Talk:Heat/Archive 9

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continuous process description

This whole "continuous process description" (within a conventional thermodynamical treatment) is a red herring you continue to raise. For quasistatic changes this is possible, but thermodynamics can in some cases also be used to compute the outcome of processes that are not quasistatic. Also for these cases, one can (in principle) define heat and work, and actually calculate them in some of such cases easily (e.g. the free expansion case is a rather trivial example). In general, this is a complicated matter, because in general you then do have to consider the real dynamics of the system, so you do have to give a "continuous process description", but this will be based on the real dynamics of the system, not on some efective thermodynamic description involving generalized forces.

In Reif's quantum mechanical picture, this is evident, he introduces the Hamiltonian of the system and says that the mean energy of the ensemble after the change is well defined given how the external parameters are changed. There is nothing to argue about that, if you have an initial condition for the ensemble, each ensemble member is in some well defined quantum state, and the time dependence of the external parameters is defiend, then the Schrödinger equation fixes the final state for each ensemble member and hence the mean energy change of the ensemble.

This means that you have to consider the dynamics of the system in terms of the molecules that make up the system. Obviously, that's not very practical to do exactly, but one can introduce stastistical methods that work well, see e.g. the last few chapters of the book by Reif.

So, it is still well defined in principle. Reif claims on page 71 that despite the complicated nature of this, it can be readily measured, giving some examples. Count Iblis (talk) 16:00, 29 April 2012 (UTC)

The problem is to distinguish work and heat transfers.

Simplest is the case of quasi-static processes. Every stage along the way is virtually at thermodynamic equilibrium, and the temperature is defined at every stage.

Next in simplicity, for processes for which a "continuous process description" is good enough, you need to keep track of the actual pressure and volume as functions of time and space, at every stage, in order to distinguish work and heat transfers. Very often in such a smoothly and mathematically well behaving "continuous process description", the temperature is also defined at every stage. Such processes can be considered as "special", as follows.

For more general processes, getting less and less smooth and mathematically well behaved, eventually you come up against what Count Iblis calls "violent process". He wrote at 17:43, 24 September 2010: "In violent process, heat transfer can happen without it being possible to define a temperature, even approximately. Cases where temperature can be defined (approximataly, in a local sense) are always special cases, albeit ones that we commonly experience."

From the definition for any actually possible natural process, to calculate the work transfer, you need the generalized forces, for example the pressure, as functions of time, as well as the external variables, for example the volume, as functions of time, to be found from some source other than thermodynamics. Of course thermodynamics doesn't supply them. You may be able to or need to measure them; or you may know how to predict them by calculations that use some phenomenological law, for example the constitutive equation of state of a gas, or Hooke's law, or somesuch; or you might like to calculate them in terms of molecules, from a classical kinetic gas theory or somesuch; or your might like to find them by use of quantum mechanical Hamiltonian theory; or you might even like to do by use of quantum field theory. But, however will get them, you need them for the calculation of work, because of the definition of work. Classical thermodynamics satisfies this need by choosing to consider only very "special", even specially idealized, processes, as noted just above by Count Iblis, for which the generalized forces are ideally defined.

If the process is violent, such that the temperature is not definable, then it is likely that the generalized forces are also not definable, because of just the very same physical characteristics of the process that make the temperature undefinable. Then we do not have the "special case" to which Count Iblis refers, although he denies that there may be a difficulty in defining the generalized forces. Such a difficulty is what Glansdorff & Prigogine 1971 mean when they say that "A continuum hydrodynamic description is then impossible."

All the quantum mechanical Hamiltonians in the world won't help in this case. The problem is not in how you go about the calculation. The problem is that the physical conditions are not right to support the definitions of the hydrodynamic variables as averages of microscopic variables. The averages don't behave well enough. To get a distinction between heat and work transfers from a quantum mechanical Hamiltonian description, the averages must behave well enough, just as they must for any other description. When the averages don't behave well enough, then it is quite likely that the generalized forces and the temperature are both undefinable; and then heat and work cannot be distinguished. There is no reason for any given process to expect the averages to behave so badly that temperature is not defined, but so well that the generalized forces are defined.

