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thermodynamics Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of ther ...
, a diathermal wall between two
thermodynamic system A thermodynamic system is a body of matter and/or radiation, confined in space by walls, with defined permeabilities, which separate it from its surroundings. The surroundings may include other thermodynamic systems, or physical systems that are ...
s allows
heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
but does not allow transfer of
matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic parti ...
across it. The diathermal wall is important because, in thermodynamics, it is customary to assume ''a priori'', for a
closed system A closed system is a natural physical system that does not allow transfer of matter in or out of the system, although — in contexts such as physics, chemistry or engineering — the transfer of energy (''e.g.'' as work or heat) is allowed. In ...
, the physical existence of transfer of energy across a wall that is impermeable to matter but is not adiabatic, transfer which is called transfer of energy as heat, though it is not customary to label this assumption separately as an axiom or numbered law.


Definitions of transfer of heat

In theoretical thermodynamics, respected authors vary in their approaches to the definition of quantity of heat transferred. There are two main streams of thinking. One is from a primarily empirical viewpoint (which will here be referred to as the thermodynamic stream), to define heat transfer as occurring only by specified
macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifying optical instruments. It is the opposite of microscopic. Overview When applied to physical phenomena a ...
mechanisms; loosely speaking, this approach is historically older. The other (which will here be referred to as the mechanical stream) is from a primarily theoretical viewpoint, to define it as a residual quantity calculated after transfers of energy as macroscopic work, between two bodies or closed systems, have been determined for a process, so as to conform with the principle of conservation of energy or the first law of thermodynamics for closed systems; this approach grew in the twentieth century, though was partly manifest in the nineteenth.


Thermodynamic stream of thinking

In the thermodynamic stream of thinking, the specified mechanisms of heat transfer are
conduction Conductor or conduction may refer to: Music * Conductor (music), a person who leads a musical ensemble, such as an orchestra. * ''Conductor'' (album), an album by indie rock band The Comas * Conduction, a type of structured free improvisation ...
and
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
. These mechanisms presuppose recognition of
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...
; empirical temperature is enough for this purpose, though absolute temperature can also serve. In this stream of thinking, quantity of heat is defined primarily through
calorimetry In chemistry and thermodynamics, calorimetry () is the science or act of measuring changes in '' state variables'' of a body for the purpose of deriving the heat transfer associated with changes of its state due, for example, to chemical react ...
. Though its definition of them differs from that of the mechanical stream of thinking, the empirical stream of thinking nevertheless presupposes the existence of adiabatic enclosures. It defines them through the concepts of heat and temperature. These two concepts are coordinately coherent in the sense that they arise jointly in the description of experiments of transfer of energy as heat.


Mechanical stream of thinking

In the mechanical stream of thinking about closed systems, heat transferred is defined as a calculated residual amount of energy transferred after the energy transferred as work has been determined, assuming for the calculation the law of conservation of energy, without reference to the concept of temperature.Bryan, G.H. (1907), p. 47.Carathéodory, C. (1909).Buchdahl, H.A. (1966), p. 43. There are five main elements of the underlying theory. *The existence of states of thermodynamic equilibrium, determinable by precisely one (called the non-deformation variable) more variable of state than the number of independent work (deformation) variables. *That a state of internal thermodynamic equilibrium of a body have a well defined internal energy, that is postulated by the first law of thermodynamics. *The universality of the law of conservation of energy. *The recognition of work as a form of energy transfer. *The universal irreversibility of natural processes. *The existence of adiabatic enclosures. *The existence of walls permeable only to heat. Axiomatic presentations of this stream of thinking vary slightly, but they intend to avoid the notions of heat and of temperature in their axioms. It is essential to this stream of thinking that heat is not presupposed as being measurable by calorimetry. It is essential to this stream of thinking that, for the specification of the thermodynamic state of a body or closed system, in addition to the variables of state called deformation variables, there be precisely one extra real-number-valued variable of state, called the non-deformation variable, though it should not be axiomatically recognized as an empirical temperature, even though it satisfies the criteria for one.


