Which statement best defines reactance?

• A. The opposition offered to the flow of an alternating current by the inductance, capacitance or both in any circuit
• B. The measure of current in a circuit
• C. The conversion of AC to DC
• D. The opposition to direct current in a resistor

Answer: (A)

Reactance is the frequency-dependent opposition to the flow of alternating current that comes from energy-storage elements like inductors and capacitors. Because inductors store energy in magnetic fields, they push back against changes in current (causing current to lag voltage), while capacitors store energy in electric fields and oppose changes in voltage (causing current to lead voltage). The strength of this opposition is quantified as X_L = 2πfL for an inductor and X_C = 1/(2πfC) for a capacitor, and it depends on frequency. When a circuit has inductance, capacitance, or both, the portion of the impedance that arises from these storage elements is called reactance, i.e., the part of the circuit’s opposition to AC that is not due to resistance.

The other options describe different ideas: the measure of current is just current, not reactance; converting AC to DC is rectification, not reactance; and opposing direct current in a resistor refers to resistance, not the reactive part.