Permittivity, relative

Before beginning a discussion of non-aqueous solvents, we must define the relative permittivity, also referred to as the dielectric constant, of a substance. In a vacuum, the Coulom-bic potential energy of a system of two unit electronic charges is given by eq. 9.1 where Sq is the (absolute) permittivity of a vacuum (8.854 x 10 Fm ), e is the charge on the electron (1.602 x 10 C) and r is the separation (in metres) between the point charges. 

If a material is placed between the charges, the force is reduced by an amount that depends upon the relative permittivity of the material. The new Coulombic potential energy is given by eq. 9.2 where e, is the relative permittivity of the material. Since it is a relative quantity, r is dimensionless. 

For example, at 298 K, , of water (the dielectric constant) is 78.7 but, as Fig. 9.1 shows, , varies with temperature. A value of 78.7 can be considered to be a high value and from eq. 9.2, we see that in aqueous solution, the force between two point charges (or two ions) is considerably reduced compared with that in a vacuum. Thus we can consider a dilute aqueous solution of a salt to contain well-separated, non-interacting ions. 

Solvent Formula Relative permittivity, Dipole moment, pidebye 

In the text, we state that in general, the trend in dipole moments follows that in values of the relative permittivities for solvents having related structures. Carry out the following two exercises, and then critically assess this statement. 

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Li) he so-called distance-dependent dielectric models. The simplest implementation of a dis-i.iiice-dependent dielectric is to make the relative permittivity proportional to the distance. Tine interaction energy between two charges qi and qj then becomes ... 

A fourth correction term may also be required, depending upon the medium that surrounds the sphere of simulation boxes. If the surrounding medium has an infinite relative permittivity (e.g. if it is a conductor) then no correction term is required. However, if the surrounding medium is a vacuum (with a relative permittivity of 1) then the following energy must be added ... 

R = 8.314 J/(mol-K) T = temperature, K and F = 9.65 x lO" C/mol. The relative permittivity is a measure of the conductance of the pure bulk material relative to a vacuum. In the salt water in the beaker example, the pure bulk material would be water, which has a relative permittivity of about 80. 

Dielectric constant (specific inductive capacity, relative permittivity) over a wide range of temperature and frequency. 

The influence of a particular dielectric on the capacitance of a condenser is conveniently assessed by the dielectric constant, also known as the relative permittivity or rarely specific inductive capacity. This is defined as the ratio of the relative condenser capacity, using the given material as a dielectric, to the capacity of the same condenser, without dielectric, in a vacuum (or for all practical intents and purposes, air). 

Relative Permittivity lEC 250. The term relative permittivity is currently more commonly used than the term dielectric constant used by the author and most of the references quoted in this chapter. As explained in this chapter, it is a ratio and thus dimensionless. 

It can be shown that the electric susceptibility is related to the relative permittivity or dielectric constant e of the particle by the relationship [88]... 

The insulating property of any insulator will break down in a sufficiently strong electric field. The dielectric strength is defined as the electric strength (V/m) which an insulating material can withstand. For plastics the dielectric strength can vary from 1 to 1000 MV/m. Materials may be compared on the basis of their relative permittivity (or dielectric constant). This is the ratio of the permittivity of the material to the permittivity of a vacuum. The ability of a... 

The dielectric con.stant i.s also referred to a.s relative permittivity by physical chemi.sts. 

The relative permittivity, the electric susceptibility and the refractive index n are related by... 

The refractive index of a medium is the ratio of the speed of light in a vacuum to its speed in the medium, and is the square root of the relative permittivity of the medium at that frequency. When measured with visible light, the refractive index is related to the electronic polarizability of the medium. Solvents with high refractive indexes, such as aromatic solvents, should be capable of strong dispersion interactions. Unlike the other measures described here, the refractive index is a property of the pure liquid without the perturbation generated by the addition of a probe species. 

So is the relative permittivity of the elastomer E is the applied electric field V is the applied voltage f is the film thickness... 

Fig. 3.1 Model of CdS deposition and recrystaUization. The changes in film structure are related to the features of the cyclic voltammogram and the capacitance plot broken line). The interpretation of the capacitance data in this way leads to a mean value of ffcds = 17 for the relative permittivity of the film. (Reprinted from [34], Copyright 2009, with permission from Elsevier)...

An alternative electrical method that has been used in the study of glass-ionomer cements has been the measurement of dielectric properties. Tay Braden (1981, 1984) measured the resistance and capacitance of setting cements at various times from mixing. From the results obtained, relative permittivity and resistivity were calculated. In general, as these cements set, their resistivity was found to fall rapidly, then to rise again. Both these results and the results of relative permittivity measurements were consistent with the cements comprising highly ionic and polar structures. 

The uncharged ion is transferred into a solvent with permittivity e = De0y where D is the relative permittivity (dielectric constant) of the medium. No work is gained or lost in this process. In the solvent, the ion is again recharged to the value of the electric potential at its surface,... 

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