Impedance is the total opposition to current flow in an alternating current (AC) circuit, measured in ohms (Ω). It combines two distinct effects: resistance, which opposes current regardless of conditions, and reactance, which opposes current only when the voltage or current is changing. If you already understand resistance from basic DC circuits, impedance is its broader, more complete cousin that accounts for everything happening in AC.
Why Resistance Alone Isn’t Enough
In a simple DC circuit with a battery and a resistor, Ohm’s law (V = IR) tells you everything you need to know. Resistance is the only thing opposing current, and it stays constant no matter what. But the moment you switch to AC, where voltage and current are constantly rising, falling, and reversing direction, two additional components start playing a role: inductors and capacitors. These components store and release energy in ways that create a second type of opposition called reactance.
Reactance behaves differently from resistance in one critical way: it depends on how fast the AC signal is oscillating, meaning it changes with frequency. Resistance stays the same whether your signal is 60 Hz household power or a 2.4 GHz Wi-Fi signal. Reactance does not. This is why engineers needed a broader concept, impedance, to describe total opposition in AC circuits.
The Two Types of Reactance
Inductors (coils of wire) resist changes in current by generating a magnetic field. Their opposition to AC is called inductive reactance, and it increases with frequency. A coil that barely slows down a 60 Hz signal can almost completely block a signal at millions of hertz. The relationship is direct: double the frequency, double the inductive reactance.
Capacitors (two conductive plates separated by an insulator) resist changes in voltage by storing charge. Their opposition is called capacitive reactance, and it works in the opposite direction. Capacitive reactance decreases as frequency goes up. At very high frequencies, a capacitor lets current pass through almost freely. At very low frequencies, it acts nearly like an open switch. This inverse relationship is why capacitors are often used to filter out low-frequency noise while allowing high-frequency signals through.
How Resistance and Reactance Combine
You might assume that you can just add resistance and reactance together to get impedance, but it doesn’t work that way. Resistance and reactance are “90 degrees apart,” meaning they affect the circuit at different points in the AC cycle. Because of this offset, they combine using the Pythagorean theorem rather than simple addition.
Picture a right triangle. Resistance sits along the bottom (the horizontal leg). Reactance runs along the vertical leg. Impedance is the hypotenuse, the longest side connecting the two. This visualization is called the impedance triangle, and it gives you the magnitude of impedance: the square root of resistance squared plus reactance squared. So a circuit with 3 ohms of resistance and 4 ohms of reactance doesn’t have 7 ohms of impedance. It has 5.
Engineers also express impedance using complex numbers, written as Z = R + jX, where R is resistance, X is reactance, and j represents the imaginary component (used instead of “i” to avoid confusion with current). This notation captures both the magnitude of the opposition and the timing relationship between voltage and current in a single expression.
Phase Angle: When Voltage and Current Fall Out of Step
In a purely resistive circuit, voltage and current peak at exactly the same moment. They’re perfectly synchronized. Add reactance into the mix and that synchronization breaks down. The peaks of voltage and current shift apart in time, a phenomenon called phase difference.
The size of that shift is the phase angle, and it’s directly tied to impedance. In a circuit dominated by inductors, voltage peaks before current (voltage “leads”). In a circuit dominated by capacitors, current peaks before voltage (current “leads”). The phase angle tells you how far apart they are, measured in degrees. A purely resistive circuit has a phase angle of zero. A circuit with only an inductor or only a capacitor has a phase angle of 90 degrees.
This matters practically because phase differences affect how efficiently power is delivered. When voltage and current are out of sync, some energy sloshes back and forth between the source and the reactive components instead of doing useful work. Industrial facilities with large motors (which are inductive) often need to correct for this by adding capacitors to bring the phase angle closer to zero.
The Core Equation
Impedance follows the same basic relationship as Ohm’s law for DC circuits, just generalized for AC. The formula is V = IZ, where V is voltage, I is current, and Z is impedance. If you know any two of these values, you can calculate the third. The unit is still the ohm, represented by the Greek letter omega (Ω), just like resistance. An impedance of 50 ohms means that for every volt applied, 1/50th of an ampere flows, accounting for both resistive losses and reactive energy storage.
Why Impedance Matching Matters
One of the most common practical applications of impedance is matching. The general rule is simple: maximum power transfers from a source to a load when their impedances match. When an amplifier with 8 ohms of output impedance drives a speaker with 8 ohms of impedance, the system delivers power as efficiently as possible, reaching 50% of total power delivered to the speaker.
When impedances don’t match, problems stack up. If the load impedance is too low, the source has to push excessive current, which can cause distortion, overheating, or damage. If the load impedance is too high, very little power actually reaches it. In audio systems, this shows up as weak, distorted, or noisy sound. In radio and telecommunications, impedance mismatches cause signals to reflect back toward the source instead of being transmitted, wasting energy and degrading performance. This is why coaxial cables, antennas, and amplifiers are all rated at standardized impedances (commonly 50 or 75 ohms) to ensure clean signal transfer.
Impedance in Body Composition Measurement
Impedance isn’t limited to electronics. A technique called bioelectrical impedance analysis (BIA) sends a small, harmless AC current through your body to estimate body composition. The principle relies on the fact that different tissues oppose electrical current differently. Muscle and organs, which contain a lot of water and electrolytes, conduct electricity relatively well and have low impedance. Fat tissue contains much less water and has higher impedance.
By measuring both the resistance and reactance components of the body’s impedance, BIA devices estimate fat mass, fat-free mass, body cell mass, and hydration levels. The reactance component specifically reflects the health of cell membranes in metabolically active tissue like muscle and organs, while resistance reflects total body water. These measurements are quick, noninvasive, and inexpensive, which is why BIA shows up in everything from bathroom scales to clinical nutrition assessments for patients with chronic diseases.