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.

represents the unit imaginary number (the positive square root of -1).*j***Inductive**reactance is assigned positive imaginary-number values. (The antenna appears long from resonance).**Capacitive**reactance is assigned negative imaginary-number values. (The antenna appears short from resonance).

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
+*j*X_{L}, in ohms, is given by:

where 6.2832 is approximately equal to 2 times pi, a constant representing the number of radians in a full AC cycle, and

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, +*j*X_{L} is found to be +*j*125.66 ohms.
If the frequency is doubled
to 4.000 MHz, then +*j*X_{L} is doubled, to +*j*251.33
ohms. If the frequency is halved to 1.000 MHz,
then +*j*X_{L} is cut in half, to +*j*62.832 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 -*j*X_{C}, in ohms,
is given by:

This formula also holds for capacitance in microfarads (μF) and frequency in megahertz (MHz).

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, -*j*X_{C} is found to be -*j*79.577
ohms. If the frequency is
doubled to 4.0000 MHz, then -*j*X_{C} is cut in half,
to -*j*39.789 ohms. If the frequency is cut in half to 1.0000
MHz, then -*j*X_{C} is doubled, to
-*j*159.15 ohms.