Macroscopic Work

Comparison of the expression on the right side of this equation with Eq. G.2.6 shows that the expression is the same as the change of E sys  

Recall that the vector is the force exerted on particle i, in the system, by the particles in the surroundings other than those responsible for an external field. Thus AFsys is equal to the total work done on the system by the surroundings, other than work done by an external field such as a gravitational field. 

It might seem strange that work done by an external field is not included in AFsys- The reason it is not included is that was defined to be a potential energy belonging only to the system, and is thus irrelevant to energy transfer from or to the surroundings. 

In thermodynamics we are interested in the quantity of work done on macroscopic parts of the system during a process, rather than the work done on individual particles. Macroscopic work is the energy transferred across the system boundary due to concerted motion of many particles on whieh the surroundings exert a force. Macroscopic mechanical work occurs when there is displacement of a macroscopic portion of the system on which a short-range contact force acts across the system boundary. This force could be, for instance, the pressure of an external fluid at a surface element of the boundary multiplied by the area of the surface element, or it could be the tension in a cord at the point where the cord passes through the boundary. 

The symbol wiiab will refer to macroscopic work measured with displacements in the lab frame. 

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The work function plays an important role in catalysis. It determines how easily an electron may leave the metal to do something useful for the activation of reacting molecules. However, strictly speaking, the work function is a macroscopic property, whereas chemisorption and catalysis are locally determined phenomena. They need to be described in terms of short-range interactions between adsorbed molecules and one or more atoms at the surface. The point we want to make is that, particularly for heterogeneous surfaces, the concept of a macroscopic work function, which is the average over the entire surface, is not very useful. It is more meaningful to define the work function as a local quantity on a scale with atomic dimensions. 

This simple empirical model for predicting the I.S. shift of a Mossbauer nucleus placed in a metallic system (alloys, as well as intermetallic compounds), uses differences in the tabulated macroscopic work functions and bulk moduli to model differences in the microscopic electronegativities and electron densities at... 

Figure 26-29 (a) Amperometric detection with macroscopic working electrode at the outlet of the capillary, (b) Electropherogram of sugars separated in 0.1 M NaOH, in which OH groups are partially ionized, thereby turning the molecules into anions. [From J. Ye and R. P Baldwin, Amperometric Detection In CapHary Electrophoresis with Normal Size Bectrodes," Anal. Chem. 1993,65,3525.]... 

Let us assume that we have a facetted surface, or a surface with perfect and defect domains. The potential barrier that an electron has to surmount when leaving the solid through a defect region or through an open facet, for example of (110) orientation, is lower than that when the electrons pass through the (111) surface. The relevant quantity in chemisorption theory is not the averaged macroscopic work function, but rather is the work function of the site where a molecule adsorbs. We therefore define the local work function of a site as the difference between the potential of an electron just outside the surface dipole layer and the... 

Fermi level of the metal [19]. This is the value that one measures with scanning tunneling microscopy (see Chapter 7) and with photoemission of adsorbed xenon (see Chapter 3). Thus, on a heterogeneous surface we have local work functions for each type of site, and the macroscopic work function is an average over these values. 

APPENDIX G FORCES, ENERGY, AND WOBiK G.4 MACROSCOPIC WORK... 

We recognize the integral /F " d/f as the macroscopic work at surface element r, because it is the integrated scalar product of the force exerted by the surroundings and the displacement. The total macroscopic work during the process is then given by... 

Heat, lab, can be defined as energy transfer to or from the system that is not accounted for by macroscopic work. This transfer occurs by means of chaotic motions and collisions of individual particles at the boundary. With this understanding, Eq. G.4.1 becomes... 

The latter correlation refers to the macroscopic work-function as defined, for example, by the threshold energy of photoionization. Literature also defines a local work-function or electrostatic potential as will be discussed below. 

The macroscopic work of deformation per unit initial volume(for work function) is obtained directly as the scalar summation of U(t,x ) over all elements ... 

Because the work function defined by eq. 19 must express the actual macroscopic work of deformation relative to the rest state of the network (and not that of isolated chains), we will adopt eq. 26 for the potential, which gives a force, eq. 24, that does not satisfy... 

There are two possible ways of changing the internal energy of a thermodynamic system (1) by the production of macroscopic work under the action of external forces applied to the system and (2) by heat exchange. 

The process of the heat exchange of contacting bodies not accompanied by the production of macroscopic work is referred to as heat exchange. The energy transferred to a body by another body during heat exchange is referred to as the amount of heat or, simply, heat. 

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