Charge displacement

The observed dielectric constant M and the dielectric loss factor k = k tan S are defined by the charge displacement characteristics of the ceramic ie, the movement of charged species within the material in response to the appHed electric field. Discussion of polarization mechanisms is available (1). 

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. 

Drawings using arrows help us see why acetone has a larger dipole moment than dimethyl ether. The arrows show the charge displacement for each polar bond. Experimental values for the dipole moments are butane, 0 D methyl ethyl ether, 1.12 D and acetone, 2.88 D. 

Claviher J, Orts JM, Gomez R, Pehn JM, Aldaz A. 1996. Comparison of electrosorption at activated polycrystaUine and Pt(531) kinked platinum electrodes. Surface voltammetry and charge displacement on potentiostatic CO adsorption. J Electroanal Chem 404 281-289. 

Climent V, Attard GA, Feliu JM. 2002a. Potential of zero charge of platinum stepped surfaces a combined approach of CO charge displacement and N2O reduction. J Electroanal Chem 532 67-74. 

Tawa, G. J., and Pratt, L. R. (1994). Tests of dielectric model descriptions of chemical charge displacements in water. ACS Symposium Series 568, 60-70. 

An electric field induces a polarization of the charge within a single molecule by the instantaneous displacement of the electrons with respect to the nucleus. In this manner an induced dipole and, hence, a dipolar moment, p, are generated. When the applied field is weak, the induced charge displacement is proportional to the strength of the field ... 

Orientation effects in benzene derivatives operate in two ways. If the substituent is inductive there are large first order charge displacements at the ortho and para positions, and these can be estimated approximately using the atom polarizabilities (which is very small at the meta position). The changes of bond order, however, and consequently of free valence, vanish in first order and hence depend on Sa. The charge g g at position s therefore increases or decreases from the value unity in the... 

There are two types of charging currents and condenser charges, which may be described as rapidly forming or instantaneous polarizations and slowly forming or absorptive polarizations. The total polarizability of the dielectric is the sum of contributions due to several types of charge displacement in the materials caused by the applied field. The relaxation time is the time required for polarization to form or disappear. The magnitude of the polarizability, k, of a dielectric is related to the dielectric constant, s, as follows ... 

A critical problem related to this is predicting the relative charge displacements which arise on coordination to a series of metal ions or of one metal ion in a series of different coordination environments. In this latter case there is a guide given by Nyholm 26) which is a corollary of Pauling s principle of electroneutrality. 

This description of antennas may seem more appropriate to a discussion of radio or television waves. We must realize, however, that at the molecular level dipoles behave exactly like antennas. Since molecules are made up of charged parts, a dipole moment /x is induced by the electric field of the radiation in any material through which radiation passes. In this discussion, the dipole moment equals the product of the effective charge displaced by the field and its distance of separation from the opposite charge. In SI, pi has units C m. We consider isotropic materials characterized by a polarizability a. As the name implies, this property measures the ease with which charge separation —polarity —is induced in a molecule by an electric field. For isotropic substances, the dipole moment and the field are related by the expression... 

Expressions of the type of the right-hand side of Eq. 2.65 can be expressed in terms of the charge displacement autocorrelation function,... 

Therefore, a symmetrical distribution of electron density exists around the atom with an equal ease of charge displacement in both the +x and -x directions. 

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