Dielectric Polarization

Dielectric Polarization

when a non polar dielectric is placed in an external electric field then the centre of positive charge in each molecule is attracted towards negative plate that is in the direction of electric field while the centres negative charges is attracted towards positive plate that is in a direction opposite to the direction of electric field. As a result the two centres of positive and negative charges get separated and a tiny dipole moment is induced in each molecule. Hence the dielectric is polarized.

         E = 0.            E = Eo

           

                          

        Fig 1.a.              Fig 1.b

In above figures, fig 1.a shows molecules of a non polar dielectric when no electric field is applied on the dielectric, while fig 1.b shows induced dipole moment on each molecule of a non polar dielectric when an external electric field is applied across it.

The polarization also occurs in polar dielectrics but the case is something different as in non polar dielectrics. As we know that the polar molecules already have some Permanent dipole moment but due to thermal agitation these dipoles are randomly oriented so that the net dipole moment of the molecules is zero as shown in fig 2.a

          E = 0.              E = Eo 

   

          Fig 2.a             Fig 2.b

but when an external electric field is applied the individual dipole moments tend to align with the external electric  field and hence there appears some net dipole moment in the direction of external field that is the dielectric is polarized. The quantity of polarization depends upon two mutually opposing factors, first one is applied electric field which tends to align the dipoles along the field and the other one is the thermal agitation which tends to disrupt the alignment of the dipoles. As a result of both mutually opposing factors a net dipole moment is induced in the dielectric as shown in fig 2.b.

Atomic or molecular polarizability:

When the external electric field is applied on a dielectric, each molecule becomes a tiny electric dipole with dipole moment parallel to the external electric field and also proportional to it. Hence the induced dipole moment acquired by the molecules may be written as

                               p = α εoEo

Where α is a constant and is known as atomic or molecular polarizability.

The atomic polarizability  α  has the dimensions of volume and for most of the atoms α is of the order of 10-29 to 10-30  m3 .