Contact difference of potential

Contact Difference of Potential.—Suppose we have two metals, a and 6, in the same container at the same temperature. For each one, we can calculate the vapor pressure of the electron gas in equilibrium with it by Eq. (2.2). Since this equation depends on the properties of the metal, we shall get a different answer in the two cases. That is, an electron gas in equilibrium with one metal, say one with a low work function, will have too great a pressure to be in equilibrium with the second metal with a larger work function. Let us use a kinetic argument to see what will happen and what sort of final equilibrium we may expect. Suppose the metal a, of low work function, has established its equilibrium pressure in the electron gas, and then the metal b is introduced into the container and brought to the same temperature. The gas pressure is too great for equilibrium with b, so that more electrons will strike its surface and... 

Neglecting the No(dCp/dNy, the Vs are the latent heats. Thus we have the important statement that the contact difference of potential between two metals equals the difference of their latent heats, or approximately of their work functions. This relation is found to be verified experimentally. The contact difference of potential can be found by purely electrostatic experiments, and the work functions by thermionic emission the results obtained in these two quite different types of experiment are in agreement. The small correction terms arising from the No(dCp/dN) s lie almost within the errors of the experiments, so that we hardly need consider them in our statement of the general theorem. 

The activity is characterized by the rate constant of exchange at 300°0 while the work function may be specified by the contact difference of potentials, taken in volts, between the oxide and gold (30). The data of the Table VI is evidence of the lack of the simple relation between the catalytic activity and the work function. With the equivalent value of the work function, the catalytic activity of some oxides differs by 5 orders while with the equivalent activity, the work function, in particular cases, differs by 1.05 volts. From this fact follows that proceeding from one oxide to the other the change of the work function is not the only main factor determining the change of catalytic activity. 

One of the main assumptions for the thin-film YSZ-based sensors is the negligible interinfluence of the thin-film layers. This means that the mechanical, physical, chemical, and electrostatic components of their interactions are close to zero [53]. In order to achieve such conditions, the following requirements must be implemented the purity of raw materials for thin films should be ultra-high (99.999%), and raw materials should have compatible coefficients of thermal expansion and parameters of the crystalline structure. They should also be characterized by the minimum value of the contact difference of potentials. 

The contact difference of potential, E, between two metals is a function of the temperature, 0, such that E = a + be + cti2. How high... 

The lower zero index is evidence that for all the activities equilibrium values are taken. As follows from the above ratios, the zFE° value is equal (with accuracy of up to Acpcont) to that of standard GE chemical reaction (1), which is a sum of reactions (5) and (6), but it is not equal to the value of AG° for reaction (14), which actually determines the E° value. Note that E° depends on contact difference of potentials notwithstanding the absence of contact between the electrodes (the external circuit is open). 

In all cases of contact structures the cause of nonequilibrium is the band bending caused by the contact difference of potentials, i.e., by the built-in electric field. The external electric field created by bias causes carrier drift within the bent bands region and a result is carrier nonequilibrium. 

DIFFUSION POTENTIAL. When liquid junctions exist where two electrolytic solutions are in contact, as in the case of two solutions of different concentrations of the same electrolyte, diffusion of ions occurs between the solutions, and the differences in rales of diffusion of different ions set up an electrical double layer, having a difference of potential, known as Ihe diffusion potential nr liquid junction potential. 

In Chap. XX, Sec. 3, we spoke about the detachment of electrons from atoms, and in Sec. 4 of that chapter we took up the resulting chemical equilibrium, similar to chemical equilibrium in gases. But electrons can be detached not only from atoms but from matter in bulk, and particularly from metals. If the detachment is produced by heat, we have thermionic emission, a process very similar to the vaporization of a solid to form a gas. The equilibrium concerned is very similar to the equilibrium in problems of vapor pressure, and the equilibrium relations can be used, along with a direct calculation of the rate of condensation, to find the rate of thermionic emission. In connection with the equilibrium of a metal and its electron gas, we can find relations between the electrical potentials near two metals in an electron gas and derive information about the so-called Volta difference of potential, or contact potential difference, between the metals. We begin by a kinetic discussion of the collisions of electrons with metallic surfaces. 

Let the terminals of the cell be of the same metal, M, as one of the electrodes. The electrostatic potentials, when the cell has reached equilibrium with the terminals disconnected from each other, are indicated above the phases there are three phase boundary potentials concerned in producing the final difference of potential, Fa— Va between the terminals. These are Va--Vs, —(VP—VS), and the metal-metal contact potential F — V their algebraic sum is the difference in electrostatic potential of the terminal wires. It is the electromotive force of the whole cell, and can, of course, be measured with a potentiometer. 

Static electricity is an important source of ignition, which has to be accounted for in process plants. It is a smface effect related to the contact and separation of bodies. One of the bodies retains a positive charge after separation, the other one a negative one. If the body consists of an electric conductor and is connected to the earth charges move freely and the body returns to the uncharged state. If there is no connection with the earth, the charge is retained showing differences of potential between 1 and 70 kV. 

This figure shows the levels of the electric potential in the electrochemical cell represented in Figure 2.5. The voltage measured at the contacts is given hy U = f M" - f M > where f M is the electric potential of the metal M and 0 " that of the metal M". This measurement in no case offers information concerning the differences of potential at the electrode-electrolyte interface, f t, - 0 and 0 o> because 0, the electric potential of the electrolyte, is not available. 

The electrodeionization process is very similar to conventional electrodialysis. The main difference is that the dUuate compartment is filled with a mixed ion exchange resin bed. Ions present in the diluate diffuse into the resin and are exchanged with or OH. When a difference of potential is applied, the ions exchanged migrate from the resin bed to the adjacent concentrate compartments. The resin allows the dissolution of diluate to keep acceptable values of electrical conductivity although the salt content is very low, because water dissociates into or OH at the point of contact of the diluate with the cationic and anionic resin beds. In this way, you... 

When two dissimilar bodies or substances meet, electrons pass from one to the other at the surface contact area. When the bodies separate, particularly if they are of an insulating material, a difference of potential occurs... 

All voltaic cells consist of a series of conducting phases in contact the electrodes are generally metallic and there are one or more liquid electrolytes. At any phase boundary, where two or more phases of different composition meet, there is a difference of potential. The e.m.f. of the cell is the algebraic sum of all these phase-boundary potentials, including any metal contact potentials that may be present. The e.m.f. of a cell is the potential difference between two pieces of metal of identical composition, the ends of the chain of conducting phases. 

An electrode consists essentially of two conductors, one electronic and the other electrolytic, in contact. At the surface of separation (e.g. metal/metal ion in solution) a difference of potential, the electrode potential, exists. In principle, the work done in bringing a unit positive charge from infinity to the interface provides a measure of this potential no such experiment is possible in practice. 

The position of the mercury meniscus in the capillary depends on the surface tension between the mercury and the sulphuric acid and this, in turn, depends on the electrical potential between the mercury and the acid. If this potential is altered, as, for example, by connecting the two mercury electrodes with a cell or with two points of a circuit between which there is a difference of potential, the meniscus will movej and for small differences of potential, the amount of movement is proportional to the difierence of potential. In order, however, that the meniscus shall take up a definite position, the two electrodes must be connected together except when making a measurement This is efiected by means of a triple contact Morse key, shown in Fig. 72. The electrical connections between the terminals a, b, c and the contacts d, V, d are indicated by means of the dotted lines. The electrodes of the electrometer are connected with the terminals b and c, so that they are connected together when the key is in its normal position. The terminals a and c are connected with the rest of the circuit, so... 

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