# What is impedance?

From the school's physics program everyone is knownsuch an element as a resistor, which is characterized by an electrical resistance, that is, a value equal to the ratio of voltage to current. The concept of electrical resistance is relevant only in circuits with a constant current. If a circuit with alternating current is used, the resistance on the inductor and on the capacitor tends to zero and to infinity, respectively, which is not true.

In this case, the concept of impedance in 1886, physicist Heaviside, was introduced to replace the classical concept of "resistance."

## What is the impedance of a two-terminal network

It is convenient to analyze the concept of impedance onsimple two-terminal network - a set of passive elements (resistor, inductor and capacitor) and two points of connection (pole) with the rest of the circuit. In such a scheme, the external properties of a two-terminal network describe impedance-a complex impedance.

Impedance - the ratio of the complex amplitudevoltage applied to the two-terminal network to the complex amplitude of the current passing through it. Unlike conventional resistance, the impedance varies from the frequency of the signal passing through the network.

In everyday life, the concept of impedance can be encountered when choosing headphones.

To study physical or other concepts, go to the Definitions section.