The magnitude is the ratio of the voltage and current amplitudes, while the phase is the voltage–current phase difference.
The reciprocal of reactance is susceptance.
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Determining the voltage-current relationship requires knowledge of both the resistance and the reactance. The reactance on its own gives only limited physical information about an electrical component or network.
There are certain specific effects that depend on the reactance alone, for example; resonance in an series RLC circuit occurs when the reactances XC and XL are equal but opposite, and the impedance has a phase angle of zero.
A capacitor consists of two conductors separated by an insulator, also known as a dielectric.
At low frequencies a capacitor is open circuit, as no current flows in the dielectric. A DC voltage applied across a capacitor causes charge to accumulate on one side; the electric field due to the accumulated charge is the source of the opposition to the current. When the potential associated with the charge exactly balances the applied voltage, the current goes to zero.
Driven by an AC supply, a capacitor will only accumulate a limited amount of charge before the potential difference changes sign and the charge dissipates. The higher the frequency, the less charge will accumulate and the smaller the opposition to the current.
For an inductor consisting of a coil with N loops this gives.
The back-emf is the source of the opposition to current flow. A constant direct current has a zero rate-of-change, and sees an inductor as a short-circuit (it is typically made from a material with a low resistivity). An alternating current has a time-averaged rate-of-change that is proportional to frequency, this causes the increase in inductive reactance with frequency.
The origin of the different signs for capacitive and inductive reactance is the phase factor in the impedance.