 - Reactance, denoted X, is a form of opposition that
electronic components exhibit to the passage of alternating current (alternating
current) because of capacitance or inductance. In some respects, reactance is like
an AC counterpart of DC (direct current) resistance.
But the two phenomena are different in important ways, and they can vary independently of
each other. Resistance and reactance combine to form impedance,
which is defined in terms of two-dimensional quantities known as complex number.
When alternating current passes through a component that contains reactance, energy is alternately
stored in, and released from, a magnetic field or an electric field. In the case of
a magnetic field, the reactance is inductive. In the case of an electric field, the
reactance is capacitive. Inductive reactance is assigned positive imaginary number values. Capacitive reactance is
assigned negative imaginary-number values.
As the inductance of a component increases, its inductive reactance
becomes larger in imaginary terms, assuming the frequency is held constant. As the
frequency increases for a given value of inductance, the inductive reactance increases in
imaginary terms. If L is the inductance in henries (H) and f is
the frequency in hertz (Hz), then the
inductive reactance +jXL, in imaginary-number ohms, is given by:
+jXL = +j(6.2832fL)
where 6.2832 is approximately equal to 2 times pi, a
constant representing the number of radians in a full AC cycle, and j represents
the unit imaginary number (the positive square root of -1). The formula also holds
for inductance in microhenries (?H) and frequency in MHz
(MHz).
As a real-world example of inductive reactance, consider a coil with an
inductance of 10.000 ?H at a frequency of 2.0000 MHz. Using the above formula, +jXL
is found to be +j125.66 ohms. If the frequency is doubled to 4.000 MHz,
then +jXL is doubled, to +j251.33 ohms. If the
frequency is halved to 1.000 MHz, then +jXL is cut in half, to +j62.832
ohms.
As the capacitance of a component increases, its capacitive reactance
becomes smaller negatively (closer to zero) in imaginary terms, assuming the frequency is
held constant. As the frequency increases for a given value of capacitance, the
capacitive reactance becomes smaller negatively (closer to zero) in imaginary terms.
If C is the capacitance in farads (F) and f is the frequency in Hz, then
the capacitive reactance -jXC, in imaginary-number ohms, is given by:
-jXC = -j (6.2832fC)-1
This formula also holds for capacitance in microfarads (?F) and frequency
in megahertz (MHz).
As a real-world example of capacitive reactance, consider a capacitor with
a value of 0.0010000 ?F at a frequency of 2.0000 MHz. Using the above formula, -jXC
is found to be -j79.577 ohms. If the frequency is doubled to 4.0000 MHz,
then -jXC is cut in half, to -j39.789 ohms. If the
frequency is cut in half to 1.0000 MHz, then -jXC is doubled, to -j159.15
ohms.
| LAST UPDATED: |
04 Jul 2000
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