|
|
| Relative Permittivity |
|
|
Permittivity is the property of a medium and affects the magnitude of force between two point charges. |
|
|
Air or vacuum has minimum value of permittivity. The absolute (or actual) permittivity of air or vacuum is 8.854 x 10-12 C2 N-1 m-2. |
|
|
The absolute permittivity ε of all other insulating materials is greater than ε0. The ratio ε / ε0 is called relative permittivity of the material and is denoted by K (or εr).
|
|
|
K = ε / ε0 = Absolute permittivity of medium / Absolute permittivity of air |
|
|
It may be noted that the relative permittivity is also called dielectric constant.
|
|
|
Force between two charges in air is, |
|
|
Fair = (1 / 4 x π x ε0 ) x (q1 x q2 / r2)
|
|
|
Force between the same two charges held same distance apart in a medium of absolute permittivity ε is
|
|
Fm = (1 / 4 x π x ε ) x (q1 x q2 / r2)
= (1 / 4 x π x ε0 x K ) x (q1 x q2 / r2)
|
|
|
Fair / Fm = ε / ε0 = K = Relative permittivity of the medium
|
|
|
Hence, relative permittivity of a medium may be defined as the ratio of force between two charges separated by a certain distance in air to the force between the same charges separated by the same distance in the medium.
|
|
For air or vacuum, K = ε / ε0 = 1.
For all other insulating materials, the value of k is more than 1.
|
|
Fm = Fair / K.
This implies that force between two charges is decreased when air is replaced by other insulating medium.
|
|
K is a number, being the ratio of two absolute permittivities.
|
|
