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Electrical impedance, or simply impedance, describes a measure of opposition to alternating current (AC). Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative amplitudes of the voltage and current, but also the relative phases. When the circuit is driven with direct current (DC) there is no distinction between impedance and resistance; the latter can be thought of as impedance with zero phase angle.
The symbol for impedance is usually and it may be represented by writing its magnitude and phase in the form . However, complex number representation is more powerful for circuit analysis purposes. The term impedance was coined by Oliver Heaviside in July 1886.^{[1]}^{[2]} Arthur Kennelly was the first to represent impedance with complex numbers in 1893.^{[3]}
Impedance is defined as the frequency domain ratio of the voltage to the current^{[citation needed]}. In other words, it is the voltage–current ratio for a single complex exponential at a particular frequency ω. In general, impedance will be a complex number, with the same units as resistance, for which the SI unit is the ohm (Ω). For a sinusoidal current or voltage input, the polar form of the complex impedance relates the amplitude and phase of the voltage and current. In particular,
 The magnitude of the complex impedance is the ratio of the voltage amplitude to the current amplitude.
 The phase of the complex impedance is the phase shift by which the current is ahead of the voltage.
The reciprocal of impedance is admittance (i.e., admittance is the currenttovoltage ratio, and it conventionally carries units of siemens, formerly called mhos).
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