Thermodynamics

Classical thermodynamics is concerned primarily with the macrostructure of matter. It addresses the

gross characteristics of large aggregations of molecules and not the behavior of individual molecules.

The microstructure of matter is studied in kinetic theory and statistical mechanics (including quantum

thermodynamics). In this chapter, the classical approach to thermodynamics is featured.

Basic Concepts and Definitions

Thermodynamics is both a branch of physics and an engineering science. The scientist is normally

interested in gaining a fundamental understanding of the physical and chemical behavior of fixed,

quiescent quantities of matter and uses the principles of thermodynamics to relate the properties of matter.

Engineers are generally interested in studying systems and how they interact with their surroundings. To

facilitate this, engineers have extended the subject of thermodynamics to the study of systems through

which matter flows.

System

In a thermodynamic analysis, the system is the subject of the investigation. Normally the system is a

specified quantity of matter and/or a region that can be separated from everything else by a well-defined

surface. The defining surface is known as the control surface or system boundary. The control surface

may be movable or fixed. Everything external to the system is the surroundings. A system of fixed mass

is referred to as a control mass or as a closed system. When there is flow of mass through the control

surface, the system is called a control volume, or open, system. An isolated system is a closed system

that does not interact in any way with its surroundings.

State, Property

The condition of a system at any instant of time is called its state. The state at a given instant of time

is described by the properties of the system. A property is any quantity whose numerical value depends

on the state but not the history of the system. The value of a property is determined in principle by some

type of physical operation or test.

Extensive properties depend on the size or extent of the system. Volume, mass, energy, and entropy

are examples of extensive properties. An extensive property is additive in the sense that its value for the

whole system equals the sum of the values for its parts. Intensive properties are independent of the size

or extent of the system. Pressure and temperature are examples of intensive properties.

A mole is a quantity of substance having a mass numerically equal to its molecular weight. Designating

the molecular weight by

M and the number of moles by n, the mass m of the substance is m = n

M. One

kilogram mole, designated kmol, of oxygen is 32.0 kg and one pound mole (lbmol) is 32.0 lb. When

an extensive property is reported on a unit mass or a unit mole basis, it is called a specific property. An

overbar is used to distinguish an extensive property written on a per-mole basis from its value expressed

per unit mass. For example, the volume per mole is , whereas the volume per unit mass is v, and the

two specific volumes are related by =

Mv

.

Process, Cycle

Two states are identical if, and only if, the properties of the two states are identical. When any property

of a system changes in value there is a change in state, and the system is said to undergo a process.

When a system in a given initial state goes through a sequence of processes and finally returns to its

initial state, it is said to have undergone a cycle.

Phase and Pure Substance

The term phase refers to a quantity of matter that is homogeneous throughout in both chemical composition

and physical structure. Homogeneity in physical structure means that the matter is all solid, or all

liquid, or all vapor (or equivalently all gas). A system can contain one or more phases. For example, a

v

v

Engineering Thermodynamics

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© 1999 by CRC Press LLC

system of liquid water and water vapor (steam) contains two phases. A pure substance is one that is

uniform and invariable in chemical composition. A pure substance can exist in more than one phase, but

its chemical composition must be the same in each phase. For example, if liquid water and water vapor

form a system with two phases, the system can be regarded as a pure substance because each phase has

the same composition. The nature of phases that coexist in equilibrium is addressed by the phase rule

(Section 2.3, Multicomponent Systems).

Equilibrium

Equilibrium means a condition of balance. In thermodynamics the concept includes not only a balance

of forces, but also a balance of other influences. Each kind of influence refers to a particular aspect of

thermodynamic (complete) equilibrium.

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