Table of Contents
What Is Impedance?
Impedance is the total opposition to alternating current in a circuit containing resistance and reactance. Unlike pure resistance, impedance is a complex quantity with both magnitude and phase. The magnitude determines how much current flows for a given voltage, while the phase angle tells you how much the current leads or lags the voltage.
In a series RLC circuit, the impedance combines the resistance (real part) with the net reactance (imaginary part). The inductive reactance (XL) causes current to lag voltage, while capacitive reactance (XC) causes current to lead voltage. The net effect depends on which reactance dominates.
Impedance Formulas
Reactance Explained
- Inductive reactance (XL): Increases with frequency. Inductors oppose changes in current, so higher frequencies create more opposition.
- Capacitive reactance (XC): Decreases with frequency. Capacitors pass high-frequency signals more easily.
- At resonance: XL = XC, net reactance is zero, and impedance equals R.
Impedance Examples
| f (Hz) | XL (Ω) | XC (Ω) | Z (Ω) | Phase |
|---|---|---|---|---|
| 100 | 6.28 | 159.2 | 158.1 | -75.5° |
| 503 | 31.6 | 31.6 | 50.0 | 0° |
| 1000 | 62.8 | 15.9 | 66.6 | 41.4° |
Frequently Asked Questions
What does a positive phase angle mean?
A positive phase angle means the circuit is inductive (XL > XC) and current lags behind voltage. A negative phase angle means the circuit is capacitive (XC > XL) and current leads voltage. At resonance, the phase angle is zero.
How does impedance affect power?
Only the resistive part of impedance dissipates real power. The power factor is cos(φ), which equals R/Z. At resonance, power factor is 1 (maximum power transfer). Reactive power oscillates between the inductor and capacitor without being consumed.
What is impedance matching?
Impedance matching is the practice of making the source impedance equal to the load impedance for maximum power transfer. In RF systems, 50 ohms is the standard impedance. Matching networks using L, C, or LC combinations transform impedances to achieve this condition.