What are capacitors?
A capacitor is a passive component (it does not have its own source of power) used to store energy in the form of an electric field which resists changes in voltage across its terminals.
How does a capacitor work?
Typically, a capacitor is made up of two conductors (ex. metal plates) separated by an insulator called a dielectric (ex. ceramic). When a power supply, such as a battery is connected to the capacitor, electrons flow from the negative terminal of the battery towards one of the conductors of the capacitor. At the same time, electrons that were already on the other conductor are attracted to the positive terminal of the battery. As a result, there is a build up of negative charge on one conductor and positive charge on the other. This potential difference between the two conductors of the capacitor creates an electric field directed from one conductor to the other. When the capacitor is completely charged, the potential difference across it is the same as that of the supply and no more current flows through the circuit. Furthermore, the capacitor can now act as a power source and therefore, supply energy to the circuit. To see how a capacitor works, see the animation below.