Modern Engineering Thermodynamics

38 CHAPTER 2: Thermodynamic Concepts
Table 2.1 Mass-Based and Mole-Based Specific Quantities
Mass-Based Specific Quantities
v = V/m
e = E/m
ke = KE/m = V 2 /2g c
pe = PE/m = gZ/g c
Mole-Based Specific Quantities
v = V/n
e = E/n
ke = KE/n = ðm/nÞ V 2 /2g c
pe = PE/n = ðm/nÞðgZ/g c Þ
HOW DO I DETERMINE THE STATE?
The state of a pure substance subjected to only one work mode is determined by the values of any pair of independent
intensive properties. If the pure substance is also homogeneous, then all its intensive properties are independent and any
two of them fix the state.
Exceptions to this intensive property notation are again temperature T (an intensive property), mass m (an
extensive property), and the number of moles n (another extensive property), as explained previously. Pressure,
p, isanatural intensive property that is not obtained by dividing something by the system mass.
The uppercase-lowercase notational scheme is also used for other thermodynamic quantities, such as kinetic energy,
potential energy, work, and heat, that are not thermodynamic properties. Total (mass dependent) values of these
quantities are given the uppercase symbols KE, PE, W, andQ, respectively. If we divide these quantities by the system
mass m, we get their specific (or per unit mass) forms, which are given the following lowercase symbols: ke = KE/m,
pe = PE/m, w = W/m, andq = Q/m. If we divide by the number of moles n in the system, we get the specific molar
values of these quantities, which are symbolized by lowercase letters with an overbar: ke = KE/n, pe = PE/n, w = W/n,
and q = Q/n. These are summarized in Table 2.1.
Later, we discuss a general principle that provides an easy way to determine the number of independent properties
in any system. In the meantime, you need to know that, for a pure substance (anything with a uniform
chemical composition in all its physical phases) subjected to only one work mode 3 (type of work), only two
independent intensive properties are required to determine its thermodynamic state.
A pure substance can be in any physical state—solid, liquid, vapor—or any combination of these states. Liquid
waterwithicecubesinaglassisapuresubstancesystem if the system boundary is drawn so that it does not
include the glass itself. If the system boundary is drawn outside the glass, then the system no longer contains a
pure substance (it contains water and glass). This illustrates the importance of carefully considering exactly what
the system is to be and where its boundaries are to be drawn.
2.5 THERMODYNAMIC EQUILIBRIUM
An equilibrium situation implies a condition of balance between opposing factions. There are many different kinds
of equilibria. A mechanical equilibrium exists when all the mechanical forces within a system are balanced so that
there is no acceleration (the study of mechanical equilibrium is called statics). A thermal equilibrium exists within a
system if there is a uniform temperature throughout the system. An electrostatic equilibrium exists within a system
whenthereisabalanceofchargethroughoutthesystem.Aphase equilibrium exists within a system when no
phase transformations (such as vaporization or melting) occur within the system. A system is said to be in chemical
equilibrium when no chemical reactions occur within the system. Since the subject matter of thermodynamics contains
all these types of phenomena, we lump all these definitions together to define thermodynamic equilibrium.
Classical equilibrium thermodynamics is based on the analysis of equilibrium states and therefore is analogous
to statics in mechanics. Since dynamic energy systems contain nonequilibrium thermodynamic states, they cannot be
analyzed by the methods of classical thermodynamics. Hence, the term thermodynamics appears to be a misnomer.
Some authors have proposed that classical thermodynamics could be more accurately titled thermostatics, tokeepit
consistent with the titles used in mechanics. However, the origin of the term thermodynamics is more closely aligned
with the concept of converting heat (the thermo part) into work (the dynamics part). Consequently, the dynamics in thermodynamics
should be thought of as the dynamics of the various processes of converting heat into work (or power).
3 A work mode may be mechanical, electrical, magnetic, etc., but only one may be present in this instance. More complex systems with
multiple work modes are discussed in “The State Postulate” section of Chapter 4.