A capacitor is a device used to store electrical charge or energy. They are made of a pair of two metal sheets that are situated in close proximity to each other and a small gap between them compared to their dimensions. The gap between the plates is either vacuum or filled with an insulating material called a dielectric.

When the two conducting plates are connected to a battery, electrons are removed from the plate connected to the positive terminal of the source and are added to the plate that is connected to the negative terminal of the source.

The building of opposite charge on the plates gradually increases and continues until the potential difference between them (V_AB) is equal to the applied voltage (V). During the charging process, the charge stored in the capacitor is directly proportional to the potential difference developing between the plates. Therefore, we can define a proportionality constant as

where C is called the capacitance of the capacitor. The SI unit of capacitance is farad (F), where, 1 F = 1 C/V. Capacitance is defined as the ability of a capacitor to store electrical charge and energy.