Some Quantitative Relationships of Dielectrics

Finally mention may be made about the influence of humidity on the electrical insulating properties of plastics. Once again the polymers may be classified into two groups, those which do not absorb water and those which do. The nonabsorbent materials are little affected by humidity whereas the insulation characteristics of the absorbent materials deteriorate seriously. These latter materials are generally certain polar materials which all appear capable of forming some sort of bond, probably a hydrogen bond, with water. Three reasons may be given for the deleterious effects of the water. 

There are a number of properties of molecules that are additive to a reasonable approximation, i.e. the value of such a property of a given molecule is an approximate sum of the values of the properties of either the atoms or bonds present. It has been shown that the dielectric constant is related to some additive properties and it is thus possible to make some estimate of dielectric properties from consideration of molecular structure. 

Pa is the nuclear polarisation (considered to be negligible and hence ignored in further discussion) 

Pq the dipole or orientation polarisation P itself is defined by the Clausius-Mosotti Equation 

118 Relation of Structure to Electrical and Optical Properties It may be shown that for electron polarisation 

It is thus seen that for polymers in which polarisations other than electronic ones are negligible (i.e. P = P ) the dielectric constant is equal to the square of the refractive index (Table 6.2). 

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Ultimately physical theories should be expressed in quantitative terms for testing and use, but because of the eomplexity of liquid systems this can only be accomplished by making severe approximations. For example, it is often neeessary to treat the solvent as a continuous homogeneous medium eharaeterized by bulk properties such as dielectric constant and density, whereas we know that the solvent is a molecular assemblage with short-range structure. This is the basis of the current inability of physical theories to account satisfactorily for the full scope of solvent effects on rates, although they certainly can provide valuable insights and they undoubtedly capture some of the essential features and even cause-effect relationships in solution kinetics. Section 8.3 discusses physical theories in more detail. 

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