Explain how coils and capacitors oppose AC, what impedance is, and why XL equals XC at resonance.
Syllabus: Reactance; inductance; capacitance; impedance; impedance transformation; resonance
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 12 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
Reactance (X) is opposition to alternating current caused by an inductor or a capacitor. It is measured in ohms. It depends on frequency, not on how big the signal is.
RememberCoil: reactance rises with frequency. Capacitor: reactance falls. Both in ohms, symbol X.
Memory hookAn inductor resists change, and faster AC changes more. A capacitor passes fast AC more easily.
Impedance (Z) is the total opposition to AC: resistance and reactance together. It is the AC version of Ohm's law, voltage divided by current.
| Term | Meaning |
|---|---|
| Impedance Z | E ÷ I, in ohms |
| Admittance Y | the inverse of impedance |
| Conductance, susceptance | inverses of resistance, reactance |
| Reluctance | opposition in a magnetic path |
RememberImpedance = voltage ÷ current. Its inverse is admittance.
XL rises with frequency and XC falls, so at one frequency they are equal. In a series LC circuit they cancel, leaving impedance very low: current flows easily.
RememberResonance: XL = XC, they cancel. Series LC impedance is very low.
A transformer, a Pi-network and a length of transmission line can all make a load look like a different impedance to the source.
RememberTransformer, Pi-network, transmission line: all can match impedances.