Non-polar and polar dielectrics –
In this topic we will discuss about Non-polar and polar dielectrics and dielectric polarization with electrical susceptibility.
Non polar and polar dielectrics –
As we know the dielectrics are insulating material which transmit electric effect without conducting electricity .
Dielectrics is of two types , Non-polar Dielectrics and polar Dielectrics .
In non-polar dielectric , in the molecule of the dielectric the centre of mass of the positive charge coincide with the centre of mass of the negative charge . Like , Hydrogen , nitrogen , oxygen , carbon dioxide , methane , benzene etc . In such kind of molecule each molecule has zero dipole moment and they are symmetric in shape .
But in case of polar dielectric . The centre of mass of positive and negative charges do not coincide because of the asymmetric shape of the molecules. It has permanent dipole moment . example – HCl , NH3 H2O, alcohol etc .
Dielectric polarization –
When a non polar dielectric is held in external electric field , the two centres of positive and negative charges in the molecules are separated . The molecules gets distorted and is said to be polarized as a small dipole moment .
The dipole moment acquired is given as , p = αϵ0E0 Where α is the constant of proportionality called molecular polarizability . Its unit is m3 .
If N is the number of atoms per unit volume then , electric polarization P = N p or, P = N qx .
Its unit is C-m-2
If we consider a small volume element in the interior of the slab as shown in figure . , each volume has no net charge ,though it has net dipole moment . This is because positive charge of each dipole sits close to the negative charge of the adjacent dipole . . The negative end of the dipole remain un-neutralized at the surface AB and positive end of the dipole at CD . then the effective electric field E = E0 -EP ;
Further EP = σ/ϵ = p/ϵ.
Electric susceptibility – As we know that the electric polarization P αE or, p= ϵ χ E
Where χ is constant called electrical susceptibility .
For vacuum χ = 0 ;
But as we know , E = E0 -EP = E0 – P/ϵ = E0 – ϵ χ E/ϵ
Or , E = E0 -χE ;
E0 = E + χ E = E( 1+ χ )
OR , E0/E = 1 + χ
K ( Dielectric constant or relative permittivity ) = 1 + χ [ since K= E0/E ]
K (ϵr) = 1 + χ …………………This is the relation between relative permittivity and susceptibility .