Impedance is a measure of the opposition to electrical flow in a circuit, represented by the symbol Z and measured in ohms. In DC systems, impedance and resistance are the same, defined as the voltage across an element divided by the current (R = V/I). In AC systems, the "reactance" enters the equation due to the frequency-dependent contributions of capacitance and inductance. Impedance in an AC system is still measured in ohms and represented by the equation Z = V/I, but V and I are frequency-dependent.
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 general, it depends upon the frequency of the sinusoidal voltage. Impedance extends the concept of resistance to alternating current (AC) circuits, and possesses both magnitude and phase, unlike resistance, which has only magnitude.
Impedance is a combination of resistance and reactance, which is anything that obstructs the flow of electrons within an electrical circuit. It affects the generation of current through the electrical circuit and is present in all the possible components of the circuit and across all possible electrical circuits. Impedance is mathematically symbolized by the letter Z and has its unit as ohm. It is a superset of both resistance and reactance combined.
In phasor terms, impedance Z is represented as a combination of resistance R and reactance X as: Z = R + jX, where reactance X is a combination of Inductive XL and capacitive XC.
