Reactance
(X)
is defined as the ratio of the AC voltage to the AC current in an AC circuit, as
caused by the presence of a reactive component in the circuit, i.e., a capacitor or an
inductor. The reactance of a capacitor or inductor
is therefore like the resistance of a
resistor, except that the reactance of a capacitor or inductor is not
constant - it varies with the frequency of the signal across the
capacitor or through the inductor. Reactance is also expressed in ohms.
The presence
of a capacitor or an inductor in a circuit impedes changes in voltage or
current within the circuit. Specifically, the voltage across a
capacitor can not change instantaneously, in the same way that current
flowing through an inductor can not change instantaneously. As
such, the presence
of a capacitor or inductor in an AC circuit introduces a phase shift
between the voltage and current signals in the circuit.
In an AC
circuit where both resistance and reactance exist, the effective ratio
of voltage to current is known as the impedance Z of the circuit, which
is given by the equation Z = R + jX, or
|Z|
= SQRT(R2 + X2).
Impedance is therefore the over-all ability of a circuit to 'impede' the
flow of current for a given voltage, and it consists of two components:
resistance and reactance.
Assume that a
sinusoidal voltage with frequency f (in Hz) is applied across a
capacitor with capacitance C. The reactance XC
of the capacitor is then given by the equation:
XC
= 1 / (2πfC) = 1 / ωC
where
ω = 2πf
is the angular frequency in radians per second. Thus, the
reactance XC
of a capacitor decreases as the frequency f increases, and increases as
f decreases. This is why a capacitor is used to block the DC
component of an AC signal. Furthermore, the
current sine wave through a capacitor is 90 degrees ahead of the voltage
sine wave across it.
On the other
hand, the reactance XL
of an inductor is given by the equation
XL
= 2πfL
= ωL,
which means that the reactance of an inductor increases as the frequency
of the signal through it increases. Furthermore, the current sine wave
through an inductor lags the voltage sine wave across it by 90 degrees.
Because the
reactance exhibited by a capacitor decreases as frequency increases, its
reactance is considered negative. Thus, a reactance that's less than 0
means that it is capacitive. Similarly, a reactance that's greater than
0 means that it is inductive. Reactance equals zero in a purely
resistive circuit.
Susceptance
(B) is the reciprocal of reactance, i.e.,
B = 1 / X.
See Also:
Resistance;
Capacitance;
Self-Inductance;
Impedance;
Ohm's Law
