Capacitors — Store, Smooth, Time

What you'll learn

• A capacitor = two metal plates with a thin insulator between them. A battery piles electrons onto one plate and pulls them off the other; they can't cross the gap, so the charge (and the energy) stays when the battery leaves.
• Battery = big tank, slow. Capacitor = small bucket that can empty in a split second (camera flash: a few seconds to charge, about a thousandth of a second to fire).
• Measured in farads (F); most are microfarads (µF, millionths of a farad) or nanofarads. Electrolytic ("can") capacitors: the stripe marks minus, the longer leg is plus, never go over the printed voltage.
• Smoothing: a charger turns AC into bumpy DC (100 bumps a second on 50 Hz mains, 120 on 60 Hz). The capacitor fills on every bump and gives energy back in every dip → an almost flat line. That's also why every chip has a 100 nF capacitor right next to its power pin.
• Timing: a resistor in front makes it fill slowly. Time constant τ = R × C. 10 kΩ × 100 µF = 1 second; after about 5 τ it's basically full (63 % after one τ). Used for blinking LEDs, delays and timers.
• Safety: big capacitors (TVs, microwaves, power supplies) can stay charged after unplugging. Never open them.