What Is Impedance?
Impedance is the total opposition a circuit presents to alternating current (AC); its unit is the ohm (Ω). It is the AC counterpart of resistance but depends on frequency: besides resistance, it includes the reactance of capacitors and inductors. At high frequency, inductor impedance rises while capacitor impedance falls. Impedance is denoted by Z.
Impedance (Z) is the total opposition to current in alternating current (AC) circuits; its unit is again the ohm (Ω). It can be thought of as the extended form of resistance in AC.
The Difference Between Resistance and Impedance
Pure resistance behaves the same in DC and AC and isn't affected by frequency. Impedance, however, includes resistance + reactance together and is frequency-dependent. In circuits with coils and capacitors, the opposition to current can't be explained by resistance alone.
What Is Reactance?
Reactance (X) is the frequency-dependent opposition that capacitors and inductors present to AC. There are two kinds: capacitive reactance (XC) and inductive reactance (XL).
| Element | As frequency rises | Behavior |
|---|---|---|
| Resistor (R) | Unchanged | Frequency-independent |
| Capacitor (XC) | Decreases | Passes high frequencies |
| Inductor (XL) | Increases | Blocks high frequencies |
Where Does It Matter?
Impedance is critical in filter design, speaker matching (e.g. "8 ohm speaker"), and antenna and RF circuits. Because impedance changes as frequency changes, it determines which frequencies a signal will pass.
Common Mistakes
- Thinking impedance is constant: impedance varies with frequency; a single value is valid only for a specific frequency.
- Applying Ohm's law blindly: in AC, V = I × Z is used instead of V = I × R.
- Confusing speaker impedance: an "8 Ω speaker" is nominal impedance, not pure resistance.
Frequently Asked Questions
What's the difference between impedance and resistance?+
What is the unit of impedance?+
How does frequency affect impedance?+
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