All tuned circuits and many i lters are made up of inductive and capacitive
elements, including discrete components such as coils and capacitors and
the stray and distributed inductance and capacitance that appear in all electronic
circuits. Both coils and capacitors offer an opposition to alternatingcurrent
l ow known as reactance, which is expressed in ohms (abbreviated
). Like resistance, reactance is an opposition that directly affects the
amount of current in a circuit. In addition, reactive effects produce a phase
shift between the currents and voltages in a circuit. Capacitance causes the
current to lead the applied voltage, whereas inductance causes the current
to lag the applied voltage. Coils and capacitors used together form tuned,
or resonant, circuits.
Capacitors. A capacitor used in an ac circuit continually charges and discharges.
A capacitor tends to oppose voltage changes across it. This translates
to an opposition to alternating current known as capacitive reactance XC.
The reactance of a capacitor is inversely proportional to the