Accounts of the diathermal wall

As mentioned above, a diathermal wall may pass energy as heat by thermal conduction, but not the matter. A diathermal wall can move and thus be a part of a transfer of energy as work. Amongst walls that are impermeable to matter, diathermal and adiabatic walls are contraries. For radiation, some further comments may be useful. In classical thermodynamics, one-way radiation, from one system to another, is not considered. Two-way radiation between two systems is one of the two mechanisms of transfer of energy as heat. It may occur across a vacuum, with the two systems separated from the intervening vacuum by walls that are permeable only to radiation; such an arrangement fits the definition of a diathermal wall. The balance of radiative transfer is transfer of heat. In thermodynamics, it is not necessary that the radiative transfer of heat be of pure black-body radiation, nor of incoherent radiation. Of course black-body radiation is incoherent. Thus laser radiation counts in thermodynamics as a one-way component of two-way radiation that is heat transfer. Also, by the elmholtz reciprocityprinciple, the target system radiates into the laser source system, though of course relatively weakly compared with the laser light. According to Planck, an incoherent monochromatic beam of light transfers entropy and has a temperature. Planck. M. (1914), Chapter . For a transfer to qualify as work, it must be reversible in the surroundings, for example in the concept of a reversible work reservoir. Laser light is not reversible in the surroundings and is therefore a component of transfer of energy as heat, not work. In radiative transfer theory, one-way radiation is considered. For investigation of
Kirchhoff's law of thermal radiation In heat transfer, Kirchhoff's law of thermal radiation refers to wavelength-specific radiative emission and absorption by a material body in thermodynamic equilibrium, including radiative exchange equilibrium. It is a special case of Onsage ...
the notions of absorptivity and
emissivity The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that most commonly includes both visible radiation (light) and infrared radiation, which is n ...
are necessary, and they rest on the idea of one-way radiation. These things are important for the study of the Einstein coefficients, which relies partly on the notion of
thermodynamic equilibrium Thermodynamic equilibrium is an axiomatic concept of thermodynamics. It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by more or less permeable or impermeable walls. In ther ...
. For the thermodynamic stream of thinking, the notion of empirical temperature is coordinately presupposed in the notion of heat transfer for the definition of an adiabatic wall.Planck. M. (1897/1903). For the mechanical stream of thinking, the exact way in which the walls are defined is important. In the presentation of Carathéodory, it is essential that the definition of the adiabatic wall should in no way depend upon the notions of heat or temperature. This is achieved by careful wording and reference to transfer of energy only as work. Buchdahl is careful in the same way. Nevertheless, Carathéodory explicitly postulates the existence of walls that are permeable only to heat, that is to say impermeable to work and to matter, but still permeable to energy in some unspecified way; they are called diathermal walls. One might be forgiven for inferring from this that heat is energy in transfer across walls permeable only to heat, and that such are admitted to exist unlabeled as postulated primitives. The mechanical stream of thinking thus regards the adiabatic enclosure's property of not allowing the transfer of heat across itself as a deduction from the Carathéodory axioms of thermodynamics, and regards transfer as heat as a residual rather than a primary concept.


References


Bibliography

*Bailyn, M. (1994). ''A Survey of Thermodynamics'', American Institute of Physics Press, New York, . *Beattie, J.A., Oppenheim, I. (1979). ''Principles of Thermodynamics'', Elsevier, Amsterdam, . * Born, M. (1921). Kritische Betrachtungen zur traditionellen Darstellung der Thermodynamik, ''Physik. Zeitschr.'' 22: 218–224. * Bryan, G.H. (1907). ''Thermodynamics. An Introductory Treatise dealing mainly with First Principles and their Direct Applications'', B.G. Teubner, Leipzig. *Buchdahl, H.A. (1957/1966). ''The Concepts of Classical Thermodynamics'', Cambridge University Press, London. * A translation may be foun
here
A partly reliable translation is to be found at Kestin, J. (1976). ''The Second Law of Thermodynamics'', Dowden, Hutchinson & Ross, Stroudsburg PA. * *Haase, R. (1971). Survey of Fundamental Laws, chapter 1 of ''Thermodynamics'', pages 1–97 of volume 1, ed. W. Jost, of ''Physical Chemistry. An Advanced Treatise'', ed. H. Eyring, D. Henderson, W. Jost, Academic Press, New York, lcn 73–117081. * Kirkwood, J.G., Oppenheim, I. (1961). ''Chemical Thermodynamics'', McGraw–Hill, New York. * * * Planck. M. (1914)
''The Theory of Heat Radiation''
a translation by Masius, M. of the second German edition, P. Blakiston's Son & Co., Philadelphia. {{refend Thermodynamics