LearnTronics
Capacitive and Inductive Reactance
Reactance — AC opposition that depends on frequency
Resistance opposes current in both DC and AC. Capacitors and inductors add another kind of opposition in AC circuits called reactance.
Recognize the three symbols first
The capacitor is the pair of separated plates. The inductor is drawn as a coil. Once those two shapes become familiar, AC schematics become much easier to read.
Symbols
XC = capacitive reactance, in ohms
XL = inductive reactance, in ohms
f = frequency in hertz
C = capacitance in farads
L = inductance in henrys
Capacitor: easier at higher frequency
A capacitor blocks steady-state DC after charging, but alternating current can repeatedly charge and discharge it. As frequency increases, a capacitor generally offers less reactance.
Inductor: harder at higher frequency
An inductor resists changes in current through its magnetic field. Faster changes mean more opposition, so inductive reactance generally increases with frequency.
The opposite frequency behaviors are worth memorizing
Capacitor: frequency up → XC down.
Inductor: frequency up → XL up.
Reactance is not quite the same thing as resistance
Both are measured in ohms because both affect how much current flows. But an ideal resistor converts electrical energy to heat, while an ideal capacitor or inductor stores energy temporarily in an electric or magnetic field and can return it to the circuit.
That energy storage is also why capacitors and inductors can shift the timing — or phase — between voltage and current.
A little history
As telegraphy, telephony, radio, and AC power developed, engineers needed ways to describe how capacitors and inductors behaved at different frequencies. Reactance became a central part of AC network analysis.