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Fill a capacitor from a battery. Why does the light start bright and then fade?
Charge the Flash lab · Last updated: 16 October 2026
A battery charges a capacitor through a lamp. At first the lamp shines brightly, then it slowly fades to nothing even though the battery is still connected. You investigate why.
What you'll discover
- A capacitor stores electric charge on two metal plates separated by an insulator.
- Charging current is biggest at the start and falls away as the capacitor fills.
- A charged capacitor can release its energy very quickly, as in a camera flash.
The science in plain words
A capacitor is two metal plates, or two layers of foil, separated by a thin insulator. When it is connected to a battery, electrons are pushed onto one plate and pulled off the other. They cannot cross the insulator, so charge builds up and the capacitor stores energy.
At the start the capacitor is empty and offers no push back, so a large current flows and the lamp in the circuit shines brightly. As charge builds up, the capacitor's own voltage rises and pushes back against the battery. The difference between the two gets smaller and smaller, so the current shrinks and the lamp fades. When the capacitor's voltage matches the battery's, the current stops completely.
The fading follows a smooth curve: the current drops quickly at first, then more and more slowly. How long it takes depends on how big the capacitor is and how much resistance is in the circuit.
Capacitors can give their stored energy back all at once. A camera flash charges a capacitor over a few seconds and then dumps the charge through the flash tube in a split second. Capacitors also smooth power supplies in nearly every electronic device.
Where it fits
- India, NCERT: Capacitors are taught in NCERT Class 12; this lab builds on current and circuits in Class 10 Chapter 11: Electricity.
- United States: Grades 9 to 12
Common questions
What does a capacitor do?
A capacitor stores electric charge and energy on two plates separated by an insulator. It can release that energy later, sometimes very quickly, as in a camera flash.
Why does the current fall as a capacitor charges?
As charge builds up, the capacitor's voltage rises and pushes back against the battery. The difference driving the current gets smaller, so the current shrinks until it stops.
What is the difference between a capacitor and a battery?
A battery makes electrical energy from chemical reactions and releases it steadily. A capacitor only stores charge it is given and can release it much faster, but it holds far less energy.
Related labs
- Simple Circuits: Close the switch. What makes the bulbs brighter: more volts, fewer bulbs, or a different wiring?
- Electric Fields: Charge a metal ball and hang a tiny pith ball beside it. How far does the invisible push move it?
- Storm over the Town: Where will the lightning strike? Watch it feel its way down, then add a lightning rod.
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Try it yourself
The Charge the Flash lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.