Inductive & Capacitive Reactance — A/L Physics

Reactance (X) The opposition to alternating current (AC) offered by an inductor or capacitor, measured in ohms (Ω). Unlike resistance, reactance depends on the frequency of the AC supply and causes a phase shift between voltage and current.

Inductive Reactance (Xₗ) The opposition to current flow caused by an inductor’s self-induced back emf. For a pure inductor, voltage leads current by 90°. As frequency increases, the back emf increases, so Xₗ increases. Example: A choke coil in a fluorescent lamp limits current without dissipating much power.

Capacitive Reactance (X꜀) The opposition to current flow caused by a capacitor’s electric field charging and discharging. For a pure capacitor, current leads voltage by 90°. As frequency increases, the capacitor charges/discharges faster, so X꜀ decreases. Example: A coupling capacitor in an audio amplifier passes high-frequency signals but blocks DC.