Pfaffian

As we have seen earlier, the thermodynamic variables p, V, T, U, S, H, A, and G (that we will represent in the following discussion as W, X, T, and Z) are state functions. If one holds the number of moles and hence composition constant, the thermodynamic variables are related through two-dimensional Pfaffian equations. The differential for these functions in the Pfaff expression is an exact differential, since state functions form exact differentials. Thus, the relationships that we now give (and derive where necessary) apply to our thermodynamic variables. 

Pfaffian Differential Expressions With Two Variables Before we undertake the arguments generalized for three or more variables, we digress to consider some examples involving only two variables. These do not provide the generality we must have to treat thermodynamic systems of three or more variables, but will provide concrete illustrations of the general behavior we will invoke in the development. 

A second Pfaffian differential of interest to us now is the one for the differential quantity of heat, 8c/KV, associated with a reversible process.11 We obtain it by combining equation (1.47) with the first law statement (equation (2.4) that relates U, vv, and q... 

In this discussion, we will limit our writing of the Pfaffian differential expression bq, for the differential element of heat flow in thermodynamic systems, to reversible processes. It is not possible, generally, to write an expression for bq for an irreversible process in terms of state variables. The irreversible process may involve passage through conditions that are not true states" of the system. For example, in an irreversible expansion of a gas, the values of p. V, and T may not correspond to those dictated by the equation of state of the gas. 

It can be shown mathematically that a two-dimensional Pfaffian equation (1.27) is either exact, or, if it is inexact, an integrating denominator can always be found to convert it into a new, exact, differential. (Such Pfaffians are said to be integrable.) When three or more independent variables are involved, however, a third possibility can occur the Pfaff differential can be inexact, but possesses no integrating denominator.x Caratheodory showed that expressions for SqKV appropriate to thermodynamic systems fall into the class of inexact but integrable differential expressions. That is, an integrating denominator exists that can convert the inexact differential into an exact differential. 

Pfaffian Differential Expressions with Three or More Variables and the Conditions for the Existence of an Integrating Denominator We extend the expression for the Pfaffian differentials in three or more variables, by writing it as... 

As we have seen before, exact differentials correspond to the total differential of a state function, while inexact differentials are associated with quantities that are not state functions, but are path-dependent. Caratheodory proved a purely mathematical theorem, with no reference to physical systems, that establishes the condition for the existence of an integrating denominator for differential expressions of the form of equation (2.44). Called the Caratheodory theorem, it asserts that an integrating denominator exists for Pfaffian differentials, Sq, when there exist final states specified by ( V, ... x )j that are inaccessible from some initial state (.vj,.... v )in by a path for which Sq = 0. Such paths are called solution curves of the differential expression The connection from the purely mathematical realm to thermodynamic systems is established by recognizing that we can express the differential expressions for heat transfer during a reversible thermodynamic process, 6qrey as Pfaffian differentials of the form given by equation (2.44). Then, solution curves (for which Sqrev = 0) correspond to reversible adiabatic processes in which no heat is absorbed or released. 

The Caratheodory treatment is grounded in the mathematical behavior of Pfaffian differential expressions (equation (2.44), and the observation that a... 

The quantities dX and d Y are called differentials, the coefficients in front of dX and dT are called partial derivatives,11 and dZ is referred to as a total differential because it gives the total change in Z arising from changes in both X and Y. If Z were to depend upon additional variables, additional terms would be included in equation (A 1.1) to represent the changes in Z arising from changes in those variables. For much of our discussion, two variables describe the processes of interest, and therefore, we will limit our discussion to two independent variables, with the exception of the description of Pfaffian differentials in... 

Reciprocity relations that guarantee exactness of three-dimensional Pfaffians are... 

That is, if these three relationships are satisfied simultaneously for a given Pfaffian, the Pfaffian is exact, and some function F(.x. y. r) exists such that the total differential dF= bQ. 

The necessary (and sufficient) condition for a three-dimensional Pfaffian to be inexact but integrable is... 

If neither of the two sets of relations holds, then the Pfaffian expression can be considered to be inexact and non-integrable. 

