If you've ever wondered why every circuit board has dozens of tiny capacitors scattered across it, here's the short answer: they're acting as frequency-selective filters. A capacitor passes signals that change, blocks signals that don't. Once you internalise that one rule, you start seeing capacitors as informational rather than mysterious.

Why blocked at DC

A capacitor is two metal plates separated by an insulator. Connect a battery and current flows briefly — electrons pile up on one plate, get pushed off the other, building up an electric field between them. Once the field's voltage matches the battery's voltage, equilibrium: no more current flows. The capacitor is "fully charged." Steady state at DC = zero current.

This is why a capacitor is described as having infinite impedance at DC. Impedance is just AC's version of resistance, and at zero frequency, it's Z = 1/(jωC) → infinity.

Why passed at AC

Reverse the battery suddenly. Now the field has to reverse: electrons stream off one plate and onto the other. Current flows during the transition. Reverse it again — more current. If you reverse it 60 times a second (60 Hz AC), current flows 60 times a second, alternating direction. To an observer at the other end of the capacitor, current is "getting through."

The math: Z = 1/(jωC). For C = 100 nF and ω = 2π × 1000 Hz, Z = 1/(j × 6283 × 1e-7) ≈ 1.6 kΩ. At 100 kHz it's 16 Ω. So the higher the frequency, the lower the impedance — capacitors are high-pass filters when wired in series.

Where you'll find them everywhere

Coupling capacitors connect two stages of an amplifier while blocking each stage's DC bias from leaking into the next. Audio amplifiers are full of them.

Decoupling capacitors (the 0.1 µF MLCCs scattered near every IC) short out any high-frequency noise on the power supply line, holding the local supply rock-steady. Without them, fast switching transients ripple through the rails and confuse other chips.

Timing capacitors in combination with resistors form RC delays — the basis of millisecond-scale timers, debouncers, and oscillators. The 555 timer chip is essentially "what useful things can I do with an RC charge curve?"

Once you start counting capacitors on a real PCB, you'll find dozens. They're tiny. They're cheap. They're doing real work.