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# Class_20new - PHYS809 Class 20 Notes Electric polarization...

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1 PHYS809Class20Notes 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 ω = - F x (20.1) where m is the mass of the charge and ω 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 ω = x E (20.2) Hence the contribution of the charge to the induced dipole moment is 2 2 . e e m ω = = 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 ω = = p x E (20.4) The molecular polarizability is defined to be 2 2 0 1 . i i i i e m γ ε ω = (20.5) Since 0 ε γ 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 0 10 cm . ε γ -

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Class_20new - PHYS809 Class 20 Notes Electric polarization...

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