Science Fair Project Encyclopedia
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:
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 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
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