What is the primary difference between resistance and reactance?

The fundamental difference lies in energy handling. Resistance dissipates electrical energy as heat, meaning it converts electrical energy into thermal energy, which is irreversible. Reactance, on the other hand, stores electrical energy (in a magnetic field for inductors, or an electric field for capacitors) during one part of the AC cycle and then returns that energy to the circuit during another part of the cycle. This energy exchange means no net power is dissipated by an ideal reactive component over a full cycle. This also leads to a phase difference between voltage and current in reactive components, which is absent in pure resistors.

Why is reactance measured in ohms, just like resistance?

Reactance is measured in ohms (\Omega) because it represents an opposition to current flow, similar to resistance. According to Ohm's Law for AC circuits, the ratio of the peak (or RMS) voltage across a component to the peak (or RMS) current through it gives a quantity measured in volts per ampere, which is defined as an ohm. Although their physical mechanisms are different (energy dissipation vs. energy storage), both resistance and reactance quantify how much a component impedes current flow for a given applied voltage, hence sharing the same unit.

How does frequency affect inductive reactance ($X_L$) and capacitive reactance ($X_C$)?

Frequency has opposite effects on inductive and capacitive reactance. Inductive reactance ($X_L = 2\pi f L$) is directly proportional to frequency ($f$). This means as frequency increases, an inductor offers greater opposition to current flow. Conversely, capacitive reactance ($X_C = 1/(2\pi f C)$) is inversely proportional to frequency. As frequency increases, a capacitor offers less opposition to current flow. This frequency dependence is crucial for designing filters and tuning circuits.

What happens to an inductor and a capacitor in a DC circuit?

In a DC circuit, where the frequency is effectively zero ($f=0$), their behavior is quite distinct. For an ideal inductor, $X_L = 2\pi (0) L = 0$. This means an inductor offers no opposition to steady DC current and acts like a short circuit after any initial transient period. For an ideal capacitor, $X_C = 1/(2\pi (0) C) \to \infty$. This implies a capacitor offers infinite opposition to steady DC current, acting like an open circuit once it is fully charged, blocking the flow of DC.

What is the significance of the phase difference in reactive circuits?

The phase difference between voltage and current is a hallmark of reactive components and is crucial for understanding power in AC circuits. In a purely inductive circuit, voltage leads current by $90^\circ$. In a purely capacitive circuit, current leads voltage by $90^\circ$. This $90^\circ$ phase shift means that when voltage is maximum, current is zero, and vice-versa. As a result, the instantaneous power oscillates between positive and negative values, with the net average power dissipated over a full cycle being zero for ideal reactive components. This phase difference is also vital for calculating the overall impedance and power factor of an AC circuit.

Reactance

Physics

NEET UG

Version 1Updated 22 Mar 2026

Explore This Topic

Definition Deep Explanation Core Principles NEET Importance Problem Strategy Practice Problems MCQ Practice Quick Revision

Reactance is the opposition offered by an inductor or a capacitor to the flow of alternating current (AC) due to the storage and release of energy in their magnetic and electric fields, respectively. Unlike resistance, which dissipates energy as heat, reactance stores and returns energy to the circuit, resulting in a phase difference between the voltage across and the current through the component…

Quick Summary

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.

Vyyuha

Your 6-Month Blueprint, Updated Nightly

AI analyses your progress every night. Wake up to a smarter plan. Every. Single.…

Key Concepts

Inductive Reactance (

X_L

) and Frequency Dependence

Inductive reactance, $X_L$ , quantifies how much an inductor opposes AC current. Its value is directly…

Capacitive Reactance (

X_C

) and Frequency Dependence

Capacitive reactance, $X_C$ , describes the opposition a capacitor offers to AC current. Its value is…

Phase Relationships in Purely Reactive Circuits

The phase relationship between voltage and current is a defining characteristic of reactive components. In a…

Inductive Reactance ($X_L$): — Opposition by inductor to AC.

X_L = \omega L = 2\pi f L

. Directly proportional to

f

. Voltage leads current by

90^\circ

. At DC (

f=0

X_L=0

(short circuit).

Capacitive Reactance ($X_C$): — Opposition by capacitor to AC.

X_C = 1/(\omega C) = 1/(2\pi f C)

. Inversely proportional to

f

. Current leads voltage by

90^\circ

. At DC (

f=0

X_C=\infty

(open circuit).

Units: — Both

X_L

and

X_C

are measured in Ohms (\Omega).

Difference from Resistance: — Reactance stores/returns energy; resistance dissipates energy. Reactance causes phase shift; resistance doesn't.

ELI the ICE man

ELI: — In an E (voltage) leads L (inductor) I (current) circuit, voltage leads current.

ICE: — In an I (current) leads C (capacitor) E (voltage) circuit, current leads voltage.

This helps remember the $90^\circ$ phase relationships for ideal inductors and capacitors.