Talk:Heat

Another edit that puts heat as a quantity of energy in transfer from one thermodynamic system to another is Heat: Difference between revisions - Wikipedia with the edit cover note "revert this misleading lede".

Some engineering textbooks (and, I seem to recall, some thermodynamics texts?) define heat as proposed in the edit.

Some texts regard convection as a mode of heat transfer, but most thermodynamics texts follow Maxwell and think of convection as a mode of transport of matter along with its internal energy, not as a mode of heat transfer. I don't have easy access to the 1978 edition of van Wylen (which is written from an engineering perspective) that is cited in the edit, but I do have access to its 2009 seventh edition, which uses the same heat definition and regards convection as a mode of heat transport/transfer because it depends upon temperature difference.

The edit proposes to change from the traditional Wikipedia thermodynamical definition which was developed over a decade or so, through extensive talk page debate and editorial consensus. It intended to make the definition suitable for a rigorously logical presentation of thermodynamics in Wikipedia. It was constructed with the following several ideas in mind.

The traditional Wikipedia definition was in terms of transfer of energy between a thermodynamic system and its surroundings. This differs from the edit's proposed definition because a thermodynamic system has a temperature, but its surroundings do so only if they also consist of another thermodynamic system that has a temperature. The source of thermal radiation from the surroundings is not required to possess a temperature, and is consequently not required to be thermodynamic system, because it is allowed to be far from thermodynamic equilibrium.

A reason for the traditional Wikipedia definition is to avoid possible circularity of definition. This entails that Wikipedia should state the first law without logical reliance on a prior statement of the second law, and without a prior definition of temperature, along with the idea that thermodynamics defines temperature, as Kelvin did, through the second law. An objection to this idea is that it is fine to make the statement of the first law depend logically on the statement of the second law and on the definition of temperature. Another objection to this idea is that, nowadays, 'temperature' is officially defined through what thermodynamics calls 'empirical temperature', not through the thermodynamic definition of temperature. Eventually, for rigorous logical consistency and validity, thermodynamics has to use the thermodynamic definition. The official empirical definition is practically close to the thermodynamic definition but it is not exactly the same.

The edit's proposed definition excludes friction as a source of heat. This is fine for the caloric theory of heat, but it hardly recognises that the caloric theory was abandoned in the first half of the nineteenth century largely because it came to be recognised that friction generates heat. This is particularly recognised in writings of Planck that have mostly not been translated into English.

Considering friction as a source of heat: The friction can be internal to the thermodynamic system of interest, as in the experiments of Mayer, and of Joule, that led Helmholtz in 1847 to enunciate his version of the principle of conservation of energy. Or the friction can be jointly between the system of interest and its surroundings, as in the 1798 cannon boring experiments of Count Rumford, or as in dragging a rough block over a rough surface; some of the heat generated will go into the thermodynamic system, and the rest into the surroundings. The first law separates these two quantities of energy through the notion of internal energy, which is an essential part of the first law. The internal energy of a thermodynamic system is defined through the concept of thermodynamic work, which is defined through the thermodynamic system of interest's state variables other than temperature and entropy. In some cases, thermodynamic work can be practically equated to ordinary physical work defined by processes external to the thermodynamic system.

The upshot is that the traditional Wikipedia definition was by exclusion of transfer of energy through thermodynamic work and through transfer of matter. This is perhaps a strained idea, but it has the advantage of strict logical validity for the presentation of thermodynamics: for a closed system, transfer of energy is either as heat or as thermodynamic work. So far as is obvious, the only modes of energy transfer that conform to this requirement for heat are thermal conduction, thermal radiation, and friction. Perhaps there are others?

The definition of a term is distinct from an explanation of it, and from examples of its use.

A prime relevant reference, for a closed system, is
F. Reif, Fundamentals of statistical and thermal physics, McGraw–Hill, New York, 1965, page 73:
"In short, (2 8-1) defines the quantity Q by the relation
                                            Q = ΔE − *W = ΔE + W                                     (2 8-2)
where W = − *W is the work done by the system. The relation (2 8-2) constitutes the general definition of the heat absorbed by a system. When the external parameters are kept fixed, (2 8-2) reduces, of course, to the definition already introduced in Sec. 2 • 6 for the case of purely thermal interaction. The relation (2 8-1) simply splits the total mean energy change into a part W due to mechanical interaction and a part Q due to thermal interaction. One of the fundamental aims of our study will be to gain a better understanding of the relationship between thermal and mechanical interactions. This is the reason for the name "thermodynamics" applied to the classical discipline dealing with such questions."

