Thermodynamic system

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The simplest state of a thermodynamic system is a state of thermodynamic equilibrium, as opposed to a non-equilibrium state. A system is defined as quantity of matter or a region in space chosen for study. Everything external to the system is surrounding. Thermodynamic system and surrounding is always separated by the boundary.[1]

The system can be separated from its surrounding by a wall or without a wall.

When the state of its content varies in space, the system can be considered as many systems located next to each other, each being a different thermodynamical system.

A thermodynamic system is subject to external interventions called thermodynamic operations; these alter the system's walls or its surroundings; as a result, the system undergoes thermodynamic processes according to the principles of thermodynamics. (This account mainly refers to the simplest kind of thermodynamic system; compositions of simple systems may also be considered.)

The thermodynamic state of a thermodynamic system is its internal state as specified by its state variables. In addition to the state variables, a thermodynamic account also requires a special kind of quantity called a state function, which is a function of the defining state variables. For example, if the state variables are internal energy, volume and mole amounts, that special function is the entropy. These quantities are inter-related by one or more functional relationships called equations of state, and by the system's characteristic equation. Thermodynamics imposes restrictions on the possible equations of state and on the characteristic equation. The restrictions are imposed by the laws of thermodynamics.

According to the permeabilities of the walls of a system, transfers of energy and matter occur between it and its surroundings, which are assumed to be unchanging over time, until a state of thermodynamic equilibrium is attained. The only states considered in equilibrium thermodynamics are equilibrium states. Classical thermodynamics includes equilibrium thermodynamics. It also considers: (a) systems considered in terms of cyclic sequences of processes rather than of states of the system; such were historically important in the conceptual development of the subject; and (b) systems considered in terms of processes described by steady flows; such are important in engineering.

In 1824 Sadi Carnot described a thermodynamic system as the working substance (such as the volume of steam) of any heat engine under study. The very existence of such thermodynamic systems may be considered a fundamental postulate of equilibrium thermodynamics, though it is only rarely cited as a numbered law.[2][3][4] According to Bailyn, the commonly rehearsed statement of the zeroth law of thermodynamics is a consequence of this fundamental postulate.[5]

In equilibrium thermodynamics the state variables do not include fluxes because in a state of thermodynamic equilibrium all fluxes have zero values by postulation. Equilibrium thermodynamic processes may involve fluxes but these must have ceased by the time a thermodynamic process or operation is complete bringing a system to its eventual thermodynamic state. Non-equilibrium thermodynamics allows its state variables to include non-zero fluxes, that describe transfers of mass or energy or entropy between a system and its surroundings.[6]

System boundary2.svg


Thermodynamic equilibrium is characterized by absence of flow of mass or energy. Equilibrium thermodynamics, as a subject in physics, considers macroscopic bodies of matter and energy in states of internal thermodynamic equilibrium. It uses the concept of thermodynamic processes, by which bodies pass from one equilibrium state to another by transfer of matter and energy between them. The term 'thermodynamic system' is used to refer to bodies of matter and energy in the special context of thermodynamics. The possible equilibria between bodies are determined by the physical properties of the walls that separate the bodies. Equilibrium thermodynamics in general does not measure time. Equilibrium thermodynamics is a relatively simple and well settled subject. One reason for this is the existence of a well defined physical quantity called 'the entropy of a body'.

Non-equilibrium thermodynamics, as a subject in physics, considers bodies of matter and energy that are not in states of internal thermodynamic equilibrium, but are usually participating in processes of transfer that are slow enough to allow description in terms of quantities that are closely related to thermodynamic state variables. It is characterized by presence of flows of matter and energy. For this topic, very often the bodies considered have smooth spatial inhomogeneities, so that spatial gradients, for example a temperature gradient, are well enough defined. Thus the description of non-equilibrium thermodynamic systems is a field theory, more complicated than the theory of equilibrium thermodynamics. Non-equilibrium thermodynamics is a growing subject, not an established edifice. In general, it is not possible to find an exactly defined entropy for non-equilibrium problems. For many non-equilibrium thermodynamical problems, an approximately defined quantity called 'time rate of entropy production' is very useful. Non-equilibrium thermodynamics is mostly beyond the scope of the present article.

