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Unformatted text preview: 1 PHYS809 Class 20 Notes Electric polarization of molecules So far we have not considered how matter responds to applied electric fields. Some molecules and atoms have intrinsic electric dipole moments (such molecules, e.g. H 2 O, are called polar molecules). An applied electric field has a tendency to align the dipole moments with the field, resulting in a net polarization. In non-polar molecules and atoms, an applied electric field can induce an electric dipole moment in the direction parallel to the applied field. A simple model can be used to estimate the induced moments and their relation to the strength of the applied field. We assume that each charge e in the molecule is bound by a restoring force 2 , m = - F x (20.1) where m is the mass of the charge and is the frequency of small oscillations about its equilibrium position. An applied field E displaces the charge from its equilibrium position by an amount 2 . m e = x E (20.2) Hence the contribution of the charge to the induced dipole moment is 2 2 . e e m = = E p x (20.3) For a set of charges in the molecule, the induced dipole moment is 2 2 . i i i i i i i e e m = = p x E (20.4) The molecular polarizability is defined to be 2 2 1 . i i i i e m = (20.5) Since has the dimensions of a volume, its magnitude must be of order the volume of the molecule or less. Sizes of molecules are typically of order a few angstroms. Hence 23 3 10 cm ....
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