Another reference for a closed system is
H.B. Callen (1985), Thermodynamics and an Introduction to Thermostatistics, 2nd edition, John Wiley, New York, pages 19–20:
"The heat flux to a system in any process (at constant mole numbers) is simply the difference in internal energy between the final and initial states, diminished by the work done in that process."Chjoaygame (talk) 08:31, 24 November 2025 (UTC)

There is no "traditional Wikipedia thermodynamical definition which was developed over a decade". Wikipedia does not develop definitions or does not engage in any kind of other creative processes. Yet this has been the practice of few editors such as the OP who insist on confusing and widely entangled subject matters, which is totally unnecessary. Wikipedia follows established science and uses existing notable sources to present the facts.

The subject matter is not difficult, but editors here engage in confusing arguments and personal views.

To state it once more. Heat is described differently in the varied fields of applications and environments. The term heat transfer is a term of engineering, while physics uses a much stricter definition for just heat. In physics heat is always a transfer of energy. That's the reason for having articles for each terminology. The general public and its history has yet a different scope of usage. There is nothing wrong with explaining all, but it needs to be clear in every treatment which scope of usage is relevant or desired. kbrose (talk) 20:11, 26 November 2025 (UTC)

The essential question is not 'whether 'heat' or 'heat transfer' is the natural term, but rather 'what kind of transfer is heat'?

The two candidate kinds are:

• transfer of a conserved quantity
  

The kind that is supported by the new edit is 'transfer of a conserved quantity'. This is how calorimetry works. It proposes that 'heat is conserved because the heat gained by the colder body is equal to the heat lost by the hotter body'. Coordinate with this is the proposition is 'the work expended by the driver in the surroundings is equal to the thermodynamic work received by the system; in this sense, work is conserved'. These are historically supported suppositions. They are convincing for suitably isolated thermal conduction, through contact between the bodies. For radiative heat transfer, it is sometimes but not always possible to restrict the radiative loss of the emitting body to be equal to the radiative gain of the receiving body. But there are kinds of heat transfer that are not covered by these historically supported suppositions.

• transfer defined by its mechanism of process
  

Twentieth century reliable sources see things differently. In the middle of the nineteenth century thermodynamics distinguished heat from work in a systematic way. For example, Bryan (1907) ^[1] and Bridgman (1941)^[2] analysed the effect of friction, as follows. Ordinary physical work can be reliably measured in the surroundings, without recourse to the state variables of the system. In contrast, thermodynamic work is measured by the state variables of the system other than temperature and entropy, excluding transfer of matter. The relevant changes in the state variables exclude those due to transfer of matter, because the associated energy cannot be split reliably into work and heat. Because of friction, the two kinds of work (ordinary physical and thermodynamic) in a process are not equal, so that "work" cannot be treated as a conserved quantity. For example, Bridgman (1941) wrote "the work received by the block from the pavement… is not equal to the work done on the pavement by the block." When friction intervenes, the process generates heat, which is consequently also not conserved. In physical terms, friction dissipates energy from ordinary physical work by generating heat.

The definition proposed in the new edit is that of the caloric theory, while the thermodynamic definition takes into account that the mechanism of heat transfer includes changes in the form of the energy in the transfer process. For a process with matter transfer excluded, the assumption that internal energy is a state variable effectively makes the first law of thermodynamics into a definition of heat. This mode of distinction between work and heat is central to the logic of thermodynamics, because it allows entropy to be defined consistently.Chjoaygame (talk) 05:50, 27 November 2025 (UTC)

In the middle of the nineteenth century, thermodynamics emerged by evolution from the caloric theory of heat.

The caloric theory was largely the work of two great physicist–chemists, Laplace and Lavoisier. It was based on thorough and careful experiments in calorimetry, heat being accurately conserved when it was measured by passing it by thermal conduction from a hotter body to a colder one. As defined by calorimetry, the heat lost by the hotter body was equal to that gained by the colder body. Conservation of heat was also accounted for in the light of the difference between latent heat, which was hidden from the thermometer, and sensible heat, which was evident to the thermometer, as elucidated by the accurate calorimetric measurements of another great physicist–chemist, Joseph Black. The caloric theory of the conservation of heat persisted into the first half of the nineteenth century, as supported by Sadi Carnot, whose heat-engine studies contributed much to the discovery of the second law of thermodynamics.

At the end of the eighteenth century, however, Benjamin Thompson, Count Rumford, in his cannon boring experiments, moved physics forward by showing that heat could be endlessly generated by friction. After all, heat was not conserved. In the first half of the nineteenth century, Robert von Mayer showed that heat could be generated by friction also in the stirring of a very viscous liquid. Near the middle of the nineteenth century, James Joule did more extensive and precisely accurate experiments on the production of heat by friction, in a liquid stirred by a paddle wheel, and in an electrical conductor, through the friction of the passage of an electrical current.

