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see impedanceimpedance,
in electricity, measure in ohms of the degree to which an electric circuit resists the flow of electric current when a voltage is impressed across its terminals.
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The imaginary part of the impedance of an alternating-current circuit.

The impedance Z of an alternating current circuit is a complex number given by Eq. (1).

The imaginary part X is the reactance. The units of reactance, like those of impedance, are ohms. Reactance may be positive or negative. For example, the impedance of an inductor L at frequency &ohgr; is given by Eq. (2), so X is positive. The impedance of a capacitor C is given by Eq. (3), so X is negative.

The reactance of a circuit may depend on both the resistors and the inductors or capacitors in the circuit. For example, the circuit in the illustration has admittance [Eq. (4)] and impedance [Eq. (5)], so that the reactance [Eq. (6)], depends on both the capacitor C and the resistor R. (4)  (5)  (6)  See Admittance, Electrical impedance

Circuit with a resistor and capacitor in parallelenlarge picture
Circuit with a resistor and capacitor in parallel



in electricity, a quantity characterizing the opposition presented to an alternating current by the capacitance and inductance of a circuit or part of a circuit. Reactance is measured in ohms.

In the case of a sinusoidal current in a circuit where inductive and capacitive circuit elements are connected in series, the reactance x can be expressed as the difference between the inductive and capacitive reactances:

Here, to is the angular frequency of the current, L is the inductance of the circuit, and C is the capacitance of the circuit. Reactance is equal to the ratio of the amplitude of the voltage on the terminals of a circuit having little resistance and the amplitude of the current through the circuit. When an alternating current flows in a circuit having only reactance, energy is transferred from the current source to the electric or magnetic field produced, respectively, by the capacitive or inductive circuit element and then back to the current source; the average power during a period is equal to zero. The presence of reactance in a circuit causes a phase difference between the voltage and the current.

When the current in a circuit is nonsinusoidal, the reactance is different for the individual harmonic components of the current.


The imaginary part of the impedance of an alternating-current circuit.


1. the opposition to the flow of alternating current by the capacitance or inductance of an electrical circuit; the imaginary part of the impedance Z, Z = R + iX, where R is the resistance, i = &#221A--1, and X is the reactance. It is expressed in ohms
2. the opposition to the flow of an acoustic or mechanical vibration, usually due to inertia or stiffness. It is the magnitude of the imaginary part of the acoustic or mechanical impedance
References in periodicals archive ?
However, all initial values, except the line reactance, have been held constant and the system is in steady-state.
The obtained value of the direct-axis synchronous reactance is later used for determining the practical static stability limit in the user real-time P-Q diagram.
A method called ORMEL96 uses the motor's nameplate data as well as statistical data, including assumed stator and rotor leakage reactance ratio, stray load loss, friction and windage losses, locked rotor current, and stator winding resistance, to compute these circuit parameters without field measurements (Kueck et al.
As for reactances, the difference is up to 4%, but usually it is below 2%.
l2] are both outside of the 1 + jb circle, they can share one type-II L-network and concurrently get matched if the serial and shunt circuit blocks have needed reactances and suceptances at the two frequencies.
To determine the frequency of the crystals making up Yb(s) simply change the sign of the constant reactance term and recalculate.
In the optimal oscillator the maximum output power is realized under the conditions of complete phase compensation with a load reactance equal to zero.
d^ may be obtained by changing the source reactance and the gate stub, but with less overall instability and less selection of passive, unstable loads.
o~, no load reactance is permitted without violating the saturation boundary conditions.
The program automatically determines the RF frequency where the circuit has simultaneously both zero reactance and negative real resistance at each of the YIG tuned frequencies.
In a simple LRC series resonant circuit, resonance occurs at a frequency [Mathematical Expression Omitted] where [Mathematical Expression Omitted] At this frequency, the capacitive and inductive reactances are equal (and opposite in phase), such that the net reactance through the circuit is zero.