Another kind of thermodynamic system is considered in engineering. It takes part in a flow process. The account is in terms that approximate, well enough in practice in many cases, equilibrium thermodynamical concepts. This is mostly beyond the scope of the present article, and is set out in other articles, for example the article Flow process.






Systems in equilibrium[edit]





Types of transfers permitted by types of wall
type of wall type of transfer
Matter Work Heat
permeable to matter Green tickY Red XN Red XN
permeable to energy but

impermeable to matter

Red XN Green tickY Green tickY
adiabatic Red XN Green tickY Red XN
adynamic and

impermeable to matter

Red XN Red XN Green tickY
isolating Red XN Red XN Red XN

A system is enclosed by walls that bound it and connect it to its surroundings.[7][8][9][10][11][12] Often a wall restricts passage across it by some form of matter or energy, making the connection indirect. Sometimes a wall is no more than an imaginary two-dimensional closed surface through which the connection to the surroundings is direct.


The system is delimited by walls or boundaries, either actual or notional, across which conserved (such as matter and energy) or unconserved (such as entropy) quantities can pass into and out of the system. The space outside the thermodynamic system is known as the surroundings, a reservoir, or the environment. The properties of the walls determine what transfers can occur. A wall that allows transfer of a quantity is said to be permeable to it, and a thermodynamic system is classified by the permeabilities of its several walls. A transfer between system and surroundings can arise by contact, such as conduction of heat, or by long-range forces such as an electric field in the surroundings.


The walls of a closed system allow transfer of energy as heat and as work, but not of matter, between it and its surroundings. The walls of an open system allow transfer both of matter and of energy.[13][14][15][16][17][18][19] This scheme of definition of terms is not uniformly used, though it is convenient for some purposes. In particular, some writers use 'closed system' where 'isolated system' is here used.[20][21]




Closed system[edit]





If the work is due to a volume expansion by dV at a pressure P then:

For a homogeneous system undergoing a reversible process, the second law of thermodynamics reads:

where T is the absolute temperature and S is the entropy of the system. With these relations the fundamental thermodynamic relation, used to compute changes in internal energy, is expressed as:



Isolated system[edit]


Truly isolated physical systems do not exist in reality (except perhaps for the universe as a whole), because, for example, there is always gravity between a system with mass and masses elsewhere.[22][23][24][25][26] However, real systems may behave nearly as an isolated system for finite (possibly very long) times. The concept of an isolated system can serve as a useful model approximating many real-world situations. It is an acceptable idealization used in constructing mathematical models of certain natural phenomena.





Selective transfer of matter[edit]






Open system[edit]

In an open system, there is an exchange of energy and matter between the system and the surroundings.The presence of reactants in an open beaker is an example of an open system.Here the boundaray is an imaginary surface enclosing the beaker and reactants. It is named closed, if borders are impenetrable for substance, but allow transit of energy in the form of heat, and isolated, if there is no exchange of heat and substances. The open system cannot exist in the equilibrium state. To describe deviation of the thermodynamic system from equilibrium, in addition to constitutive variables that was described above, a set of internal variables that are called internal variables have been introduced. The equilibrium state is considered to be stable. and the main property of the internal variables, as measures of non-equilibrium of the system, is their trending to disappear; the local law of disappearing can be written as relaxation equation for each internal variable






where is a relaxation time of a corresponding variables. It is convenient to consider the initial value are equal to zero.

