Reactance — Core Principles

Core Principles

Reactance is the opposition offered by inductors and capacitors to the flow of alternating current (AC). Unlike resistance, which dissipates energy, reactance stores and returns energy, leading to a phase difference between voltage and current.

There are two types: inductive reactance ($X_L$) and capacitive reactance ($X_C$). Inductive reactance, $X_L = 2\pi f L$, is directly proportional to frequency ($f$) and inductance ($L$). It causes voltage to lead current by $90^\circ$.

Capacitive reactance, $X_C = 1/(2\pi f C)$, is inversely proportional to frequency ($f$) and capacitance ($C$). It causes current to lead voltage by $90^\circ$. Both are measured in ohms (\Omega). At DC ($f=0$), an inductor acts as a short circuit ($X_L=0$), and a capacitor acts as an open circuit ($X_C=\infty$).

Reactance is a key component of impedance in AC circuits and is fundamental to understanding resonance, filters, and power factor correction.

Often confused with

Side-by-side differences the NEET paper likes to test.

Reactance vs Resistance

Aspect	Reactance	Resistance
Definition	Opposition to current flow that dissipates electrical energy as heat.	Opposition to AC current flow that stores and returns electrical energy.
Energy Handling	Dissipates energy (converts to heat).	Stores energy (magnetic or electric field) and returns it to the circuit.
Phase Relationship	Voltage and current are in phase (phase difference = $0^\circ$).	Voltage and current are $90^\circ$ out of phase (voltage leads current in inductor, current leads voltage in capacitor).
Frequency Dependence	Generally independent of frequency (for ideal resistors).	Strongly dependent on frequency ($X_L \propto f$, $X_C \propto 1/f$).
Components	Resistors.	Inductors (inductive reactance, $X_L$) and Capacitors (capacitive reactance, $X_C$).
Average Power Dissipation	Non-zero (P = $I_{RMS}^2 R$).	Zero (for ideal reactive components over a full cycle).

Resistance and reactance both oppose current flow and are measured in ohms, but their fundamental nature differs significantly. Resistance is a measure of energy dissipation, converting electrical energy into heat, and keeps voltage and current in phase.

Reactance, on the other hand, is a measure of energy storage and release, causing a $90^\circ$ phase shift between voltage and current. Reactance is also highly frequency-dependent, unlike ideal resistance.

Understanding this distinction is crucial for analyzing AC circuits, especially when calculating power and impedance.

Why it is tested: For NEET, distinguishing between resistance and reactance is fundamental. Questions often test the conceptual understanding of energy dissipation versus storage, phase relationships, and frequency dependence. Misconceptions between these two can lead to errors in calculating impedance, power factor, and understanding resonance in RLC circuits. It's a core concept that underpins many AC circuit problems.