Try it
Understand the result
What's going on
A capacitor blocks steady DC but lets AC through - sort of. It doesn’t just let AC pass freely; it resists it by an amount that depends on frequency. That frequency-dependent resistance is called reactance, and for a capacitor, it goes down as frequency goes up.
The formula
Xc = 1 / (2π × f × C)
Xc is reactance in ohms, f is frequency in hertz, C is capacitance in farads.
Show the derivation, mnemonic, and worked example
Build it up
A capacitor resists change by storing charge and pushing back. At low frequency, the voltage barely changes before the capacitor keeps up - so it blocks current effectively, and reactance is high. At high frequency, the voltage is constantly reversing before the capacitor can catch up, so more current slips through - reactance drops. A bigger capacitor stores more charge for the same voltage swing, so it resists less at any given frequency too.
Worked example
Worked example
f = 1 kHz, C = 100 nF
Xc = 1 / (2π × 1,000 × 1e-7)
Xc ≈ 1,592 Ω