Within each solution surface are numerous subsets of points that also satisfy the differential equation bQ = dF = 0. These subsets are referred to as solution curves of the Pfaffian. The curve z — 0, y + y2 = 25.00 is one of the solution curves for our particular solution surface with radius = 5.00. Others would include x = 0, y2 + z2 — 25.00, and r — 0,. v2 + r2 = 25.00. Solution curves on the same solution surface can intersect. For example, our first two solution curves intersect at two points (5, 0, 0) and (-5, 0. 0). However, solution curves on one surface cannot be solution curves for another surface since the surfaces do not intersect. That two solution surfaces to an exact Pfaffian differential equation cannot intersect and that solution curves for one surface cannot be solution curves for another have important consequences as we see in our discussion of the Caratheodory formulation of the Second Law of Thermodynamics. 

When the Pfaffian expression is inexact but integrable, then an integrating factor A exists such that AbQ = d5, where dS is an exact differential and the solution surfaces are S = constant. While solution surfaces do not exist for the inexact differential 8Q, solution curves do exist. The solution curves to dS = 0 will also be solution curves to bQ = 0. Since solution curves for dS on one surface do not intersect those on another surface, a solution curve for 8Q — 0 that lies on one surface cannot intersect another solution curve for bQ = 0 that lies on a different surface. 

Thus, exact or integrable Pfaffians lead to non-intersecting solution surfaces, which requires that solution curves that lie on different solution surfaces cannot intersect. For a given point p. there will be numerous other points in very close proximity to p that cannot be connected to p by a solution curve to the Pfaffian differential equation. No such condition exists for non-integrable Pfaffians, and, in general, one can construct a solution curve from one point to any other point in space. (However, the process might not be a trivial exercise.)... 

In two dimensions, the Pfaffian differential expression reduces to the form we saw earlier... 

For the two-dimensional Pfaffian, condition (c) above is no longer applicable. That is, the Pfaffian differential expression is either (a), exact, or (b), integrable with an integrating factor. 

Pfaffian differentials 22-3. 608-11 in Second law of thermodynamics 63-7 three or more variables 67 two variables 64-6 solution curves and surfaces 610-11 in three dimensions 609 in two dimensions 611 phase equilibria criteria for 231-7... 

This formula can easily be deduced from a theory due to P. W. Kasteleyn [4] (1961) which allows the number of 1-factors of any planar graph G with an even number of vertices to be expressed as the value of the Pfaffian PfS = j/det S of some skew-symmetric matrix S connected with G. Elementary proofs of Eq. (2) (not using Kasteleyn s formula) for plane graphs in which every face F is a (4k + 2)-gon (where k depends on F) were also given by D. Cvetkovic, I. Gutman and N. Trinajstic [5] (1972) and H. Sachs [6] (1986). 

Normally the apparatus of equilibrium thermodynamics can be used for the remoteness in the second and third sense and a corresponding choice of space of variables, though in each specific case this calls for additional check. Because for the spaces that do not contain the functions of state (in the descriptions of nonequilibrium systems these are the spaces of work-time or heat-time) the notion of differential loses its sense, and transition to the spaces with differentiable variables requires that the holonomy of the corresponding Pfaffian forms be proved. The principal difficulties in application of the equilibrium models arise in the case of remoteness from equilibrium in the first sense when the need appears to introduce additional variables and increase dimensionality of the problem solved. 

Equation (1.73) is a Pfaffian equation and a, is an independent variable. Caratheodory s theory states that starting from a known original state, there may be other states that cannot be reached by an adiabatic process along the path 8q = 0. This shows the existence of an integrating factor for 8q hence we have... 

Equation (1.107) is more useful if it is integrated with the Pfaffian form however, this is not a straightforward step, since intensive properties are functions of all the independent variables of the system. The Euler relation for... 

Equation (1.4.24) is representative of the so-called Pfaffian forms to be introduced later. Note that Eqs. 

However, in general, W is not a function of state thus, one cannot ordinarily obtain a unique differential dW of W. In recognition of this fact we shall henceforth write 3W as the element of work. This symbol represents a shorthand notation of the quantity - (fxdx + fydy + fzdz) appearing on the right-hand side of (1.6.2) it represents another example of the so-called Pfaffian form introduced in Section 1.4(h). 

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