Mumbai university > FE > SEM 1 > Applied Physics 1

Marks: 5M

Year: May 2013

Electronic Polarization:-

• Electronic polarization occurs due to displacement of the center of the negatively charged electron cloud relative to the positive nucleus of an atom by the electric field.

• When an external electric field is applied, like by placing the dielectric material between the two plates of the parallel plate capacitors, the positively charged nucleus is attracted towards the negative side of the electric field and the negatively charged electron cloud is shifted towards the positive plate.

• In fig. 3.12A, you can see that the centroid of positively charged nucleus and negatively charged electron cloud coincide.

• Fig. 3.12B shows the atom when placed in an external electric field. As you can see from the figure that the centroid of negatively charged electron cloud moves away from the positively charged nucleus thus generating a dipole moment.

• The induced dipole moment is given by

• Mono-atomic gases exhibit this kind of polarization.

• Electronic Polarizability is proportional to the volume of atoms and independent of the temperature.

Ionic Polarization:-

• Ionic Polarization occurs due to relative displacements between positive and negative ions in an ionic crystal.

• If a crystal or molecule consists of atoms of more than one kind, the distribution of charges around an atom in the crystal or molecule leans to positive or negative.

• As a result, when lattice vibrations or molecular vibrations induce relative displacements of the atoms, the centers of positive and negative charges are also displaced

• The locations of these centers are affected by the symmetry of the displacements.

• When the centers don't correspond, polarizations arise in molecules or crystals.

• The displacement is independent of temperature.

Orientational Polarization:-

• Orientational Polarization is found in medium constituting of polar molecules.

• Polar molecules have their own individual dipole moment.

• However, the molecules are oriented randomly, thus resulting in zero net moment.

• When an electric field is applied, the molecules arrange themselves along the direction of the applied electric field, giving rise to orientational polarization.

• Orientational polarizability $(α_O)$ is inversely proportional to the temperature and is given by,

$$α_O = \frac{μ^2}{3K_B T}$$