Capacitive Reactance — the NEET Physics formula with its derivation, variables, validity constraints and worked solver.
Capacitive Reactance Applies to AC circuits containing capacitors. The reactance describes the opposition to alternating current flow offered by the capacitor. Start with the definition of current through a capacitor: i = C dv dt . Assume a sinusoidal voltage source v = V m ( t) . Substitute v into the current equation: i = C d dt (V m ( t)) . Differentiate: i = C V m ( t) = C V m ( t + /2) . Identify the peak current amplitude I m = C V m . Apply Ohm's Law analogy for peak values: V m = I m X C . Solve for Reactance: X C = V m I m = 1 C . Ideal capacitor assumed (no dielectric loss or ESR). Valid for steady-state sinusoidal AC sources. f > 0 for finite reactance (at f=0, reactance is infinite). Linear capacitance behavior assumed. Confusing Capacitive Reactance with Inductive Reactance (Xc is inversely proportional to frequency, while Xl is directly proportional). Using Capacitance (C) directly in Ohm's Law instead of Reactance (Xc). Assuming Xc exists in DC circuits (in DC, f=0, so Xc is infinite/open circuit). Ignoring the phase shift; current leads voltage by 90 degrees in a purely capacitive circuit.