What is impedance?
From the school curriculum of physics, everyone knows such an element as a resistor, which is characterized by 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 direct current. If a circuit with alternating current is used, the resistance on the inductance coil and on the capacitor tends to zero and to infinity, respectively, which is not true.
In this case, the concept of impedance in 1886 was introduced by the physicist Heaviside to replace the classic concept of "resistance."
What is a two-pole impedance
It is convenient to analyze the concept of impedance on a simple two-port network — a set of passive elements (resistor, inductor and capacitor) and two connection points (poles) with the rest of the circuit. In such a scheme, the external properties of a two-port network are described by impedance — complex (total) resistance.
Impedance is the ratio of the complex amplitude of a voltage applied to a two-terminal network to the complex amplitude of the current passing through it.Unlike conventional impedance, the impedance varies with the frequency of the signal passing through the network.
In everyday life, the concept of impedance can be encountered when choosing headphones.