Orientational Polarisation

ORIENTATIONAL POLARISATION

Orientational polarisation takes place only in polar dielectrics. Polar dielectrics have molecules with permanent dipole moments even in the absence of an electrical field as shown in fig. 1.7.

When the polar dielectrics are subjected to an electric field, the molecular dipoles are oriented in the direction of electric field.

The contribution to polarisation due to orientation of molecular dipoles is called orientational polarisation.

This polarisation depends on temperature. When the temperature is increased, thermal energy tends to disturb the alignment.

From the Langevin's theory of paramagnetism, net intensity isnolinogorq Nu2B

of magnetisation = N μ²B / 3kT

Since the same principle can be applied to the application of electric field in dielectrics, we may write

Orientational polarisation

P₀ = N μ²E / 3kT ...(1)

But, orientational polarisation is proportional to applied field   and it is given by

Comparing the equations (1) and (2),

we get α₀ = μ² / 3kT

where ∝₀ is orientational polarisability.

Example

Orientational polarisation occurs only in polar dielectrics such as H₂O, HCl and CO etc.

Conclusion

The orientational polarisability is inversely proportional to absolute temperature of the material.

Note

Orientational polarisation occurs only in polar dielectrics ie., dielectric with molecules having permanent dipoles.