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Reactance is similar to resistance, except that it applies only to the AC characteristic of an electric current. Reactance varies, depending on the whether the considered device is inductive or capacitive. Simply put, an inductor's reactance increases with frequency, while a capacitor's reactance decreases with frequency.

Here's the formula for an inductor's reactance:

Xi = 2pifL

Xi is your reactance, in Ohms.
f is your frequency, in Hertz.
L is your device's inductance, in Henries.

Here's the formula for a capacitor's reactance:

Xc = 1/(2pifC)

Xi is your reactance, in Ohms.
f is, again, your frequency in Hertz.
C is your device's capacitance, in Farads.

Re*act"ance (?), n. [React + -ance.] (Elec.)

The influence of a coil of wire upon an alternating current passing through it, tending to choke or diminish the current, or the similar influence of a condenser; inductive resistance. Reactance is measured in ohms. The reactance of a circuit is equal to the component of the impressed electro-motive force at right angles to the current divided by the current, that is, the component of the impedance due to the self-inductance or capacity of the circuit.


© Webster 1913

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