Thermodynamics is a quantitative subject. It allows us to derive relations between the values of numerous physical quantities. Some physical quantities, such as a mole fraction, are dimensionless; the value of one of these quantities is a pure number. Most quantities, however, are not dimensionless and their values must include one or more units.
This chapter reviews the SI system of units, which are the preferred units in science applications. The chapter then discusses some useful mathematical manipulations of physical quantities using quantity calculus, and certain general aspects of dimensional analysis.
There is international agreement that the units used for physical quantities in science and technology should be those of the International System of Units, or SI (standing for the French Systeme International d’Unit ` es´ ).
The Physical Chemistry Division of the International Union of Pure and Applied Chemistry, or IUPAC, produces a manual of recommended symbols and terminology for physical quantities and units based on the SI. The manual has become known as the Green Book (from the color of its cover) and is referred to here as the IUPAC Green Book.
This book will, with a few exceptions, use symbols recommended in the third edition (2007) of the IUPAC Green Book;1 these symbols are listed for convenient reference in Appendices C and D. The SI is built on the seven base units listed in Table 1.1 on the next page.
These base units are independent physical quantities that are sufficient to describe all other physical quantities. One of the seven quantities, luminous intensity, is not used in this book and is usually not needed in thermodynamics.
The official definitions of the base units are given in Appendix A. Table 1.2 lists derived units for some additional physical quantities used in thermodynamics.
The derived units have exact definitions in terms of SI base units, as given in the last column of the table.
The units listed in Table 1.3 are sometimes used in thermodynamics but are not part
of the SI. They do, however, have exact definitions in terms of SI units and so offer no
problems of numerical conversion to or from SI units.