DEFINITION - Reactance, denoted X, is a form of opposition that
electronic components exhibit to the passage of alternating current
(AC) because of capacitance or inductance. In some
respects, reactance is like an AC counterpart of direct current (DC)
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 a 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.
When the inductance of a component increases, its inductive reactance
becomes larger presuming the frequency is held constant.
As the frequency increases for a given value of inductance, the inductive reactance increases.
If "L" is the inductance in henries (H) and "f" is
the frequency in hertz (Hz), then the inductive reactance
+jXL, in ohms, is given by:
As a real-world example of inductive reactance, consider a coil with an inductance of 10.000 μH 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 (closer to zero), presuming the frequency is held constant. As the frequency increases for a given value of capacitance, the capacitive reactance becomes smaller (closer to zero).
If "C" is the capacitance in farads (F) and "f" is the frequency in Hz,
then the capacitive reactance -jXC, in ohms,
is given by:
As a real-world example of capacitive reactance, consider a capacitor with a value of 0.0010000 μF 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.