Even though I have been a naughty boy today, I will be a reformed character, a good boy tomorrow, if you give me an ice-cream now. Perhaps the tiger will change his stripes and the leopard his spots if they are studied by the quantum mechanical Hamiltonian formalism? No, the quantum mechanical Hamiltonian formalism will not reform a misbehaving natural process so as to make the generalized forces "well defined in principle", will not make a "violent process" become "special".

As far as I can work out, Count Iblis is asserting that there is a principle that, for any given violent process, one can expect that even when the averages are so badly behaved that temperature cannot be defined, somehow they will be always be well enough behaved for the generalized forces to be defined, so as to allow a distinction between heat and work transfers. It seems that he even thinks that for a violent process, somehow doing the calculation by use of a quantum mechanical Hamiltonian formalism will in general improve the behaviour of the averages in a selective way that makes the generalized forces definable but leaves the temperature undefinable.

Perhaps there may sometimes, or even often, be cases for which the averages for the generalized forces are better behaved than the averages for the temperature. In such cases, it may be possible to distinguish heat and work transfers even when the temperature is not definable. But there is no general principle that says so.

Even if somehow it can be shown that it is not universal, the association of heat transfer with temperature is very commonly found and should be made prominent in the article including the lead, not downplayed for the sake of making it easy to conduct mathematical derivations and tickling some mathematical sensibilities of elegance. If it is proposed that there is a physical principle that says that the averages for the generalized forces are in general better behaved than those for the temperature, let us hear it, not arbitrarily presume it. The Maxwell-Boltzmann distribution is pretty fast, perhaps fastest, in establishing itself in many cases. Is there a practical example of a heat calculation that does not refer to temperature? If so, the definition offered on page 227 by Kittel & Kroemer 1980 is not admitting it.

The reality is that the behaviour of the averages is not determined by the formalism of the calculation; it is determined by the physics of the process.Chjoaygame (talk) 04:42, 30 April 2012 (UTC)

point of view of Pippard 1957

I refer to Pippard, A.B. (1957/1966), Elements of Classical Thermodynamics, reprinted with corrections, Cambridge University Press, London. Pippard 1957 is listed in Reif's 1965 longer bibliography at the end of his book but is not suggested in the shorter reading lists at the ends of the chapters. Pippard 1957 is listed by Buchdahl 1966 in his bibliography of ten references. Pippard 1966 is listed in Callen 1985 in his list of six books on thermodynamics on page 485, with the comment: "A scholarly and rigorous treatment." Callen 1960 occurs in several of Reif's 1965 end-of-chapter suggested reading lists.

I will not try here to summarize Pippard. Enthusiastic editors may like to read what he says.Chjoaygame (talk) 20:13, 1 May 2012 (UTC)Chjoaygame (talk) 20:38, 1 May 2012 (UTC)

Defining temperature when there is (non-quasistatic) heat flow

Let's discuss here this issue raised by SBHarris above in more detail. We know that we can avoid having to consider this by focussing on initial and final states that are in thermal equilibrium with well defined temperatures. If the work done by the system is known (and it is well defined in princile as pointed out in the book by Reif), then the heat absorbed follows from the First Law (which is thus taken the definition of heat). Then during the heat flow, a simple thermodynamic description isn't available, except in the quasistatic limit.

But we are not satisfied with this and we want to dig deeper. We should be able to bring two objects with different temperatures into contact with each other and see that the temperatures come closer to each other until they become equal and thermal equilibrium is reached. And this is precisely when heat flows between the systems. However, during this process the systems are not in thermal equilibrium, so you could question if ou could assign temperatures when the process of heat transfer is going on. Now, we know that in practice, there isn't much of a problem here, you can measure temperatures when heat transfer is going on. So, under not too extreme circmstances, we should be able to define temperatures.

Suppose then that during the process of heat transfer one object has temperature of T (in some sense). Without making that precise at this moment, we should note the following. If we were to interrupt the flow of heat (put an insulator between the two objects and let the object reach internal thermal equilibrium), then we could see a difference. In case of the heat flowing between the objects, there is obviously a flux of energy from the object, which is absent in the case of full internal thermal equilibrium.

