Glossary term
Impedance
The complex opposition that an electrical circuit presents to alternating current, combining resistance and reactance.
Definition
quantityImpedance is the complex ratio of voltage to current in an AC circuit, representing both resistive and reactive opposition to current flow.
Impedance generalizes resistance for sinusoidal steady-state analysis. It is written as Z = R + jX, where R is resistance and X is reactance. The magnitude of impedance determines the voltage-current amplitude ratio, while its phase determines whether current leads or lags voltage. Impedance is central to power systems, electronics, filters, transmission lines, sensors, motors, antennas, and signal integrity.
Impedance describes how strongly an electrical network opposes alternating current. Unlike DC resistance, impedance includes both energy dissipation and energy storage. A resistor dissipates energy as heat. An inductor stores energy in a magnetic field. A capacitor stores energy in an electric field. In AC analysis these effects combine into a complex quantity:
where R is resistance, X is reactance, and j is the imaginary unit. The magnitude |Z| gives the ratio of voltage amplitude to current amplitude. The phase angle of Z gives the phase shift between voltage and current.
Frequency dependence
Impedance usually depends on frequency. A pure resistor has constant impedance R. An inductor has impedance:
so its opposition increases with frequency. A capacitor has impedance:
so its opposition decreases with frequency. Real components also have parasitic resistance, capacitance, inductance, dielectric loss, skin effect, and temperature dependence, so measured impedance departs from ideal formulas.
Engineering use
Impedance controls current flow, voltage division, filtering, resonance, power factor, fault current, signal reflections, bandwidth, and noise susceptibility. In power systems, source impedance affects short-circuit levels and voltage drop. In electronics, input and output impedance determine loading between stages. In transmission lines, impedance mismatch causes reflections. In sensors, impedance can determine whether a measurement circuit disturbs the quantity being measured.
Impedance is also central to apparent power and power factor. When current and voltage are not in phase, not all apparent power becomes real power. Reactive elements exchange energy with the source each cycle, increasing current for the same useful power transfer.
Common mistakes
A common mistake is treating impedance as a single fixed number without specifying frequency and operating condition. Another is quoting only magnitude while ignoring phase. For filters, motors, cables, transformers, antennas, and power converters, phase can be just as important as magnitude. Good documentation states the frequency range, measurement method, temperature, signal amplitude, bias condition, and whether impedance is differential, common-mode, input, output, source, load, or characteristic impedance.