Reactance is the opposition to current flow that occurs specifically in AC (alternating current) circuits, caused by inductors and capacitors. It’s measured in ohms (Ω), just like resistance, and is represented by the symbol X. While resistance opposes current in both AC and DC circuits, reactance only appears when the current is alternating, because it arises from the phase difference between voltage and current waveforms.
How Reactance Differs From Resistance
Resistance and reactance both limit current flow, but they handle energy in completely different ways. A resistor converts electrical energy into heat. The energy is gone, dissipated into the environment. Reactance doesn’t do this. When an inductor or capacitor limits current, it stores energy temporarily (in a magnetic field or electric field) and then returns it to the circuit. No energy is lost to heat.
This distinction matters in real-world power systems. The energy that sloshes back and forth in reactive components without doing useful work is called reactive power, measured in volt-amps-reactive (VAR) rather than watts. It doesn’t power your appliances or generate heat, but it still occupies capacity in the electrical system. Utilities care a lot about reactive power because it forces them to supply more current than the load actually consumes.
Inductive Reactance
An inductor (a coil of wire) resists changes in current by generating a magnetic field. In an AC circuit, where current is constantly changing direction, this creates inductive reactance. The formula is:
XL = 2πfL
Here, f is the frequency of the AC signal in hertz and L is the inductance in henrys. Two things jump out from this formula. First, inductive reactance increases with frequency. A coil that barely impedes a 60 Hz power signal can present serious opposition to a 1 MHz radio signal. Second, larger inductance means more opposition. This makes intuitive sense: a bigger coil stores more energy in its magnetic field, pushing back harder against rapid changes in current.
In an inductor, voltage leads current by 90 degrees. That means the voltage waveform peaks a quarter cycle before the current does. This phase shift is one of the defining characteristics of reactance and is why reactance can’t simply be added to resistance like ordinary numbers.
Capacitive Reactance
A capacitor stores energy in an electric field between two plates. Its reactance works opposite to an inductor’s:
XC = 1 / (2πfC)
Here, C is the capacitance in farads. Notice the inverse relationship: capacitive reactance decreases as frequency rises. At low frequencies, a capacitor has time to fully charge and effectively blocks current. At high frequencies, the constant reversal of current means charge barely builds up on the plates, so current flows more freely. A capacitor in a DC circuit (frequency of zero) has infinite reactance, which is why capacitors block DC entirely.
In a capacitor, current leads voltage by 90 degrees, the exact opposite of an inductor. This opposing phase behavior is what makes resonance possible.
Impedance: The Full Picture
Real circuits contain both resistance and reactance. The combination of the two is called impedance (Z), expressed as:
Z = R + jX
R is the resistance, X is the net reactance, and j represents the imaginary unit (used because reactance involves a phase shift, not just a magnitude). You can’t simply add resistance and reactance together with normal arithmetic because they affect the current at different points in time. Impedance captures both the magnitude of opposition to current and the timing relationship between voltage and current.
For practical calculations, the magnitude of impedance in a series circuit is the square root of R² + X², where X is the difference between inductive and capacitive reactance. If a circuit has 30 ohms of resistance and 40 ohms of net reactance, the total impedance magnitude is 50 ohms.
Resonance: When Reactances Cancel
Something useful happens when you put an inductor and capacitor in the same circuit. Since inductive reactance increases with frequency and capacitive reactance decreases with frequency, there’s a specific frequency where the two are exactly equal. At that point, they cancel each other out, and the circuit’s impedance drops to just its resistance. This is resonance.
At resonance, the circuit allows maximum current to flow at that one frequency while opposing current at all other frequencies. This frequency selectivity is the foundation of radio tuning. When you tune a radio to a station, you’re adjusting a capacitor (the tuning capacitor) so the circuit resonates at the station’s broadcast frequency. The desired signal passes through easily while signals at other frequencies are suppressed. The same principle drives bandpass filters, crossover networks in speakers, and frequency-selective circuits throughout electronics.
Practical Uses of Reactance
Reactance shows up any time you need a circuit to treat different frequencies differently. A low-pass filter uses an inductor’s rising reactance at high frequencies to block high-frequency signals while letting low frequencies through. A high-pass filter uses a capacitor’s falling reactance at high frequencies to do the reverse. Audio crossover networks in speaker systems split a music signal so that bass goes to the woofer and treble goes to the tweeter, all by exploiting the frequency-dependent behavior of inductors and capacitors.
In power systems, engineers use capacitor banks to offset the inductive reactance of motors and transformers. This reduces the reactive power flowing through the grid and improves efficiency. Radio frequency chokes, which are inductors designed to present high reactance at radio frequencies but near-zero reactance at DC, are used to block unwanted high-frequency signals while allowing DC power to pass.
Unintended Reactance at High Frequencies
Every physical wire has a tiny amount of inductance, and any two conductors near each other create a small capacitance. At low frequencies, these parasitic effects are negligible. But as frequency climbs into the hundreds of kilohertz and beyond, they start to matter. A wire that behaves like a simple conductor at 60 Hz can act like a noticeable inductor at 100 MHz. Similarly, traces on a circuit board running close together develop parasitic capacitance that creates unintended signal paths.
This is why high-frequency circuit design requires careful attention to layout, cable length, and shielding. Components that work perfectly at audio frequencies can behave unpredictably at radio frequencies because their parasitic reactance becomes significant relative to the intended circuit values.