The essential contribution to the thermodynamics of the open non-equilibrium systems was brought by Prigogine, when he and his collaborators investigated the systems of chemically reacting substances. The stationary states of such systems exists due to exchange both particles and energy with the environment. In section 8 of the third chapter of his book [28], Prigogine has specified three contributions to the variation of entropy of the considered open system at the given volume and constant temperature . The increment of entropy can be calculated according to the formula






The first term on the right hand side of the equation presents a stream of thermal energy into the system; the last term -- a stream of energy into the system coming with the stream of particles of substances that can be positive or negative, is chemical potential of substance . The middle term in (1) depicts energy dissipation (entropy production) due to the relaxation of internal variables . In the case of chemically reacting substances, which was investigated by Prigogine, the internal variables appear to be measures of incompleteness of chemical reactions, that is measures of how much the considered system with chemical reactions is out of equilibrium. The theory can be generalized[29][30], to consider any deviation from the equilibrium state as an internal variable, so that we consider the set of internal variables in equation (1) to consist of the quantities defining not only degrees of completeness of all chemical reactions occurring in the system, but also the structure of the system, gradients of temperature, difference of concentrations of substances and so on.

The Prigogine approach to the open system allow describing the growth and development of living odjects in thermodynamic terms.

Adiabatic system[edit]

An adiabatic system is the one which doesn't allow any heat to be transferred into or out of the system. The equation is only valid for an adiabatic system which is also undergoing a reversible process provided it is a closed system having an ideal gas. If it fails to satisfy any of these conditions then only is true and it cannot be represented in an equation like .

See also[edit]


  1. ^ What is system, surrounding and boundary? "Mechanical Engineering Discussion Forum"
  4. ^ Marsland, R. III, Brown, H.R., Valente, G. (2015). Time and irreversibility in axiomatic thermodynamics, Am. J. Phys., 83(7): 628–634.
  8. ^ Tisza, L. (1966), pp. 109, 112.
  9. ^ Haase, R. (1971), p. 7.
  10. ^ Adkins, C.J. (1968/1975), p. 4
  12. ^ Tschoegl, N.W. (2000), p. 5.
  13. ^ Prigogine, I., Defay, R. (1950/1954). Chemical Thermodynamics, Longmans, Green & Co, London, p. 66.
  14. ^ Tisza, L. (1966). Generalized Thermodynamics, M.I.T Press, Cambridge MA, pp. 112–113.
  15. ^ Guggenheim, E.A. (1949/1967). Thermodynamics. An Advanced Treatment for Chemists and Physicists, (1st edition 1949) 5th edition 1967, North-Holland, Amsterdam, p. 14.
  16. ^ Münster, A. (1970). Classical Thermodynamics, translated by E.S. Halberstadt, Wiley–Interscience, London, pp. 6–7.
  19. ^ Silbey, R.J., Alberty, R.A., Bawendi, M.G. (1955/2005). Physical Chemistry, fourth edition, Wiley, Hoboken NJ, p. 4.
  21. ^ ter Haar, D., Wergeland, H. (1966). Elements of Thermodynamics, Addison-Wesley Publishing, Reading MA, p. 43.
  22. ^ I.M.Kolesnikov; V.A.Vinokurov; S.I.Kolesnikov (2001). Thermodynamics of Spontaneous and Non-Spontaneous Processes. Nova science Publishers. p. 136. ISBN 978-1-56072-904-4.
  23. ^ "A System and Its Surroundings". ChemWiki. University of California - Davis. Retrieved 9 May 2012.
  24. ^ "Hyperphysics". The Department of Physics and Astronomy of Georgia State University. Retrieved 9 May 2012.
  25. ^ Bryan Sanctuary. "Open, Closed and Isolated Systems in Physical Chemistry". Foundations of Quantum Mechanics and Physical Chemistry. McGill University (Montreal). Retrieved 9 May 2012.
  26. ^ Material and Energy Balances for Engineers and Environmentalists (PDF). Imperial College Press. p. 7. Archived from the original (PDF) on 15 August 2009. Retrieved 9 May 2012.
  28. ^ Prigogine, I. (1955/1961/1967). Introduction to Thermodynamics of Irreversible Processes. 3rd edition, Wiley Interscience, New York.
  29. ^ Pokrovskii V.N. (2005) Extended thermodynamics in a discrete-system approach, Eur. J. Phys. vol. 26, 769-781.
  30. ^ Pokrovskii V.N. (2013) A derivation of the main relations of non-equilibrium thermodynamics. Hindawi Publishing Corporation: ISRN Thermodynamics, vol. 2013, article ID 906136, 9 p.


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