*This article is about electronics. For a disscussion of "reactive" or "reactance" in chemistry, see reactivity.*

In the analysis of an alternating-current electrical circuit (for example a RLC series circuit), **reactance** is the imaginary part of impedance, and is caused by the presence of inductors or capacitors in the circuit. Reactance is denoted by the symbol X and is measured in ohms.

If X > 0, the reactance is said to be *inductive*

If X = 0, then the circuit is purely resistive, i.e. it has no reactance.

If X < 0, it is said to be *capacitive*.

The reciprocal of reactance is **susceptance**.

The relationship between impedance, resistance, and reactance is given by the equation:

Often it is enough to know the magnitude of the impedance:

where

*Z* is impedance, measured in ohms

*R* is resistance, measured in ohms

*X* is reactance, measured in ohms

**Inductive reactance** (symbol X_{L}) is caused by the fact that a current is accompanied by a magnetic field; therefore a varying current is accompanied by a varying magnetic field; the latter gives an electromotive force that resists the changes in current. The more the current changes, the more an inductor resists it: the reactance is proportional with the frequency (hence zero for DC). There is also a phase difference between the current and the applied voltage.

Inductive reactance has the formula

where

*X*_{L} is the inductive reactance, measured in ohms

*f* is the frequency, measured in hertz

*L* is the inductance, measured in henry

**Capacitive reactance** (symbol X_{C}) reflects the fact that electrons can not pass through a capacitor, yet effectively alternating current (AC) can: the higher the frequency the better. There is also a phase difference between the alternating current flowing through a capacitor and the potential difference across the capacitor's electrodes.

Capacitive reactance has the formula

where

*X*_{C} is the capacitive reactance measured in ohms

*f* is the frequency, measured in hertz

*C* is the capacitance, measured in farad

## SI electricity units

## External links

Last updated: 06-02-2005 13:01:21

Last updated: 08-19-2005 07:20:04