Voltage Drop in Alternating-current Circuits: Impedance
In alternating-current circuits, opposition to current flow does occur because of resistance (just as in direct-current circuits). Alternating current circuits also present a second kind of opposition to current flow: reactance. This "total" opposition (resistance "plus" reactance) is called impedance. The impedance in an alternating-current circuit depends on the spacing and dimensions of the elements and conductors, the frequency of the alternating current, and the magnetic permeability of the elements, the conductors, and their surroundings.
The voltage drop in an AC circuit is the product of the current and the impedance (Z) of the circuit. Electrical impedance, like resistance, is expressed in ohms. Electrical impedance is the vector sum of electrical resistance, capacitive reactance, and inductive reactance. It is expressed by the formula, analogous to Ohm's law for direct-current circuits.
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