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Combination of electronic, ionic and dipolar contributions to the permittivity.

Electrical frequency regime characterisation of the permittivity in terms of the static and high frequency permittivities and a relaxation time - the Debye equations.

The Kramers-Kronig relations.

Local field effects. Polarisability. The Clausius-Mossotti equation. Monatomic low pressure gas.

Schematic of combined contributions to real part of permittivity

Kramers-Kronig handout

Local field effects

Cavity field handout

What you need to know

You need to have an understanding of how all of the individual polarisation contributions combine to give an overall picture of the frequency dependent permittivity and how the static and high frequency constants are defined.

You need to know the basic form of the Debye equation description of the frequency dependent permittivity in the electrical frequency regime.

You should be aware of the significance of the Kramers-Kronig relations (but you do not need to recall their precise form).

You should understand the significance of local field effects and the inherent difficulty in making a direct link between macroscopic measurements and microscopic models of polarisation.

You should be aware of the Clausius-Mossotti relationship.

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