Capacitors

In one line

A capacitor stores energy in the electric field between two conductive plates — it charges, holds, and releases. It can dump a big burst of energy fast, or smooth a noisy signal.

<figure style="text-align:center;margin:1.5rem 0"> <svg viewBox="0 0 260 130" role="img" aria-label="Capacitor: two parallel plates with + and − charge" style="width:100%;max-width:300px;height:auto;color:var(--c-iot)"> <line x1="14" y1="65" x2="110" y2="65" stroke="currentColor" stroke-width="2"/> <line x1="110" y1="32" x2="110" y2="98" stroke="currentColor" stroke-width="5"/> <line x1="146" y1="32" x2="146" y2="98" stroke="currentColor" stroke-width="5"/> <line x1="146" y1="65" x2="246" y2="65" stroke="currentColor" stroke-width="2"/> <text x="96" y="28" font-size="15" fill="currentColor" text-anchor="middle">+</text> <text x="160" y="28" font-size="15" fill="currentColor" text-anchor="middle">−</text> <text x="130" y="120" font-size="11" fill="var(--text-3)" text-anchor="middle">charge stored in the field between the plates</text> </svg> </figure>

How it works

Apply a voltage and electrons pile up on one plate. No current flows through it at steady state — it holds the charge.

• Capacitance (farads) — how much charge per volt.
• It resists changes in voltage — perfect for smoothing a wobbly supply and filtering.

Where it shows up in digital health

• Defibrillators charge a large capacitor over seconds, then release ~150–360 J in milliseconds — far faster than a battery. The "whine" before the shock is the capacitor charging.
• ECG / EEG front-ends use capacitors in filters to strip mains hum.
• Decoupling capacitors steady the supply next to every chip.

Watch for

Electrolytic capacitors are polarised and fail — sometimes dramatically — if reversed. A degraded capacitor is a classic cause of drifting measurements and resets.