How do capacitors store and release energy?

1. A camera flash is a huge burst of light that lasts only a blink. A small battery cannot push out energy that fast. So the camera saves energy up first, in a part called a capacitor.
2. Inside a capacitor are two metal plates, very close together, with a thin layer between them that electricity cannot cross. In real ones the plates are long strips, rolled up to fit in a little can.
3. Connect a battery. It pushes electrons onto one plate and pulls them off the other. The plate fills quickly at first, then more slowly, because the electrons already there push back on the newcomers.
4. Disconnect the battery and the charge stays put. The crowded electrons are pulled toward the other plate, but the insulating layer stops them. The energy waits, stored between the plates.
5. Now connect the plates through a lamp. The electrons rush around through the wire to the other plate, all in a moment. Their stored energy comes out as a short, bright flash.
6. Slow in, fast out: that is a capacitor's trick. Capacitors fire camera flashes, sense your finger on a touch screen, and smooth out bumpy electricity inside almost every gadget.

This capacitor is cut open to show its roll. The magnifier shows the layers of the roll, zoomed in. Tap each part.

• Metal can. The aluminum can protects the roll inside. A printed stripe on the sleeve shows which leg is the minus one.
• Rolled-up plates. The two plates are long strips of metal foil with a thin separator between them, rolled up tightly. Unrolled, they could be longer than your arm.
• Legs. Each leg is joined to one of the two plates. Electrons flow in through one leg and out through the other, but never straight through the capacitor.
• Plate with extra electrons. The battery pushes extra electrons onto this plate, so it becomes negatively charged. The more electrons arrive, the harder they push back.
• Plate with missing electrons. The battery pulls electrons away from this plate, leaving it positively charged. It attracts the electrons crowded on the other plate.
• Insulating layer. A very thin layer between the plates that electricity cannot cross. It keeps the opposite charges apart, and that is what lets the capacitor hold its energy.

Tap Charge and watch the electrons crowd onto the plate. Tap Flash to let the energy out. Try a short charge and a long one, then a bigger capacitor.

Experiment: You charge for one second and flash. Then you charge for five seconds and flash. How do the two flashes compare?

1. Tap Charge, wait about one second, then tap Flash.
2. Tap Charge again and wait until the meter stops rising. Then tap Flash.
3. Try the large capacitor. Does it take longer to fill?

What we found: The longer you charge, the more electrons crowd onto the plate and the brighter the flash, until the capacitor is full. It fills fast at first and then slows down. A bigger capacitor holds more, but takes longer to fill.

Turn on JavaScript to try it yourself.

Take three photos. Each needs a different amount of flash: flash when the meter is inside the green band. Too little is too dark, and too much is too bright.

A capacitor is one of the simplest electronic parts: two metal plates very close together, with a thin insulating layer between them so that they never touch. It does not make electricity. It stores a little of it, and it can give it back very quickly.

When you connect a battery, the battery pushes electrons onto one plate and pulls electrons off the other. One plate ends up with too many electrons and the other with too few. These opposite charges pull on each other across the gap, but the insulating layer keeps them apart. That pull is where the energy is stored.

Filling a capacitor is fast at first and then slows down. The first electrons arrive easily. Later ones are pushed back by the crowd already on the plate, a bit like blowing up a balloon that gets harder to blow as it fills. When the capacitor pushes back as hard as the battery pushes, it is full.

Give the electrons a path, such as a wire through a lamp, and they rush around to the other plate. All the stored energy comes out in a moment. That is the difference from a battery: a battery holds much more energy but gives it out steadily, while a capacitor holds less and can release it in a burst. That is why camera flashes use capacitors. The balloon comparison is not perfect, because nothing is pumped in from outside: electrons are moved from one plate to the other. One warning: big capacitors inside appliances can stay charged after they are unplugged, so never open one up.

Key words

Capacitor

A part that stores electrical energy on two plates and can release it quickly.

Charge

Extra electrons on one plate and missing electrons on the other. Also: to fill a capacitor.

Capacitance

How much charge a capacitor can hold for each volt. A bigger capacitor has more.

Dielectric

The thin insulating layer between a capacitor's plates.

Time constant

How long a capacitor takes to get about two thirds full.

Discharge

To let a capacitor's stored charge flow out through a circuit.

Check your thinking

1. How does a capacitor store energy?

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   By holding extra electrons on one plate and too few on the other. The opposite charges pull on each other across the gap. That pull holds the energy.

2. Why does a capacitor fill more slowly as it gets fuller?

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   The electrons already on the plate push back on the new ones. Electrons repel each other. The fuller the plate, the harder it is to add more.

3. What happens when the capacitor flashes?

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   The stored energy rushes out all at once. The electrons race around to the other plate, and their energy becomes a flash of light.

4. Why does a camera flash use a capacitor instead of taking energy straight from the battery?

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   A capacitor can release its energy much faster than a battery can. A battery holds more energy, but gives it out slowly. A capacitor can dump its energy in a blink.

5. What stops the charges on the two plates from simply jumping across?

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   The insulating layer between the plates. Electricity cannot cross the insulating layer, so the charges stay apart until you give them another path.

6. Which change doubles the time constant of a charging circuit?

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   Doubling the resistance. The time constant is R times C. The battery voltage changes how far the capacitor charges, not how long it takes.