### The Unit of Capacitance

When a battery is connected between the plates of a capacitor, the potential difference between the plates builds up at a rate that depends on the capacitance. The greater the capacitance, the slower the rate of change of voltage in the plates. The unit of capacitance is an expression of the ratio between the current that flows and the rate of voltage change between the plates as the plates become charged. A capacitance of 1 farad (1 F) represents a current flow of 1 A while there is a voltage increase of 1 V/s. A capacitance of 1 F also results in 1 V of potential difference for an electric charge of 1 C.

The farad is a huge unit of capacitance. You’ll almost never see a capacitor with a value of 1 F. Commonly employed units of capacitance are the microfarad (μF) and the picofarad (pF). A capacitance of 1 μF represents 0.000001 (10−6) F, and 1 pF is a millionth of a microfarad, or 0.000000000001 (10−12) F.

Physically small components can be made to have fairly large capacitance values. Conversely, some capacitors with small values take up large physical volumes. The physical size of a capacitor, if all other factors are held constant, is proportional to the voltage that it can handle. The higher the rated voltage, the bigger the component.