Capacitance

A device that can store charge is said to have capacitance. Typically parallel charged plates are used to describe capacitance. In this case, capacitance (C) is the ratio of the charges on the plates to the potential difference across the plates: C = Q/V. Capacitance is measured in farads which is denoted by F.

For two parallel plates which have charge +Q and -Q, the electric field between them, E, is given by

E = Q/(ε0A)
	

where ε₀ is the permittivity of a vacuum which is 8.85×10^-12 Fm^-1.

The potential difference is

V = Ed 
  = (Q/(ε0A))d

So

C = Q/V 
  = Q/((Q/(ε0A))d) 
  = ε0A/d

If the space between the plates is filled with a dielectric then the capacitance is increased by the relative permittivity of the material. A dielectric is a material whose atoms are polarised by an electric field.

References

Fischer-Cripps. A.C., The Electronics Companion. Institute of Physics, 2005.