Reactance vs Resistance: Why AC Circuits Behave Differently

Updated on January 8, 2026

If you've ever tried to troubleshoot an AC circuit using the same mental model you use for DC, you've probably run into a wall. The LED you calculated to run at 20 mA is glowing at half brightness. The filter isn't rolling off where you expected. The capacitor seems to be "doing something" that a resistor just wouldn't do. The reason for all of this is reactance — and understanding it properly will change how you design and debug circuits forever.

Let's start with what's actually happening inside a resistor, an inductor, and a capacitor when AC flows through them, because the physics is where the intuition lives.

Resistance: Pure Dissipation, No Surprises

A resistor opposes current by converting electrical energy into heat. That's it. The voltage across a resistor is always perfectly in phase with the current through it — they rise together, they fall together, they hit zero at the same instant. Ohm's Law works directly: V = IR, and it works at any frequency, including DC (which is just AC at 0 Hz).

Resistance is real in the mathematical sense too — it's a real number, measured in ohms, and it doesn't change with frequency. A 100Ω resistor looks like 100Ω whether you're driving it at 1 Hz or 1 MHz.

This simplicity makes resistors predictable, but it also means they can only dissipate energy. They can't store it, and they can't return it to the circuit. That limitation is exactly what makes inductors and capacitors so useful — and so confusing at first.

Capacitors: Opposition That Decreases With Frequency

A capacitor stores energy in an electric field between its plates. When you apply a voltage, charge builds up on those plates. When you remove it, the charge flows back out. In DC, once the capacitor is fully charged, current stops flowing entirely — it's an open circuit.

In AC, the voltage is constantly reversing, which means the capacitor is constantly charging and discharging. Current flows continuously, not because charge is crossing the gap between the plates (it never does), but because it's constantly accumulating and depleting on each side. Here's the key insight: the faster the voltage changes, the more current flows. At high frequencies, the capacitor barely has time to charge before the polarity flips, so it looks almost like a short circuit. At very low frequencies, it has plenty of time to charge up and resist further current — it looks more like an open.

This frequency-dependent opposition is called capacitive reactance, denoted XC, and it's calculated as:

X_C = 1 / (2π × f × C)

where f is frequency in hertz and C is capacitance in farads. Plug in a 10 µF capacitor at 60 Hz and you get about 265Ω. At 600 Hz, it drops to 26.5Ω. At 6 kHz, just 2.65Ω. The reactance falls with rising frequency — the capacitor becomes more and more "transparent" to high-frequency signals.

But here's what makes capacitive reactance fundamentally different from resistance: the voltage and current are not in phase. Specifically, the current through a capacitor leads the voltage by 90°. Current peaks a quarter cycle before the voltage does. You can remember this with the mnemonic ICE — in a Capacitor, I leads E (where E is the old notation for EMF/voltage).

Inductors: Opposition That Increases With Frequency

An inductor stores energy in a magnetic field. When current flows through a coil of wire, it builds a magnetic field. When you try to change that current — either increase or decrease it — the magnetic field pushes back (this is Lenz's Law in action). The inductor actively resists changes to current flow.

In DC, once current is steady, an inductor looks like just a length of wire — the only opposition is the small DC resistance of the coil. But with AC, the current is constantly changing, and so the magnetic field is constantly in flux. The faster the current tries to change, the harder the inductor pushes back. This means inductive reactance increases with frequency — the exact opposite of a capacitor.

Inductive reactance XL is calculated as:

X_L = 2π × f × L

where L is inductance in henries. A 10 mH inductor at 1 kHz has XL = 62.8Ω. At 10 kHz, it's 628Ω. At 100 kHz, 6.28 kΩ. It scales linearly with frequency.

The phase relationship is also opposite to a capacitor: in an inductor, voltage leads current by 90°, or equivalently, current lags voltage by 90°. The mnemonic ELI covers this — in an L (inductor), E leads I. Together, ELI the ICE man is how electrical engineers have remembered these relationships for generations.

