Capax

Meet Capax, a capacitor who blocks steady current but lets fast wiggles straight through.

A capacitor is two plates with a gap between them, so steady current cannot cross it. With alternating current the plates charge and discharge again and again, and current flows back and forth in the circuit. The faster the switching, the more easily it flows.

That opposition is the capacitive reactance, X_C = 1/(2πfC), measured in ohms. A 1.0 μF capacitor has X_C ≈ 3200 Ω at mains frequency of 50 Hz. At 5000 Hz it drops to about 32 Ω, a hundred times less.

Doubling the capacitance halves the reactance too, because a bigger capacitor can soak up more charge before its voltage pushes back. At zero frequency, which is steady direct current, the reactance would be infinite, so no current flows at all.

Speakers use this. A capacitor in front of a small tweeter blocks the deep bass but passes the high notes, so each speaker only plays the sounds it handles best.