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practical value. In many important practical cases, as in heat engines or
refrigerators, the systems consist of many subsystems at different
temperatures and pressures. In practice, thermodynamic calculations deal
effectively with these complicated dynamic systems that provide the

equilibrium thermodynamic variables are nearly enough well-defined.

Central to thermodynamic analysis are the definitions of the system, which

is of interest, and of its surroundings. The surroundings of a
thermodynamic system consist of physical devices and other
thermodynamic systems that can interact with them. 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 three fundamental kinds of physical entities in
thermodynamics, states of a system, thermodynamic processes of a system,

and thermodynamic 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 a macroscopic physical object, explicitly
specified in terms of macroscopic physical and chemical variables that

describe its macroscopic properties. The macroscopic state variables of
thermodynamics have been recognized in the course of empirical work in
physics and chemistry.
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.,
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.
Thermodynamic operations are not the only initiators of thermodynamic
processes. Of course, the changes in the intensive or extensive variables of

the surroundings can also initiate thermodynamic processes.
A thermodynamic system can also be defined in terms of the cyclic
processes that it can undergo. 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.

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