![]() ![]() Wietlisbach avoided using differential equations by expressing AC currents and voltages as exponential functions with imaginary exponents (see § Validity of complex representation). Perhaps the earliest use of complex numbers in circuit analysis was by Johann Victor Wietlisbach in 1879 in analysing the Maxwell bridge. Instruments used to measure the electrical impedance are called impedance analyzers. The reciprocal of impedance is admittance, whose SI unit is the siemens, formerly called mho. In multiple port networks, the two-terminal definition of impedance is inadequate, but the complex voltages at the ports and the currents flowing through them are still linearly related by the impedance matrix. The notion of impedance is useful for performing AC analysis of electrical networks, because it allows relating sinusoidal voltages and currents by a simple linear law. ![]() However, Cartesian complex number representation is often more powerful for circuit analysis purposes. Its symbol is usually Z, and it may be represented by writing its magnitude and phase in the polar form | Z| ∠θ. Impedance can be represented as a complex number, with the same units as resistance, for which the SI unit is the ohm ( Ω). ![]() Impedance extends the concept of resistance to alternating current (AC) circuits, and possesses both magnitude and phase, unlike resistance, which has only magnitude. In general, it depends upon the frequency of the sinusoidal voltage. Quantitatively, the impedance of a two-terminal circuit element is the ratio of the complex representation of the sinusoidal voltage between its terminals, to the complex representation of the current flowing through it. In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit. ![]()
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