Impedance is a measure of the opposition to electrical flow. It is a combination of resistance and reactance, and is essentially anything and everything that obstructs the flow of electrons within an electrical circuit. 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. The impedance of a circuit element can be defined as the ratio of the phasor voltage across the element to the phasor current through the element, as determined by the relative amplitudes and phases of the voltage and current.
In phasor terms, impedance Z is represented as a combination of resistance R and reactance X as: Z = R + j X, where reactance X is a combination of Inductive XL and capacitive XC.
