Impedance
Definition
Impedance (symbol Z, measured in ohms Ω) is the total opposition to alternating current (AC) flow in a circuit, combining both resistance (R, opposition from conductors) and reactance (X, opposition from capacitors and inductors). Impedance is calculated as Z = √(R² + X²) and is a complex quantity with magnitude and phase angle. In purely resistive circuits (heaters, incandescent bulbs), impedance equals resistance. In circuits with motors, transformers, or capacitors, reactance adds to the impedance. Speaker impedance (typically 4Ω, 8Ω, or 16Ω) must match amplifier output impedance for optimal power transfer. Impedance affects power factor, voltage drop, and energy efficiency in AC systems.
Why is Impedance Important?
In electrical engineering and everyday applications, Impedance is a fundamental concept for understanding how electrical systems work. Whether you are an engineer designing circuits, an electrician sizing wires, or a homeowner estimating energy costs, this metric is essential for safety, efficiency, and accurate calculations.
Our electrical conversion calculators help you quickly convert between units and verify calculations, reducing errors and saving time in both professional and DIY electrical work.
What is Impedance?
Impedance (symbol Z) is the total opposition to alternating current (AC) flow in a circuit, measured in ohms (Ω). It extends the concept of resistance to AC circuits by combining resistance (R) and reactance (X) — the opposition caused by capacitors and inductors that varies with frequency.
Impedance vs Resistance
| Property | Resistance (R) | Impedance (Z) |
|---|---|---|
| Applies to | DC and AC (resistive loads) | AC circuits only |
| Unit | Ohms (Ω) | Ohms (Ω) |
| Frequency dependent? | No | Yes — changes with AC frequency |
| Components | R only | R + jX (resistance + reactance) |
| Phase shift | None (0°) | Can be 0° to ±90° |
Impedance Formula
Z = √(R² + X²)
Where X = reactance (inductive reactance XL or capacitive reactance XC)
| Component | Reactance Formula | Behavior |
|---|---|---|
| Inductor | XL = 2πfL | Impedance increases with frequency |
| Capacitor | XC = 1 ÷ (2πfC) | Impedance decreases with frequency |
| Resistor | R (constant) | Impedance unaffected by frequency |
Common Impedance Ratings
| Application | Impedance | Why It Matters |
|---|---|---|
| Speaker (home audio) | 4, 6, or 8 Ω | Must match amplifier output impedance |
| Headphones | 16–600 Ω | Low Ω (16–32) for phones; high Ω for studio |
| Coaxial cable (TV/internet) | 75 Ω | Standard for video/RF — prevents signal reflection |
| Ethernet cable (Cat 5/6) | 100 Ω | Impedance matching for data integrity |
| Antenna cable (ham radio) | 50 Ω | Standard for radio transmission |