In the middle of the nineteenth century, the work of Helmholtz, Rankine, Clausius, and Kelvin created the new theory of thermodynamics.

The new theory created the idea that the internal energy of a body could be changed and measured purely through energy transfers that avoided thermal conduction, radiative transfer, and friction. On the other hand, thermal conduction transferred heat through a conductive medium, thermal radiation transferred heat without a conductive medium, and friction generated heat from externally supplied mechanical energy. The first law of thermodynamics was established on this basis. Calorimetry remained useful, but not necessary, for the estimation of internal energy. But the caloric theory was abandoned.

With the avoidance of thermal conduction, radiative transfer, and friction, the new theory determined the internal energy of a body purely through its ‘macroscopic’, or ‘external’ quantities or 'parameters', such as volume and electric polarisation. Preventing microscopic energy transfers to a body leaves the only transfers as the macroscopic ones, of thermodynamic work, defined by the external quantities, and of matter with its associated energy.

Thermal conduction, radiative transfer, and friction, are ‘microscopic’ mechanisms. Temperature is a purely ‘internal’, ‘microscopic’, or ‘intensive’ property of a body. It has a thermodynamically conjugate ‘extensive’ property, entropy. Chjoaygame (talk) 03:31, 25 November 2025 (UTC)

The first version of the Wikipedia article contained the following:

"Conduction is the most common means of heat transfer in a solid. On a microscopic scale, conduction occurs as hot, rapidly moving or vibrating atoms and molecules interact with neighboring atoms and molecules, transfering some of their energy (heat) to these neighboring atoms.
"Convection is usually the dominant form of heat transfer in liquids and gases. In convection, heat transfer occurs by the movement of hot or cold portions of the fluid. For example, when water is heated on a stove, hot water from the bottom of the pan rises, heating the water at the top of the pan. Two types of convection are commonly distinguished, free convection, in which gravity and buoyancy forces drive the fluid movement, and forced convection, where a fan, stirrer, or other means is used to move the fluid.
"Radiation is the final means of heat transfer. Radiative heat transfer is the only form of heat transfer that can occur in the absense of any form of material and as such is the only means of heat transfer through a vacuum. Thermal radiation is a direct result of the movements of atoms and molecules in a material. Since these atoms and molecules are composed of charged particles (protons and electrons), their movements result in the emission of electromagnetic radiation, which carries energy away from the surface. At the same time, the surface is constantly bombarded by radiation from the surroundings, resulting in the transfer of energy to the surface. Since the amount of emitted radiation increases with increasing temperature, a net transfer of energy from higher temperatures to lower temperatures results."

The revised lead read:
“Heat is related to energy in a similar fashion to how work is related to energy. Heat is said to flow from areas of high Temperature to areas of low temperature. Essentially, all objects have a certain amount of energy within them that is related to the random motion of their atoms. This internal energy is directly proportional to the temperature of the object. When two bodies of different temperature come in to thermal contact, they will exchange internal energy until the temperature is equalized. The amount of energy transfered is the amount of heat exchanged. It is a common misconception to confuse heat with internal energy, but there is a difference, and understanding the difference is a necessary part of understanding the First law of thermodynamics.”

I can’t date that oldest version for sure: perhaps it was 22:48, 8 August 2001? I got it by searching for the first revision to it, dated 15:43, 25 February 2002.

We may note: the reference to the first law involving internal energy, without explicit mention of the second law, but with extensive reference to temperature; the assumption of conservation of heat in the transfer; no mention of the nineteenth century abandonment of the caloric theory; no mention of the Bryan (1907) – Caratheodory (1909) revolution in thermodynamical theory, to a 'mechanical' theory of heat. Chjoaygame (talk) 12:53, 1 December 2025 (UTC)

At 15:19, 25 May 2004, the following was added as the first sentence of the lead:
"Heat (abbreviated q, also called heat change) is the transfer of thermal energy between two bodies which are at different temperatures. The SI unit for heat is the joule."
This is a loose statement that heat, called 'thermal energy', is conserved in a transfer between two bodies at different temperatures, the fundamental principle of calorimetry.

At 10:41, 25 August 2004, the following was added as the last sentence of the lead:
"Infrared radiation is often linked to heat, since objects at room temperature or above will emit radiation mostly concentrated in the mid-infrared band (see black body)."
Thus the lead explicitly mentioned thermal radiation.

At 10:39, 17 February 2005, convection was removed from the list of heat transfer mechanisms. At 14:09, 17 February 2005, that removal was undone.

At 08:42, 25 January 2006, generation of heat was added to the lead: "electromagnetic dissipation (as in electric stoves,) or mechanical dissipation (such as friction.)"Chjoaygame (talk) 13:55, 2 December 2025 (UTC)