what is Clausius–Mossotti equation

The term μ²/3kT stems from the thermal averaging of the electric dipole moment in
the presence of the applied field. The corresponding expression without
the contribution from the permanent dipole moment is called the Clausius–Mossotti
equation

 

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The Clausius–Mossotti equation is used when there is no contribution from permanent
electric dipole moments to the polarization, either because the molecules are nonpolar
or because the frequency of the applied field is so high that the molecules cannot
orientate quickly enough to follow the change in direction of the field.