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 is a concept associated with the transmission of waves and electrical signals. There are many kinds of waves, and impedance is different in each of them, hence this disambiguation page. However, impedance is also a unifying concept. It is related to the load that is imposed on the source that generates a wave. It governs the reflection and transmission of waves incident on a change of medium. If the impedances in the two media match, there will be no reflection. One simple approach identifies two parameters for a wave: the restoring force that tries to return to equilibrium and the inertia of the medium displaced. Then the impedance is

Z = the square root of (restoring force) times (inertia)

while the velocity of the wave is

v = the square root of (restoring force) divided by (inertia)

For example, for a sound wave the restoring force is the modulus of elasticity, while the inertia is just the density. For some waves (light waves, for example) it is not obvious how this simple picture for mechanical waves can be applied, but the same general picture holds.

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