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# Exact differential

Thermodynamic equations
Laws of thermodynamics
Conjugate variables
Thermodynamic potential
Material properties
Maxwell relations
Bridgman's equations
Exact differential
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In mathematics, a differential dQ is said to be exact, as contrasted with an inexact differential, if the differentiable function Q exists. However, if dQ is arbitrarily chosen, a corresponding Q might not exist.

## Overview

In one dimension, a differential

$dQ = A(x)dx\,$

is always exact. In two dimensions, in order that a differential

$dQ = A(x, y)dx + B(x, y)dy\,$

be an exact differential in a simply-connected region R of the xy-plane, it is necessary and sufficient that between A and B there exists the relation:

$\left( \frac{\partial A}{\partial y} \right)_{x} = \left( \frac{\partial B}{\partial x} \right)_{y}$

In three dimensions, a differential

$dQ = A(x, y, z)dx + B(x, y, z)dy + C(x, y, z)dz\,$

is an exact differential in a simply-connected region R of the xyz-coordinate system if between the functions A, B and C there exist the relations:

$\left( \frac{\partial A}{\partial y} \right)_{x,z} \!\!\!= \left( \frac{\partial B}{\partial x} \right)_{y,z}$   ;   $\left( \frac{\partial A}{\partial z} \right)_{x,y} \!\!\!= \left( \frac{\partial C}{\partial x} \right)_{y,z}$   ;   $\left( \frac{\partial B}{\partial z} \right)_{x,y} \!\!\!= \left( \frac{\partial C}{\partial y} \right)_{x,z}$

These conditions, which are easy to generalize, arise from the independence of the order of differentiations in the calculation of the second derivatives. So, in order for a differential dQ, that is a function of four variables to be an exact differential, there are six conditions to satisfy.

In summary, when a differential dQ is exact:

• the function Q exists;
• $\int_i^f dQ=Q(f)-Q(i)$, independent of the path followed.

In thermodynamics, when dQ is exact, the function Q is a state function of the system. The thermodynamic functions U, S, H, A and G are state functions. Generally, neither work nor heat is a state function. An exact differential is sometimes also called a 'total differential', or a 'full differential', or, in the study of differential geometry, it is termed an exact form.

## Partial Differential Relations

For three variables, x, y and z bound by some differentiable function F(x,y,z), the following total differentials exist[1]:667&669

$d x = {\left ( \frac{\partial x}{\partial y} \right )}_z d y + {\left ( \frac{\partial x}{\partial z} \right )}_y dz$
$d z = {\left ( \frac{\partial z}{\partial x} \right )}_y d x + {\left ( \frac{\partial z}{\partial y} \right )}_x dy$.
Note: The subscripts outside the parenthesis indicate which variables are being held constant during differentiation. Due to the definition of the partial derivative, these subscripts are not required, but they are included as a reminder.

Substituting the first equation into the second and rearranging, we obtain[1]:669

$d z = {\left ( \frac{\partial z}{\partial x} \right )}_y \left [ {\left ( \frac{\partial x}{\partial y} \right )}_z d y + {\left ( \frac{\partial x}{\partial z} \right )}_y dz \right ] + {\left ( \frac{\partial z}{\partial y} \right )}_x dy$,
$d z = \left [ {\left ( \frac{\partial z}{\partial x} \right )}_y {\left ( \frac{\partial x}{\partial y} \right )}_z + {\left ( \frac{\partial z}{\partial y} \right )}_x \right ] d y + {\left ( \frac{\partial z}{\partial x} \right )}_y {\left ( \frac{\partial x}{\partial z} \right )}_y dz$,
$\left [ 1 - {\left ( \frac{\partial z}{\partial x} \right )}_y {\left ( \frac{\partial x}{\partial z} \right )}_y \right ] dz = \left [ {\left ( \frac{\partial z}{\partial x} \right )}_y {\left ( \frac{\partial x}{\partial y} \right )}_z + {\left ( \frac{\partial z}{\partial y} \right )}_x \right ] d y$.

Since y and z are independent variables, dy and dz may be chosen without restriction. For this last equation to hold in general, the bracketed terms must be equal to zero.[1]:669

### Reciprocity Relation

Setting the first term in brackets equal to zero yields[1]:670

${\left ( \frac{\partial z}{\partial x} \right )}_y {\left ( \frac{\partial x}{\partial z} \right )}_y = 1$.

A slight rearrangement gives a reciprocity relation,[1]:670

${\left ( \frac{\partial z}{\partial x} \right )}_y = \frac{1}{{\left ( \frac{\partial x}{\partial z} \right )}_y}$.

