Charged alternating fields

In the case of symmetrical molecules such as carbon tetrachloride, benzene, polyethylene and polyisobutylene the only polarisation effect is electronic and such materials have low dielectric constants. Since electronic polarisation may be assumed to be instantaneous, the influence of frequency and temperature will be very small. Furthermore, since the charge displacement is able to remain in phase with the alternating field there are negligible power losses. 

Polarization can be classified as electronic (electron cloud distortion), atomic, molecular, ionic, and crystalline. The point of maximum polarization in a system would occur when all dipoles reacted to the applied field and aligned. This is difficult to obtain even in a static situation. In an alternating field situation, the dielectric remains the same or decreases as the frequency increases past the microwave region (11). In the microwave region, attainment of equilibrium is more difficult, and there is an observable lag in the dipole orientation which is commonly called relaxation. The polarization then acquires a component out of phase with the field thermal dissipation of some of the energy of the field. This dissipation and its relation to the normal charging current can be related by Equation 1 where c is the measured dielectric constant of the material and e"... 

An electrified liquid flows faster through a capillary because the charge on the tip repels the liquid flowing from it. The influence of an electric field on viscosity (except an alternating field) is very small with very pure liquids, and is probably zero for non-polar liquids. With polar liquids there may be a small effect. Sellerio o found the viscosity of an insulating liquid (castor oil) increased in an electric field. The flow of a liquid between solid surfaces close together seems to be influenced by electric potential gradients set up at the interfaces. 

The equations of set (B) reflect the fact that magnetic charges do not exist. This set is also valid for alternating fields. 

Electrical phenomena occur in space as well as in atoms. Eadia-tion consists of electromagnetic waves in which there is a propagation of alternating fields. A close connexion exists between these fields and the movement of charges in atoms and molecules. The laws of such interactions must now be explored, partly because they are of the greatest importance in their own right and partly because they reveal many intimate details about the economy of the molecules themselves. 

Minor et al. [32] have analyzed the time dependence of both the electroosmotic flow and electrophoretic mobility in an electrophoresis cell. They concluded that, for most experimental conditions, the colloidal particle reaches its steady motion after the application of an external field in a much shorter time than electroosmotic flow does. Hence, if electrophoresis measurements are performed in an alternating field with a frequency much larger than the reciprocal of the characteristic time for steady electroosmosis (t 10° sec), but smaller than that of steady electrophoresis (t 10 sec), the electroosmotic flow cannot develop. In such conditions, electroosmosis is suppressed, and the velocity of the particle is independent of the position in the cell. Figure 3.6 is an example we measured the velocity of polystyrene particles in the center of a cylindrical cell using a pulsed field with the frequency indicated when the frequency is above 10 Hz, the velocity (average between the field-on and field-off values) tends to the true electrophoretic velocity measured at the stationary level. Another way to overcome the electroosmosis problem is to place both electrodes providing the external field, inside the cell, completely surrounded by electroneutral solution as no net external field acts on the charged layer close to the cell walls, the associated electroosmotic flow wiU not exist [33]. 

Fig. VI 1.2. Particle charging and charge sign reversal (a, b, c) exposed to steady electric field (d) exposed to alternating field (a) on insulator (b) on conductor (c) on conductor with sorption of ions.

In an alternating field, a metal surface may also undergo charge reversal. In this case, the charges formed on the particle and on the metal surface will be opposite in sign, and interaction between these charges may increase the adhesive force. Hence metal surfaces are more difficult to clean with an ac field than are insulators or semiconductors. 

Fig. X.5. Charging and discharging of the particles. a,b,c) On application of a steady electric field d) on application of an alternating field, a) On an insulator b) on a conductor c) on a conductor with the sorption of ions.

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