1 Complex ratio of sinusoidal voltage to current in an electric circuit or component. Its real part is the resistance dissipative or wattful impedance and its imaginary part reactance non-dissipative or wattless, which may be positive or negative according to whether the phase of the current lags or leads on that of the voltage. Resistance, reactance and impedance are all measured in ohms. Expressed symbolically, the impedance Z is given by R+jX where R is resistance, X is reactance and j=yj

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