Circuits That Change
AC, Reactance & Impedance
14 min · 2 worked examplesFree
Why Ohm's law needs extending once signals move, and how a capacitor can oppose current without dissipating any power.
After this lesson you can
- Say why reactance depends on frequency
- Combine resistance and reactance into impedance
- Explain the difference between real and apparent power
Opposition that depends on frequency
A resistor opposes current equally at every frequency. A capacitor and an inductor do not.
Capacitive reactance is Xc = 1/(2πfC), in ohms. It falls as frequency rises, a capacitor blocks DC completely and passes high frequencies easily.
Inductive reactance is XL = 2πfL, and it does the opposite, rising with frequency. An inductor passes DC freely and chokes off fast changes.
Both are measured in ohms and both limit current, but neither dissipates energy. A resistor turns electrical energy into heat; a reactance stores it and gives it back each cycle. That difference is why a component can carry substantial current and stay cold.
In the full lesson
- Opposition that depends on frequency
- Impedance
- Real and apparent power
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