From the relation derived in previous article,we can say
that electric flux density depends on electric field strength. Now the value of
electric flux density depends on the value of electric field strength E along
with the dielectric property of the medium which is known as permittivity.
Note : Permittivity can be defined as the ease with which a
dielectric medium permits an electric flux to be established in it.
1.1Absolute Permittivity
The ratio of the electric flux density D to electric field
strength E at any point is defined as the absolute permittivity.
It is denoted by ε and measured in units farads/metre, (F/m).
1.2 Permittivity and Free Space
It is also called as electric space constant.
Note: The ratio of the electric flux density in a vacuum (or
free space) to the corresponding electric field is defined as permittivity of
the free space.
It is denoted by ε0 and measured in unit farads/m (F/m).
The value of εr is less than the value of permittivity of any
medium. Experimentally, its value has been derived as,
1.3 Relative Permittivity
To define the permittivity of the dielectric medium, the
vacuum or free space is considered to be a reference medium. So, relative
permittivity of vacuum with respect to itself is unity.
The ratio of electric flux density in a dielectric medium
to that produced in a vacuum by the same electric field strength under
identical conditions is called relative permittivity.
It is denoted by εr and has no units.
It can also be defined as the ratio of the absolute
permittivity of the dielectric medium to the permittivity of the free space. The
relative permittivity of the air is also taken as unity, though its actual
value is 1.006. most of the other materials have value of relative permittivity
between 1 to 10.
The relative permittivity of air is assumed to be one for
all practical purposes.
Note: higher the value of , easier is the flow of electric
flux through the materials.
In practice, paper and mica are extensively used for
manufacturing of capacitors.
Note: The relative permittivity is nothing but the
dielectric constant of the material.