Impedance: The Full Picture

When a real circuit mixes resistors, capacitors, and inductors, you can't just add their ohm values together — the phase angles matter. This is where impedance comes in. Impedance (Z) is the total opposition to AC current, expressed as a complex number:

Z = R + jX

Here, R is the resistance (the real part) and X is the net reactance (the imaginary part, marked by the j operator — electrical engineers use j instead of i to avoid confusion with current). Capacitive reactance is negative imaginary (−jXC) and inductive is positive imaginary (+jXL).

The magnitude of impedance — what actually limits how much current flows — is:

|Z| = √(R² + X²)

And the phase angle between voltage and current is:

θ = arctan(X / R)

When X is positive (inductive dominance), current lags voltage. When X is negative (capacitive dominance), current leads. When X = 0, you have pure resistance — or resonance, which happens when XL exactly cancels XC.

Resonance: When Reactances Cancel

At resonance, the inductive and capacitive reactances are equal and opposite, so they cancel out completely. The circuit impedance drops to just the series resistance R (in a series RLC circuit). Current peaks dramatically. This is the principle behind radio tuning, bandpass filters, antenna matching, and a dozen other things in electronics.

The resonant frequency of a series or parallel LC circuit is:

f_r = 1 / (2π × √(LC))

For a 100 µH inductor and 100 pF capacitor, that's about 1.59 MHz — smack in the AM radio band. No coincidence there.

At resonance, the voltage across an inductor or capacitor can actually be higher than the source voltage — sometimes dramatically so. This voltage magnification factor is called the Q factor (quality factor). A high-Q resonant circuit can have voltages across the reactive components that are 50× or 100× the input. This is why RF engineers worry about component ratings at resonance even when input power is low.

Why This Matters for Real Engineering Work

Understanding the resistance-reactance distinction isn't just academic. It has immediate, practical consequences:

Filter design — every passive filter (RC low-pass, LC bandpass, etc.) works because reactance changes with frequency while resistance doesn't. The frequency at which XC = R in an RC filter defines the -3 dB cutoff point. Change the capacitor value, you change the frequency. The math is completely transparent once you understand reactance.

Power factor — in power systems, reactive loads (motors, transformers, fluorescent ballasts) draw current that's out of phase with voltage. You're moving current back and forth without doing useful work. Utilities charge large industrial consumers for poor power factor because it wastes grid capacity. Capacitor banks are installed specifically to cancel inductive reactance and bring the phase angle back toward zero.

Impedance matching — maximum power transfer between a source and a load happens when the source impedance is the complex conjugate of the load impedance. In RF work, this is the difference between a transmitter that delivers full power to an antenna and one that reflects half of it back as heat. Matching networks use inductors and capacitors specifically because they can transform impedances without dissipating power.

Decoupling and bypass capacitors — every digital IC needs bypass capacitors because the fast switching creates high-frequency current spikes that a power trace can't supply quickly enough (the inductance of the trace itself causes a voltage dip). A capacitor placed close to the IC provides local charge storage. Its low impedance at high frequencies (small XC) lets it supply current faster than the distant power supply can. Getting the capacitor value right means calculating the frequency of the transient and choosing C so that XC is acceptably low there.

Bringing It Together

The fundamental divide between resistance and reactance is this: resistance dissipates energy as heat and is independent of frequency; reactance stores and returns energy and is entirely defined by frequency. Resistance is always real; reactance is always imaginary in the complex-number sense, which isn't mystical — it's just saying that the current and voltage are 90° out of phase.

Impedance is the synthesis of both, and it's what you calculate when you want to know how a real component or circuit section responds to AC at a given frequency. Get comfortable converting between polar form (magnitude and phase angle) and rectangular form (R + jX), and AC circuit analysis will start to feel as natural as DC.

Once you've internalized that a capacitor's impedance falls with frequency while an inductor's rises, and that both of them shift phase in opposite directions, you have the core toolkit for understanding filters, resonators, matching networks, and power factor — essentially everything that makes AC electronics distinct from the simpler DC world.

A good engineering calculator that handles complex impedance, reactance at any frequency, and resonant frequency computation will save you time during design and verification. But the deeper payoff is building the physical intuition that lets you look at a circuit and predict its behavior before you run a single calculation.