{X_L} contribute to the total reactance \scriptstyle{X} as follows.

{X = X_L + X_C = \omega L \frac {1} {\omega C}}
where:

\scriptstyle{X_C} is the capacitive reactance, measured in ohms;

\scriptstyle{X_L} is the inductive reactance, measured in ohms;

\omega is the angular frequency, 2\pi times the frequency in Hz.
Although \scriptstyle{X_L} and \scriptstyle{X_C} are both positive by convention, the capacitive reactance \scriptstyle{X_C} makes a negative contribution to total reactance.
Hence:

if \scriptstyle X > 0, the total reactance is said to be inductive;

if \scriptstyle X = 0, then the impedance is purely resistive;

if \scriptstyle X < 0, the total reactance is said to be capacitive.
Capacitive reactance
Capacitive reactance is an opposition to the change of voltage across an element. Capacitive reactance \scriptstyle{X_C} is inversely proportional to the signal frequency \scriptstyle{f} (or angular frequency ω) and the capacitance \scriptstyle{C}.^{[1]}

X_C = \frac {1} {\omega C} = \frac {1} {2\pi f C} ^{[2]}
A capacitor consists of two conductors separated by an insulator, also known as a dielectric.
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
Inductive reactance is an opposition to the change of current through an element. Inductive reactance \scriptstyle{X_L} is proportional to the sinusoidal signal frequency \scriptstyle{f} and the inductance \scriptstyle{L}.

X_L = \omega L = 2\pi f L
The average current flowing through an inductance \scriptstyle{L} in series with a sinusoidal AC voltage source of RMS amplitude \scriptstyle{A} and frequency \scriptstyle{f} is equal to:

I_L = {A \over \omega L} = {A \over 2\pi f L}.
Because a square wave has multiple amplitudes at sinusoidal harmonics, the average current flowing through an inductance \scriptstyle{L} in series with a square wave AC voltage source of RMS amplitude \scriptstyle{A} and frequency \scriptstyle{f} is equal to:

I_L = {A \pi^2 \over 8 \omega L} = {A\pi \over 16 f L}
making it appear as if the inductive reactance to a square wave was about 19% smaller X_L = {16 \over \pi} f L than the reactance to the AC sine wave:
Any conductor of finite dimensions has inductance; the inductance is made larger by the multiple turns in an electromagnetic coil. Faraday's law of electromagnetic induction gives the counteremf \scriptstyle{\mathcal{E}} (voltage opposing current) due to a rateofchange of magnetic flux density \scriptstyle{B} through a current loop.

\mathcal{E} =  in the impedance.

\begin{align} \tilde{Z}_C &= {1 \over \omega C}e^{j({\pi \over 2})} = j\left({ \frac{1}{\omega C}}\right) = jX_C \\ \tilde{Z}_L &= \omega Le^{j{\pi \over 2}} = j\omega L = jX_L\quad \end{align}
For a reactive component the sinusoidal voltage across the component is in quadrature (a \scriptstyle{\pi/2} 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
References

Shamieh C. and McComb G., Electronics for Dummies, John Wiley & Sons, 2011.

Meade R., Foundations of Electronics, Cengage Learning, 2002.

Pohl R. W. Elektrizitätslehre. – BerlinGöttingenHeidelberg: SpringerVerlag, 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, SpringerVerlag, 1959.


^ Irwin, D. (2002). Basic Engineering Circuit Analysis, page 274. New York: John Wiley & Sons, Inc.

^ http://hyperphysics.phyastr.gsu.edu/hbase/hframe.html
External links

Interactive Java Tutorial on Inductive Reactance National High Magnetic Field Laboratory

Reactance calculator
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