Electric potential gradient, effect

Kelvin effect The electrical potential gradient caused by a temperature gradient along a conducting wire. Also known as the Thomson Effect. 

Mass transfer may be influenced by gradients in variables other than concentration and pressure. In the pharmaceutical sciences, gradients in electrical potential and in temperature are two important examples of these other driving forces. Section IV.B.l describes the effect of electrical potential gradients on the transport of ions, and Section IV.B.2 discusses mass transport in the presence of temperature gradients, known as combined heat and mass transfer. 

An applied electrical potential gradient can induce diffusion (electromigration) in metals due to a cross effect between the diffusing species and the flux of conduction electrons that will be present. When an electric field is applied to a dilute solution of interstitial atoms in a metal, there are two fluxes in the system a flux of conduction electrons, Jq, and a flux of the interstitials, J. For a system maintained at constant temperature with Fq = -V = E, Eq. 2.21 gives... 

If one looks along the strip in the direction of the current, with the magnetic lield directed downward, then, with si rips of antimony, cohall, zinc, or iron, the electric potential drop is toward the right and the effect is said to he positive. With gold, silver, platinum, nickel, bismuth, copper, and aluminum, it is toward the left, and Ihe effect is called negative. The transverse electric potential gradient per anil magnetic lield intensity per unit current density is called the Hall coefficient" for the metal in question Thus, the Hall coeflicienL is delined us... 

A F is the difference in free energy of the top and the foot of the barrier. The flux is determined by the difference in the free energies of activation for the different unit processes. B. J. Zwolinski, H. Eyeing and C. E. Reese (190) have extended the approach of Danielli. Their derivations are only valid in the case that the energy barriers are regularly divided, and if the distance between these barriers is very small. These authors consider also the effect of external forces. They obtain if there is an electrical potential gradient and a concentration gradient across the membrane , ... 

Equation (5.4) is correct if the counterions, A and B, have the same mobility, that is, DA-DB-D. However, the Nernst-Planck model, which takes into account the effect of electrical potential gradients, challenges Eq. (5.4) if counterions of different mobilities are present. 

In the sedimentation-equilibrium method a lower centrifugal field is applied and the processes of sedimentation and diffusion are brought to equilibrium [13]. In this case the governing equation contains sedimentation equilibrium concentrations of species at different positions from the axis of rotation, but one does not need to know D. It should be pointed out that sedimentation and diffusion are more complicated when the species are electrically charged. This is because the smaller counterions sediment at a slower rate than do the colloidal-sized species. This creates an electric potential gradient that tends to speed up the counter-ions and to drag the colloidal species. The reverse effect occurs for diffusion. 

It is evident from the last equation that the effects of the gradient and the electric field can be either additive or subtractive, because each term on the right-hand side can be of either sign. In fact, a flow of charged particles produced by a chemical potential difference across a diffusion medium can lead to charge flow and the creation of an electric potential which effectively cancels the effects of the chemical potential difference... 

Surface tension gradient effects add to the better known phenomena of density-gradient-driven convection, concentration-gradient-driven diffusion and electrical-potential-gradient-driven ion migration, which appear in the existing theory of cells and electrodes. The potential difference of a working cell is affected by all the near electrode effects mentioned here. The experimental and analytical difficulty is to separate the variables. Indeed the fluid mechanical effects stir the electrochemical reaction, and make cause and effect difficult to discern. 

A similar treatment can be used to calculate the electric field and the electric potential in the metal. However, in a metal, both the electric field and the electric potential drop to zero at a very short distance from the semiconductor/metal interface. This occurs because metals do not support electric fields, and all of the excess charge density resides on the surface of the metallic phase. The surface dipole layer is therefore effectively screened from test charges at any finite distance into the metallic phase, and the width of the electric potential gradient is extremely small. Because charge carriers can pass freely through this extremely thin barrier, only the electric field in the semiconductor significantly affects the electrical properties of semiconductor/metal contacts. 

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. 

Fig. 4 Imposing an electrical potential gradient across a charged membrane produces a convective solvent flow in the direction of counter-ion transport (i.e., from anode-to-cathode in the case of skin). This electroosmotic effect (EO) adds to electrorepulsion (ER) to enhance the transport of cationic compounds during iontophoresis (4a) while acting against the electromigration of anions (4b).

The other two contributions to mass transfer arise from forces on the molecule considered. When the molecule or particle is charged and an external electrical field is applied, electrostatic forces are effective and a positive molecule descends the electrical potential gradient, that is, goes in the direction of lower potentials, whereas a negative ion... 

When an electrical potential gradient is imposed on the stack, alternate compartments become enriched and depleted in sodium chloride. A typical module of an electrodialytic salt plant has 1500 pairs of membranes, each with an effective membrane area of 1 m2. The current density is 3.65 A/dm2 at 620 V with a membrane spacing of about 0.75 mm. A brine concentrate containing about 118 g/1 of chloride can be attained. The overall current efficiency is 73% for Na+ and 85% for Cl". Typically, the membranes are divinylbenzene cross-linked polystyrene with sul-phonic acid, or quaternary ammonium exchange groups the exchange capacity is 1.8 to 2.8 meq/g at 25 °C34). 

Several processes occur simultaneously within the membrane phase of an operating cell. Sodium, chloride and hydroxide ions all migrate under the combined effects of concentration and electrical potential gradients with sodium ions as the major current carrier. The flow of sodium ions in a field is accompanied by a net elect roosmo tic flow of water in the same direction. Chloride ion flux is much smaller than that of sodium... 

The practically pure-metal behaviour of thermopower can easily be understood in terms of the fact that a thermal gradient plays a more important part here than an electrical potential gradient. The barriers between the metallic phases therefore play only a secondary role in thermopower. The negative peaks at low temperatures stilt await a convincing explanation, but do at least suggest that the conductivity may not necessarily be due (solely) to holes, but (at least partially) to electrons as well. They may also indicate an increase in the importance of charge energy, quantum effects (SIMIT) or localisation at low temperatures. 

Yuan (2006) reported a study on the effect of Fe on EK remediation of clay contaminated with PCE. That work investigated the effect of iron wall position and Fe°quantity on the remediation efficiency and EK performance of PCE-contaminated clay under an electric potential gradient of 2V/cm for 5 days. The iron wall was composed of 2-16g of Fe° mixed with Ottawa sand at a ratio of 1 2. Its positions were located at the anode, the middle, and the cathode end of the EK cell, respectively. Test results showed that a relatively higher remediation of 66% of PCE was found as the iron wall located at the cathode side, which corresponded to a factor... 

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