A capacitor is two metal plates with a tiny gap between them. When you push electrons onto one plate, they crowd together and shove electrons off the other plate, so one side ends up a little negative and the other a little positive. The plates can't touch, so that charge just sits there, stored up like water held behind a dam. The clever part is what happens next: when you give it a path, the whole lot rushes out in one quick burst. That burst is how a camera makes its bright flash. Open the simulator, hold the charge button to fill the plates, then hit discharge and watch the flash fire.
Most people think a capacitor is just a tiny battery holding a reserve of electricity. In fact a battery makes electrons by chemistry, while a capacitor makes nothing and only separates charge it already has across a gap.
What's actually happening
People often picture a capacitor as a tiny battery, a little tank that holds a reserve of electricity. That picture is wrong in a way that matters. A battery makes electrons through a slow chemical reaction; a capacitor makes nothing at all. It just separates charge it already has, pulling electrons onto one plate and leaving the facing plate short of them, and then holds that imbalance still, frozen across a gap too thin to cross.
Because the plates never touch, the charge has nowhere to go, and the energy sits in the invisible electric field stretched across the gap between them. How much it can hold per volt is its capacitance, measured in farads, and it depends on dull physical things: bigger plates hold more, a thinner gap holds more. Push charge in through a resistor and it fills gradually, fast at first and then easing off as the plates fill up and start pushing back. A bigger capacitor is a bigger bucket, so it takes longer to fill.
The thing a battery can never do is the dump. A battery trickles its energy out over hours; a capacitor can release its entire stored charge in a few millionths of a second, which means a colossal current for an instant. That single talent is why a camera flash charges a capacitor for a second between shots and then shorts it through the bulb for a blinding pulse, why a defibrillator banks its charge and then delivers it to a heart in one controlled jolt, and why the flash on a disposable camera can still bite your fingers days after the battery is dead.
A capacitor's one talent is dumping its whole stored charge in microseconds, which is why a camera flash and a defibrillator both rely on it.
- 1Get a large capacitor (say 1000 µF), a battery, an LED, and a resistor. Briefly touch the capacitor across the battery to charge it — a second is plenty.
- 2Now disconnect the battery and connect the charged capacitor across the LED through the resistor.
- 3The LED lights and slowly fades over a few seconds as the capacitor empties — you are watching stored charge drain out with no battery anywhere in the circuit.
Common questions
A battery makes electrons through a slow chemical reaction and trickles energy out over hours. A capacitor makes nothing; it just separates charge it already has and can dump the entire lot in millionths of a second.
It fills through a resistor on a time constant τ = R·C, so a larger capacitance takes longer to charge. It is a bigger bucket, filling fast at first then easing off as the plates push back.
The big capacitor inside a disposable camera holds its charge for days after the battery dies. Repair guides warn you to short it out first, because it can give your fingers a real zap.