Key Differences Between Reactance and Impedance

Impedance is the overall opposition to alternating current, expressed as a complex quantity Z=R+jXZ, where R is resistance and X is reactance.

Reactance is the part of an AC circuit’s opposition caused by energy storage elements—inductors and capacitors—that depends on frequency. It is the imaginary component of impedance.

Impedance is primarily used in AC analysis. In DC circuits (0 Hz), the inductive and capacitive reactance both become zero or infinite respectively (depending on the component), so only resistance remains, and impedance effectively reduces to plain resistance.

• Inductive reactance:$X_L$=2πfL

• Capacitive reactance: $X_C = \frac{1}{2\pi f C}$

The SI unit for impedance is the ohm (Ω).

In an AC circuit, resistance (R) is the real part (in ohms), and reactance (X) is the imaginary part (also in ohms), so the resulting impedance ZZZ has the unit of ohms as well.

• Reactance also has the same SI unit as resistance: the ohm (Ω).

• The difference is conceptual rather than in unit: a resistor dissipates energy (real power), while a purely reactive element (inductor or capacitor) does not dissipate power but stores and releases energy, yet both are quantified using ohms.

1. Inductive Reactance ($X_L$) – from inductors

2. Capacitive Reactance ($X_C$) – from capacitors

1. Resistive Impedance (purely real part)

2. Inductive Impedance (imaginary part due to inductors)

3. Capacitive Impedance (imaginary part due to capacitors)

A resistor and capacitor in series, powered by an AC source, together present a total impedance. The resistor contributes real (resistive) opposition, while the capacitor adds capacitive reactance, combining into a complex impedance.