Permittivity

Symbol: e
Unit: farad meter-1 (F m-1)

The permittivity of a substance is a characteristic which describes how it affects any electric field set up in it. A high permittivity tends to reduce any electric field present. We can increase the capacitance of a capacitor by increasing the permittivity of the dielectric material.

The permittivity of free space (or a vacuum), e0, has a value of 8.9 10-12 F m-1.

The permittivity of a material is usually given relative to that of free space, it is known as relative permittivity, er. Permittivity is then calculated

by multiplying e0 by this figure.

Material

Relative permittivity, er

Vacuum 1 (by definition)
Air 1.0005
Polythene 2.35
Perspex 3.3
Mica 7
Water 80
Barium Titanate 1200

We can increase the capacitance of a capacitor by increasing the permittivity of the dielectric material.