Impedance, Electrical
a quantity characterizing the opposition presented by a circuit to an alternating current. Often called simply impedance, it is measured in ohms. In the case of a sinusoidal alternating current, the impedance is expressed by the ratio of the amplitude of the voltage applied to the circuit terminals and the amplitude of the current throught the circuit; here, the impedance is equal to 
, where r is the resistance and x is the reactance. In the case of a nonsinusoidal alternating current, the impedance is determined separately for each kth har monic component: 
.
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