There are two more permutations of the foregoing derivation that give a total of three reciprocity relations between x, y and z. Reciprocity relations show that the inverse of a partial derivative is equal to its reciprocal.

### Cyclic Relation

Setting the second term in brackets equal to zero yields[1]:670

${\left ( \frac{\partial z}{\partial x} \right )}_y {\left ( \frac{\partial x}{\partial y} \right )}_z = - {\left ( \frac{\partial z}{\partial y} \right )}_x$.

Using a reciprocity relation for $\tfrac{\partial z}{\partial y}$ on this equation and reordering gives a cyclic relation (the triple product rule),[1]:670

${\left ( \frac{\partial x}{\partial y} \right )}_z {\left ( \frac{\partial y}{\partial z} \right )}_x {\left ( \frac{\partial z}{\partial x} \right )}_y = -1$.

If, instead, a reciprocity relation for $\tfrac{\partial x}{\partial y}$ is used with subsequent rearrangement, a standard form for implicit differentiation is obtained:

${\left ( \frac{\partial y}{\partial x} \right )}_z = - \frac { {\left ( \frac{\partial z}{\partial x} \right )}_y }{ {\left ( \frac{\partial z}{\partial y} \right )}_x }$.

## Some useful equations derived from exact differentials in two dimensions

(See also Bridgman's thermodynamic equations for the use of exact differentials in the theory of thermodynamic equations)

Suppose we have five state functions z,x,y,u, and v. Suppose that the state space is two dimensional and any of the five quantites are exact differentials. Then by the chain rule

$(1)~~~~~ dz = \left(\frac{\partial z}{\partial x}\right)_y dx+ \left(\frac{\partial z}{\partial y}\right)_x dy = \left(\frac{\partial z}{\partial u}\right)_v du +\left(\frac{\partial z}{\partial v}\right)_u dv$

but also by the chain rule:

$(2)~~~~~ dx = \left(\frac{\partial x}{\partial u}\right)_v du +\left(\frac{\partial x}{\partial v}\right)_u dv$

and

$(3)~~~~~ dy= \left(\frac{\partial y}{\partial u}\right)_v du +\left(\frac{\partial y}{\partial v}\right)_u dv$

so that:

$(4)~~~~~ dz = \left[ \left(\frac{\partial z}{\partial x}\right)_y \left(\frac{\partial x}{\partial u}\right)_v + \left(\frac{\partial z}{\partial y}\right)_x \left(\frac{\partial y}{\partial u}\right)_v \right]du$

$+ \left[ \left(\frac{\partial z}{\partial x}\right)_y \left(\frac{\partial x}{\partial v}\right)_u + \left(\frac{\partial z}{\partial y}\right)_x \left(\frac{\partial y}{\partial v}\right)_u \right]dv$

which implies that:

$(5)~~~~~ \left(\frac{\partial z}{\partial u}\right)_v = \left(\frac{\partial z}{\partial x}\right)_y \left(\frac{\partial x}{\partial u}\right)_v + \left(\frac{\partial z}{\partial y}\right)_x \left(\frac{\partial y}{\partial u}\right)_v$

Letting v = y gives:

$(6)~~~~~ \left(\frac{\partial z}{\partial u}\right)_y = \left(\frac{\partial z}{\partial x}\right)_y \left(\frac{\partial x}{\partial u}\right)_y$

Letting u = y, v = z gives:

$(7)~~~~~ \left(\frac{\partial z}{\partial y}\right)_x = - \left(\frac{\partial z}{\partial x}\right)_y \left(\frac{\partial x}{\partial y}\right)_z$

using ($\partial a/\partial b)_c = 1/(\partial b/\partial a)_c$ gives the triple product rule:

$(8)~~~~~ \left(\frac{\partial z}{\partial x}\right)_y \left(\frac{\partial x}{\partial y}\right)_z \left(\frac{\partial y}{\partial z}\right)_x =-1$

• Closed and exact differential forms for a higher-level treatment
• Differential
• Inexact differential
• Integrating factor for solving non-exact differential equations by making them exact

## References

1. ^ a b c d e f g Çengel, Yunus A.; Boles, Michael A. [1989] (1998). "Thermodynamics Property Relations", Thermodynamics - An Engineering Approach, 3rd, McGraw-Hill Series in Mechanical Engineering (in English), Boston, MA.: McGraw-Hill. ISBN 0-07-011927-9.
• Perrot, P. (1998). A to Z of Thermodynamics. New York: Oxford University Press.
• Zill, D. (1993). A First Course in Differential Equations, 5th Ed. Boston: PWS-Kent Publishing Company.