impedance

impedance in electricity, measure in ohms of the degree to which an electric circuit resists the flow of electric current when a voltage is impressed across its terminals. Impedance is expressed as the ratio of the voltage impressed across a pair of terminals to the current flow between those terminals. When a circuit is supplied with steady direct current, the impedance equals the total resistance of the circuit. The resistance depends upon the number of electrons that are free to become part of the current and upon the difficulty that the electrons have in moving through the circuit. When a circuit is supplied with alternating current, the impedance is affected by the inductance and capacitance in the circuit. When supplied with alternating current, elements of the circuit that contain inductance or capacitance build up voltages that act in opposition to the flow of current. This opposition is called reactance, and it must be combined with the resistance to find the impedance. The reactance produced by inductance is proportional to the frequency of the alternating current. The reactance produced by capacitance is inversely proportional to the frequency of the alternating current. In order for a source of electricity that has an internal impedance to transfer maximum power to a device that also has an impedance, the two impedances must be matched. For example, in the simple case of pure resistances, the resistance of the source must also equal the resistance of the device. Impedance matching is important in any electrical or electronic system in which power transfer must be maximized.

impedance (symbol Z) Property of a component in an electric circuit that opposes the passage of current. In a direct current (DC) circuit, the impedance corresponds to the resistance (R). In an alternating current (AC) circuit with capacitance or inductance, the additional property of reactance (X) has to be allowed for, as expressed in the equation Z² = R² + X². All of these quantities are measured in ohms.

im·ped·ance / imˈpēdns/ • n. the effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.