Reactance
- 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:
where
Z is impedance, measured in ohms
R is resistance, measured in ohms
X is reactance, measured in ohms
Often it is enough to know the magnitude of the impedance:
For a purely inductive or capacitive element, the magnitude of the impedance simplifies to just the reactance.
Inductive reactance (symbol XL) 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
XL is the inductive reactance, measured in ohms
f is the frequency, measured in hertz
L is the inductance, measured in henry
Capacitive reactance (symbol XC) 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
XC is the capacitive reactance measured in ohms
f is the frequency, measured in hertz
C is the capacitance, measured in farad
SI electricity units
| SI electromagnetism units edit | |||
|---|---|---|---|
| Name | Symbol | Dimensions | Quantity |
| ampere (SI base unit) | A | A | Current |
| coulomb | C | A新 | Electric charge, Quantity of electricity |
| volt | V | J/C = kg搶2新−3嫂−1 | Potential difference |
| ohm | Ω | V/A = kg搶2新−3嫂−2 | Resistance, Impedance, Reactance |
| ohm metre | Ω搶 | kg搶3新−3嫂−2 | Resistivity |
| watt | W | V嫂 = kg搶2新−3 | Electrical power |
| farad | F | C/V = kg−1搶−2嫂2新4 | Capacitance |
| farad per metre | F/m | kg−1搶−3嫂2新4 | Permittivity |
| reciprocal farad | F−1 | kg1搶2嫂−2新−4 | Elastance |
| siemens | S | Ω−1 = kg−1搶−2新3嫂2 | Conductance, Admittance, Susceptance |
| siemens per metre | S/m | kg−1搶−3新3嫂2 | Conductivity |
| weber | Wb | V新 = kg搶2新−2嫂−1 | Magnetic flux |
| tesla | T | Wb/m2 = kg新−2嫂−1 | Magnetic flux density |
| ampere per metre | A/m | m−1嫂 | magnetic induction |
| ampere-turns per weber | A/Wb | kg−1搶−2新2嫂2 | Reluctance |
| henry | H | Wb/A = V新/A = kg搶2新−2嫂−2 | Inductance |
| henry per metre | H/m | kg搶新−2嫂−2 | Permeability |
| (dimensionless) | χ | - | Magnetic susceptibility |