When we study dielectrics or non conductors (substances that do not allow electric current to pass through them) then we come across a term known as polarization which is usually denoted by the symbol P. Its a vector.

For many substances this P happens to follow the following equation

For many substances this P happens to follow the following equation

**P**= Є0 χe**E**.E is the net electric field intensity inside the dielectric.

Є0 is defined as the permittivity of free space. Its unit is C2/Nm2. It is a universal constant. In SI units

Є0 = 8.85 X 10 ^ (-12) C2/Nm2.

χe is a dimensionless constant called the electric susceptibility of the medium which depends on the material of the substance and some external conditions like temperature.

The quantity Є0(1 + χe) appears in some important equations of electromagnetism and so physicists found it appropriate to give it a especial name. That name is permittivity (Є )and is defined as

Є = Є0(1 + χe).

Obviously the unit of Є is the same as that of Є0.

The quantity Є / Є0 is also given a especial name viz the relative permittivity or the dielectric constant of the medium and is denoted by Єr or K.

Є / Є0 = (1 + χe) = Єr = K.

Since Єr is a pure ratio therefore it is a unit less and a dimensionless quantity.

Є0 is defined as the permittivity of free space. Its unit is C2/Nm2. It is a universal constant. In SI units

Є0 = 8.85 X 10 ^ (-12) C2/Nm2.

χe is a dimensionless constant called the electric susceptibility of the medium which depends on the material of the substance and some external conditions like temperature.

The quantity Є0(1 + χe) appears in some important equations of electromagnetism and so physicists found it appropriate to give it a especial name. That name is permittivity (Є )and is defined as

Є = Є0(1 + χe).

Obviously the unit of Є is the same as that of Є0.

The quantity Є / Є0 is also given a especial name viz the relative permittivity or the dielectric constant of the medium and is denoted by Єr or K.

Є / Є0 = (1 + χe) = Єr = K.

Since Єr is a pure ratio therefore it is a unit less and a dimensionless quantity.