What is reactance?

• A. The opposition presented to DC by resistors
• B. The opposition presented to AC by inductance or capacitance
• C. The loss of energy due to resistance
• D. The energy stored in a capacitor

Answer: (B)

Reactance is the opposition to alternating current that comes from energy-storage elements in a circuit. When current is alternating, inductors and capacitors store and release energy each cycle, which makes them resist the flow of AC in a way that depends on frequency. For an inductor, this opposition grows with frequency (X_L = ωL), because a faster-changing current builds more magnetic energy. For a capacitor, it shrinks with frequency (X_C = 1/(ωC)), since at higher frequencies the capacitor can charge and discharge more quickly and offer less impedance to the AC. Together, these effects form the reactive part of impedance, distinct from real power loss in resistors. The other ideas describe energy loss or energy storage without describing how energy storage elements create frequency-dependent opposition to AC, so this description best captures what reactance is.