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In terms, state (TR:672) (LH:11) (TL:683) is an ensemble of physical quantities, such as temperature, pressure, composition, etc., which characterize the system, but neither by its surroundings nor by its history.[1]


In 1850, Rudolf Clausius, in his “On the Moving Force of Heat and the Laws of Heat which may be Deduced Therefrom”, building on William Thomson, who in turn built on Sadi Carnot, began to refer to the working body (or working substance), e.g. the water in the piston and cylinder, of the heat engine, as being in different "states", e.g. liquid state or gas state, among other specifics; the first use of the term "state" is as follows:

“Thomson mentions distinctly the obstacles which lie in the way of an unconditional acceptance of Carnot's theory, referring particularly to the investigations of Joule, and dwelling on one principal objection to which the theory is liable. If it be even granted that the production of work, where the body in action remains in the same state after the production as before, is in all cases accompanied by a transmission of heat from a warm body to a cold one, it does not follow that by every such transmission work is produced, for the heat may be carried over by simple conduction; and in all such cases, if the transmission alone were the true equivalent of the work performed, an absolute loss of mechanical force must take place in nature, which is hardly conceivable.”
— Rudolf Clausius (1850), “On the Moving Force of Heat and the Laws of Heat which may be Deduced Therefrom” (pgs. 16-17)[2]

In 1851, William Thomson, building on Clausius, commented the following about the internal energy U of a body:

“U is the mechanical energy of a body in a given state.”
William Thomson (1851), “Article” [3]

In the decades to follow, it was worked out that the state of each body or system, is quantified by a thermodynamic potential, e.g. internal energy U, Helmholtz energy H, or Gibbs energy G, etc. depending on the constraints of the system, e.g. whether it is a constant pressure or a constant volume system, etc.

End matter

See also


  1. Perrot, Pierre. (1998). A to Z of Thermodynamics. Oxford.
  2. Clausius, Rudolf. (1865). The Mechanical Theory of Heat (translator: Thomas Hirst) (§1:14-89). Macmillan, 1867.
  3. (a) Thomson, William. (1851). “Article”, Transactions of the Royal Society of Edinburgh, 20:475.
    (b) Clausius, Rudolf. (1875). The Mechanical Theory of Heat (translator: Walter Browne) (pg. 31). Macmillan, 1879.

External links

  • State – Hmolpedia 2020.