sähköimpedanssi

Sähköimpedanssi (electrical impedance) is a measure of a material's opposition to the flow of alternating electric current. It combines both resistive and reactive components, expressed as a complex quantity Z = R + jX, where R is resistance and X is reactance. Reactance arises from capacitive and inductive properties of the medium and depends on the signal frequency.

The concept originates from early 20th‑century studies of AC circuit behavior. Measurement is performed with impedance analyzers, LCR meters, or specialized techniques such as four‑probe methods to minimise contact resistance. In biomedical contexts, bioelectrical impedance analysis (BIA) estimates body composition; higher fat tissue presents higher impedance, whereas muscle tissue conducts better due to its water content.

In electronics, component impedance is critical for signal integrity and matching network design. For example, passive components like resistors, capacitors, and inductors display frequency‑dependent impedance that influences filter characteristics. Prototypes and printed circuit board designers use impedance control to maintain performance in high‑speed digital and RF circuits.

Industrial applications include material characterization in polymers and ceramics, where impedance spectra reveal dielectric relaxation processes. Electrochemical impedance spectroscopy (EIS) is used to study battery degradation and corrosion in metals.

Important parameters are magnitude |Z| and phase angle θ, often plotted on a Nyquist or Bode plot to visualize frequency response. Typical impedance values vary widely: a household resistor might exhibit a few ohms, while a biological tissue measurement can range from several thousand to tens of thousands of ohms, depending on frequency.

Sähköimpedanssi remains a fundamental tool bridging physics, engineering, and life sciences, enabling quantitative analysis of conductive, dielectric, and electrochemical systems.