In this case a non polar dielectric like methane is taken and placed in some external electric field. Center of positive charge of individual molecules is pulled automatically in the same direction as that of electric field towards the plate having negative charge. Similarly, the centre of the negative charged electrons is dragged in the opposite direction of the electric field, towards the plate having positive charge. So the centres of positive as well as negative charges are set apart. Due to this the molecules are deformed from their original shape and hence separate at last. So, due to the above process each molecule gets some dipole moment . After sometime these molecules will get polarized when the forces of attraction between the centres of positive and negative charges and the force due to electric field will reach at some stable state.
Now the individual molecules will exist as separate tiny dipoles.
In the above equation alpha is proportionality constant. Its is also called as Atomic polarizability.
By using the equation 2.22
= Cm / (C2 N-1m-2 ) * (NC-1)
Now from the above result it is cleared that the Alpha i.e. atomic polarizability has similar dimensions as that of volume. Mostly values of alpha vary from 10-29 to 10-30.
We are going to made some experiments on a rectangular slab. Let’s name it as ABCD. Keep it in the influence of the electric field present between the two plates. For this we have to think that all the atoms must be polarized in a uniform manner. Let the displacement between the charges be x. So the equation will be:
If the total number of atoms present per unit volume be taken as N. But N is equal to the total dipole moment density .
New equation formed will be:
Or we can write it as
Here P is the density of the dipole. Its other name is Electric polarization. Its units are C -m-2
Lets us think about the interior of the slab. The cancellation of the charges due to their equal magnitude of positive and negative charge, the volume charge density will be Zero. Some amount of positive charge will generate in the surface CD.It can be seen in the figure below. The equation of Effective electric field present inside a dielectric which is polarized is given by:
E=Eo – E p —-2.23
Eo= i / = Qi/a *
= p/ —-2.24
The nature of the dielectric slab will affect the value of E. Now it is proved from the definition and the equation given below.The charge in external electric field divided by the reduced value of electric field gives us the constant i.e. k , which is the constant for material of the rectangular slab ABCD.
(b) Electric Susceptibility: It states that the reduced value of electric field effects the electric polarization P due to a relation of direct proportion between them.
P is directly proportional to E.
P= * * E ——2.26
In the above equation is the electric susceptibility. The main purpose of to make the dimensionless. Its main purpose is to tell about the complete electric manner of acting of a dielectric. Value of Susceptibility varies with the variation of dielectrics. The value of Electric susceptibility for vacuum is equal to zero.
E= Eo – P / – * * E /
By using the equation no 2.25
Or E= Eo – E
Or Eo=E+E=E (1+ )
K=1+ ——– 2.27
Dielectric constant and the susceptibility are related to each other according to the equation 2.27
In polar dielectrics individual molecules have their individual dipole moments. But when the External electric field’s influence remains inactive then the net dipole moment of the elements of dielectric becomes zero.
Lets consider the case in which external electric fields influence is present. Due to this value of dipole moment in the individual elements increase by a little amount. Then another affect will take place, a twisting force will be generated which will try to change the alignment of electric dipoles parallel to that of the electric field. The alignment depends upon the amount of the electric field passed. Stronger the electric field higher will be the alignment. This also depends upon the temperature. The alignment of the dipoles increases if we increase the temperature.