Exact differential

From Exampleproblems

A differential dQ is said to be ‘exact’ if the function Q exists. It is always possible to calculate the differential dQ of a given function Q(x, y, z). However, if dQ is arbitrarily given, the function Q generally does not exist.

A differential dQ = A(x)dx, a function of only one variable, is always exact. In order that a differential dQ = A(x, y)dx + B(x, y)dy, a function of two variables, be an exact differential in a region R of the xy-plane, it is necessary and sufficient that between A and B there exists the relation:

A differential dQ = A(x, y, z)dx + B(x, y, z)dy + C(x, y, z)dz is an exact differential in a region R of the xyz-coordinate system if between the functions A, B and C there exists the relations:

; ;

These conditions, which are easy to generalize, arise from the independece of the order of differentions 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;
• , 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, F 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.

Differentials which are not exact are often denoted with a δ rather than a d. For example, in thermodynamics, δQ  and δW  denote infinitesimal amounts of heat energy and work, respectively.

Some useful equations derived from exact differentials in two dimensions

(See also Bridgman's 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

but also by the chain rule:

and

so that:

which implies that:

Letting v = y gives:

Letting u = y, v = z gives:

using ( gives:

Sources

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.