A capacitor is a device for storing charge. It is usually made up of two plates separated by a thin insulating material known as the dielectric. One plate of the capacitor is positively charged, while the other has negative charge.
The charge stored in a capacitor is proportional to the potential difference between the two plates. For a capacitor with charge Q on the positive plate and -Q on the negative plate, the charge is proportional to the potential:
If C is the capacitance, Q = CV
The capacitance is a measure of the amount of charge a capacitor can store; this is determined by the its geometry and by the kind of dielectric between the plates. For a parallel plate capacitor made up of two plates of area A and separated by a distance d, with no dielectric material, the capacitance is given by :
C = ε0A/d where A is cross sectional area of Plate of Capacitor and d is separation between the plates.
ε0= Permitivity of free space = 8.85×10-12C2/Nm2
Mind that capacitance has units of farads (F). A 1 F capacitor is exceptionally large; typical capacitors have capacitances in the rage of pF to microfarad range.
Dielectrics, the insulating materials placed between the plates of a capacitor, cause the electric field inside the capacitor to be reduced for the same amount of charge on the plates. This is because the molecules of the dielectric material get polarized in the field, and they align themselves in a way that sets up another field inside the dielectric opposite to the field from the capacitor plates. The dielectric constant is the ratio of the electric field without the dielectric to the field with the dielectric.