The Thermodynamics reference article from the English Wikipedia on 24-Apr-2004
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Thermodynamics is the study of energy, its conversions between various forms, the ability of energy to do work, and the spontaneity of processes. It is closely related to statistical mechanics from which many thermodynamic relationships can be derived.

While dealing with processes in which systems exchange matter or energy, equilibrium thermodynamics is not concerned with the rate at which such processes take place. In this connection, a central concept in thermodynamics is that of a quasistatic process, which are idealized, "infinitely slow" processes. Time-dependent thermodynamic processes are studied by non-equilibrium thermodynamics.

Because it is not concerned with the concept of time, it has been suggested that a better name for equilibrium thermodynamics would have been thermostatics.

Thermodynamic laws are of very general validity, and they do not depend on the details of the interactions or the systems being studied. This means they can be applied to systems about which one knows nothing other than the balance of energy and matter transfer between them and the environment. Examples of this include Einstein's prediction of spontaneous emission around the turn of the 20th century and the current research into the thermodynamics of black holes.

Table of contents
1 The basic concepts of Thermodynamics
2 The Laws of Thermodynamics
3 Basics
4 Thermodynamic Systems
5 Thermodynamic State
6 Units
7 See also

The basic concepts of Thermodynamics

The basic abstraction of thermodynamics is the division of the world into systems delimited by real or ideal boundaries. The systems not directly under consideration are lumped into the environment. It is possible to subdivide a system into subsystems, or to group several systems together into a larger system.

There are three kinds of systems depending on the kinds of exchanges taking place between a system and its environment:

The Laws of Thermodynamics

Alternative statements can be given for each law which are mathematically equivalent.

If A and B are in thermodynamic equilibrium, and B and C are in thermodynamic equilibrium, then A and C are also in thermodynamic equilibrium.

While this is a fundamental concept of thermodynamics, the need to state it explicitly as a law was not perceived until the first third of the 20th century, long after the first three laws were already widely in use. Hence the zero numbering. There is still some discussion about its status.

Thermodynamic equilibrium includes thermal equilibrium (associated to heat exchange and parameterized by temperature), mechanical equilibrium (associated to work exchange and parameterized generalized forces such as pressure), and chemical equilibrium (associated to matter exchange and parameterized by chemical potential).

The work exchanged in an adiabatic process depends only on the initial and the final state and not on the details of the process.


The heat flowing into a system equals the increase in internal energy of the system minus the work done by the system.

It is impossible to obtain a process such that the unique effect is the subtraction of a positive heat from a reservoir and the production of a positive work.


A system operating in contact with a thermal reservoir cannot produce positive work in its surroundings (Lord Kelvin)


A system operating in a cycle cannot produce a positive heat flow from a colder body to a hotter body (Clausius)

The entropy of a closed system never decreases (see Maxwell's demon)

All processes cease as temperature approaches zero.

As temperature goes to 0, the entropy of a system approaches a constant

These laws have been humorously summarised as Ginsberg's theorem: (1) you can't win, (2) you can't break even, and (3) you can't get out of the game.

Or, alternatively: (1) you can't get anything without working for it, (2) the most you can accomplish by working is to break even, and (3) you can only break even at absolute zero.


The following is a list of the major concepts in thermodynamics, together with the algebraic symbols used to represent them.

The rest of this discussion is about reversible transformation of systems in equilibrium. For irreversible processes or systems out of equilibrium, see nonequilibrium thermodynamics.

Substances describable by temperature alone

Blackbody radiation is an example, since photon number is not conserved. Such a state is completely described by its temperature, although if phase transitions or spontaneous symmetry breaking occur other variables may be needed to discriminate among the phases. (This problem does not arise for blackbody radiation.) Given the internal energy as a function of temperature, we can define F=U-TS.

Substances describable by temperature and pressure alone

Most "pure" nonmagnetic substances fall into this category. This state is completely described by its temperature and pressure, except at phase transitions and perhaps spontaneous symmetry breaking in the ordered phase. Given U and V (or the density ρ) as a function of T and P, we can define the Helmholtz energy as before and the Gibbs energy as G=U-TS+PV and the enthalpy as H=U+PV.

Substances describable by temperature, pressure and chemical potential

If there are more than one kind of atom/molecule, a substance would fall into this category. This state is completely described by its temperature, pressure and chemical potentials, except at phase transitions and perhaps spontaneous symmetry breaking in the ordered phase.

Substances describable by temperature and magnetic field

If a substance is a ferromagnet or a superconductor, for example, it would fall into this category. It is completely described by its temperature and magnetic field, except at phase transitions and perhaps spontaneous symmetry breaking in the ordered phase.

Thermodynamic Systems

A thermodynamic system is that part of the universe that is under consideration. A real or imaginary boundary separates the system from the rest of the universe, which is referred to as the surroundings. Often thermodynamic systems are characterized by the nature of this boundary as follows:

Thermodynamic State

A key concept in thermodynamics is the state of a system. When a system is at equilibrium under a given set of conditions, it is said to be in a definite state. For a given thermodynamic state, many of the system's properties have a specific value corresponding to that state. The values of these properties are a function of the state of the system and are independent of the path by which the system arrived at that state. The number of properties that must be specified to describe the state of a given system is given by Gibbs phase rule. Since the state can be described by specifying a small number of properties, while the values of many properties are determined by the state of the system, it is possible to develop relationships between the various state properties. One of the main goals of Thermodynamics is to understand these relationships between the various state properties of a system. Equations of state are examples of some of these relationships.

See also: thermodynamic properties

Thermodynamics also touches upon the fields of:


"In this house, we OBEY the LAWS of THERMODYNAMICS!" &mdash Homer Simpson


See also

General subfields within physics
Classical mechanics | Condensed matter physics | Electromagnetism | Field theory (physics) | Continuum mechanics | General relativity | Particle physics
Quantum mechanics | Quantum field theory | Solid state physics | Electronic Structure of Materials | Special relativity | Standard Model | Statistical mechanics | Thermodynamics