Introduction[edit]Historically, thermodynamics arose from the
study of two distinct kinds of transfer of energy, asheatand
aswork, and the relation of those to the system's macroscopic
variables of volume, pressure and temperature.[13][14]As it
developed, thermodynamics began also to study transfers of
matter.The plain term 'thermodynamics' refers to a macroscopic
description of bodies and processes.[15]Reference to atomic
constitution is foreign to classical thermodynamics.[16]The
qualified term 'statistical thermodynamics' refers to descriptions
of bodies and processes in terms of the atomic or other microscopic
constitution of matter, using statistical and probabilistic
reasoning.Thermodynamic equilibriumis one of the most important
concepts for thermodynamics.[17]The temperature of a thermodynamic
system is well defined, and is perhaps the most characteristic
quantity of thermodynamics. As the systems and processes of
interest are taken further from thermodynamic equilibrium, their
exact thermodynamical study becomes more difficult. Relatively
simple approximate calculations, however, using the variables of
equilibrium thermodynamics, are of much practical value. Many
important practical engineering cases, as in heat engines or
refrigerators, can be approximated as systems consisting of many
subsystems at different temperatures and pressures. If a physical
process is too fast, the equilibrium thermodynamic variables, for
example temperature, may not be well enough defined to provide a
useful approximation.Central to thermodynamic analysis are the
definitions of thesystem, which is of interest, and of
itssurroundings.[8][18]The surroundings of a thermodynamic system
consist of physical devices and of other thermodynamic systems that
can interact with it. An example of a thermodynamic surrounding is
a heat bath, which is held at a prescribed temperature, regardless
of how much heat might be drawn from it.There are four fundamental
kinds of physical entities in thermodynamics,statesof a
system,wallsof a system,[19][20][21][22][23]thermodynamic
processesof a system, andthermodynamic operations. This allows two
fundamental approaches to thermodynamic reasoning, that in terms of
states of a system, and that in terms of cyclic processes of a
system.A thermodynamic system can be defined in terms of its
states. In this way, a thermodynamic system is amacroscopicphysical
object, explicitly specified in terms of macroscopic physical and
chemical variables that describe its macroscopic properties.
Themacroscopic state variablesof thermodynamics have been
recognized in the course of empirical work in physics and
chemistry.[9]Always associated with the material that constitutes a
system, its working substance, are the walls that delimit the
system, and connect it with its surroundings. The state variables
chosen for the system should be appropriate for the natures of the
walls and surroundings.[24]A thermodynamic operation is an
artificial physical manipulation that changes the definition of a
system or its surroundings. Usually it is a change of the
permeability or some other feature of a wall of the system,[25]that
allows energy (as heat or work) or matter (mass) to be exchanged
with the environment. For example, the partition between two
thermodynamic systems can be removed so as to produce a single
system. A thermodynamic operation usually leads to a thermodynamic
process of transfer of mass or energy that changes the state of the
system, and the transfer occurs in natural accord with the laws of
thermodynamics. Besides thermodynamic operations, changes in the
surroundings can also initiate thermodynamic processes.A
thermodynamic system can also be defined in terms of thecyclic
processesthat it can undergo.[26]A cyclic process is a cyclic
sequence of thermodynamic operations and processes that can be
repeated indefinitely often without changing the final state of the
system.For thermodynamics and statistical thermodynamics to apply
to a physical system, it is necessary that its internal atomic
mechanisms fall into one of two classes: those so rapid that, in
the time frame of the process of interest, the atomic states
rapidly bring system to its own state of internal thermodynamic
equilibrium; and those so slow that, in the time frame of the
process of interest, they leave the system unchanged.[27][28]The
rapid atomic mechanisms account for the internal energy of the
system. They mediate the macroscopic changes that are of interest
for thermodynamics and statistical thermodynamics, because they
quickly bring the system near enough to thermodynamic equilibrium.
"When intermediate rates are present, thermodynamics and
statistical mechanics cannot be applied."[27]Such intermediate rate
atomic processes do not bring the system near enough to
thermodynamic equilibrium in the time frame of the macroscopic
process of interest. This separation of time scales of atomic
processes is a theme that recurs throughout the subject.For
example, classical thermodynamics is characterized by its study of
materials that haveequations of state or characteristic equations.
They express equilibrium relations between macroscopic mechanical
variables and temperature and internal energy. They express the
constitutive peculiarities of the material of the system. A
classical material can usually be described by a function that
makes pressure dependent on volume and temperature, the resulting
pressure being established much more rapidly than any imposed
change of volume or temperature.[29][30][31][32]The present article
takes a gradual approach to the subject, starting with a focus on
cyclic processes and thermodynamic equilibrium, and then gradually
beginning to further consider non-equilibrium systems.Thermodynamic
facts can often be explained by viewing macroscopic objects as
assemblies of very many microscopic oratomicobjects that
obeyHamiltonian dynamics.[8][33][34]The microscopic or atomic
objects exist in species, the objects of each species being all
alike. Because of this likeness, statistical methods can be used to
account for the macroscopic properties of the thermodynamic system
in terms of the properties of the microscopic species. Such
explanation is calledstatistical thermodynamics; also often it is
referred to by the term 'statistical mechanics', though this term
can have a wider meaning, referring to 'microscopic objects', such
as economic quantities, that do not obey Hamiltonian
dynamics.[33]
