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4.3 Microscopic Approach
Earlier we saw, polarization in a system with N dipoles per unit volume can be expressed as
i.e.
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(4.17) |
This gives
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(4.18) |
Hence, greater is δ greater is χ and hence larger is εr, i.e. more polarizable a medium is, more is its dielectric constant.
Further, polarizability of an ion or atom of type, i, in a dipole is defined as
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(4.19) |
where Eloc is the local electric field experienced by an atom or ion or molecule which can be different than the applied field. However, magnitude of local field can be modified quite significantly the polarization of surrounding medium.
As a result P can expressed as
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(4.20) |
Note that equation (4.20) relates a macroscopic parameter, P, to the microscopic parameters N, α and Eloc.
As a result, in general, susceptibility is
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(4.21) |
where α is the sum of all types of polarizabilities due to different mechanisms (αelectronic + αionic + αdipolar+ αinterface) as we will see later.
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