In electrical circuits, reactance is the opposition presented to alternating current by inductance and capacitance.
Greater reactance gives smaller current for the same applied voltage.
Reactance is used to compute amplitude and phase changes of sinusoidal alternating current going through a circuit element.
Finally, the main circuit elements that have reactance (capacitors and inductors) have a frequency dependent reactance, unlike resistors which have the same resistance for all frequencies, at least in the ideal case.
The term reactance was first suggested by French engineer M. Hospitalier in L'Industrie Electrique on 10 May 1893.
Capacitive reactance is an opposition to the change of voltage across an element.
One is to use a uniform notion of reactance as the imaginary part of impedance, in which case the reactance of a capacitor is the negative number,[3][4][5] Another choice is to define capacitive reactance as a positive number,[6][7][8] In this case however one needs to remember to add a negative sign for the impedance of a capacitor, i.e.
, the magnitude of the capacitor's reactance is infinite, behaving like an open circuit (preventing any current from flowing through the dielectric).
As frequency increases, the magnitude of reactance decreases, allowing more current to flow.
Driven by an AC supply (ideal AC current source), a capacitor will only accumulate a limited amount of charge before the potential difference changes polarity and the charge is returned to the source.
The higher the frequency, the less charge will accumulate and the smaller the opposition to the current.
Inductive reactance is a property exhibited by an inductor, and inductive reactance exists based on the fact that an electric current produces a magnetic field around it.
In the context of an AC circuit (although this concept applies any time current is changing), this magnetic field is constantly changing as a result of current that oscillates back and forth.
It is this change in magnetic field that induces another electric current to flow in the same wire (counter-EMF), in a direction such as to oppose the flow of the current originally responsible for producing the magnetic field (known as Lenz's law).
Hence, inductive reactance is an opposition to the change of current through an element.
For an ideal inductor in an AC circuit, the inhibitive effect on change in current flow results in a delay, or a phase shift, of the alternating current with respect to alternating voltage.
Specifically, an ideal inductor (with no resistance) will cause the current to lag the voltage by a quarter cycle, or 90°.
In electric power systems, inductive reactance (and capacitive reactance, however inductive reactance is more common) can limit the power capacity of an AC transmission line, because power is not completely transferred when voltage and current are out-of-phase (detailed above).
That is, current will flow for an out-of-phase system, however real power at certain times will not be transferred, because there will be points during which instantaneous current is positive while instantaneous voltage is negative, or vice versa, implying negative power transfer.
Power providers utilize capacitors to shift the phase and minimize the losses, based on usage patterns.
in series with a sinusoidal AC voltage source of RMS amplitude
is equal to: Because a square wave has multiple amplitudes at sinusoidal harmonics, the average current flowing through an inductance
in series with a square wave AC voltage source of RMS amplitude
is equal to: making it appear as if the inductive reactance to a square wave was about 19% smaller
(voltage opposing current) due to a rate-of-change of magnetic flux density
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).
are assumed both positive by definition, then the intermediary formula changes to a difference:[7] but the ultimate value is the same.
The phase of the voltage across a purely reactive device (i.e. with zero parasitic resistance) lags the current by
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