The number of electrons that can be stored under a given electrical pressure (voltage) is called its capacitance or capacity. The property of capacitance is exhibited by capacitor. Basically, a capacitor consists of two metallic plates separated by a non-conducting substance between them.

In the circuit shown left when the switch is open the capacitor is not charged i.e. it does not contain any charges but when the switch is closed current starts to flow and capacitor gets charged. This process of charging takes place because the emf forces electrons into the top plate of the capacitor from the negative end of the battery and pulls others out of the bottom plate toward the positive end of the battery.

The unit uF stands for microfarad (one millionth) and pF stands for Pico-farad (one million, millionths). These are the two common values of capacitance you will encounter in electronics.

Time constant of capacitance
The time required for a capacitor to get completely charged depends upon two factors:
1. The capacitance value
2. The resistance value.

The time constant of a resistance - capacitance circuit is:
T = R X C
Where T = time in seconds
where R = resistance in ohms
where C = capacitance in farads