If the object is a gas in a box, and we focus on a point inside the box, close to the boundary, then the moleculs there have a certain velocity disribution. In case of thermal equilibrium, there is no net transport of energy; the velocity distribution is Maxwellian. When the two objects are in thermal contact, there is a net flux of energy that moves thoough the box. This means that the velocity distribution is not of a purely Maxwellian form. We can understand this as follows.

When the two objects are in thermal contact, we have conduction of heat from one object to another and in a first approximation, you can describe this situation using time dependent temperatures, but if we want to take into account that the objects themselves have to conduct heat internally, then the objects not being in thermal equilibrium can to first approximation be described as there being local thermal equilibrium. So, to a good approximation we should have a Maxwellian velocity distribution where the temperature is position dependent.

Now, this is still not yet consistent with heat being conducted through the gas to the boundary of the box. Because we still have have a Maxwellian velocity distribution at every point, and then the flux of energy is exactly zero. However, once we take into account the finite mean free path of molecules in the gas, this changes things. If you imagien a plane parallel to the boundary of the box just inside the box, and look at the flux of kinetic energy of molecules that in both directions (left to right or vice versa). If we assume to first approximation that the velocity distribution is Maxwellian, then the fact that the molecules originate from one mean free path in one direction or the other, makes a difference. The flux from the two directions do not cancel, one flux is from a Maxwell distribution at a slightly higher temperature than the other. So, we then have net flux of energy toward the boundary of the box.

The velocity distribution is then not precisely Maxwellian due to molecules traveling finite distances, and that then yields the non-zero flux of heat. This is then not a fully self-consistent way of looking at things, because we started with assuming that you do have a position dependent Maxwellian distribution and we end up with something slightly different. Of course, a purely Maxwellian distribution does not yield a nonzero energy flux, so however one arrives at this conclusion, the end result is that the flow heat is related to local termal equilibrium breaking down; the velocity distribution is not purely Maxwellian.

That's why saying that mere temperature differences explain heat flow is not sufficient. In terms of the two boxes this is analogous to saying that if you have just two objects at a different temperature, no heat will flow unless you bring them into thermal contact. But the act of doing that leads to non-equilibrium in the objects themselves. Before you brought the objects into thermal contact, the non-equilibrium of the two objects with each other could be described exactly with assigning two different temperatures to the objects. Bring them into thermal contact and this exact description will break down. And as I explained above, you can look at what happens inside the objects, exact local thermal equilibrium in an object is not sufficient to capture the heat flow.

Count Iblis (talk) 23:10, 28 April 2012 (UTC)

Dear Count Iblis, you are creating a straw man here, not focusing on the question at issue.

The question at issue is whether one can always define work in cases when temperature cannot be defined.

The question you try to distract us with here is whether "mere temperature differences explain heat flow".

That is an entirely different question. It is a straw man. Would I be so silly as to let you distract me into a discussion of your straw man?Chjoaygame (talk) 23:35, 28 April 2012 (UTC)

What is up with you guys? All this stuff about 'heat flowing' went out with the 'caloric' theory for the very good reason that 'heat' is not a conserved quantity, it is only energy in all its forms that is conserved - that is what the measurements show. Count Iblis, just what do you mean by "And this is precisely when heat flows between the systems"?

More 'stuff' about work. Chjoaygame, you write "The question at issue is whether one can always define work in cases when temperature cannot be defined". What has work got to do with heat? Work is about energy that doesn't have a related temperature because it can be completely defined by force time distance (F x D), it is a macroscopic concept. Heat, on the other hand is a microscopic concept, it is the microscopic kinetic energy of atoms and molecules. Just to clarify, microscopic kinetic energy does not 'flow', it can be transferred in a number of ways, exchange of momentum is one and the closely related exchange of electromagnetic momentum is another

You are wasting time trying to find a form of words to make the facts fit your 'heat flow' theories, far better explain the observations, don't you think? --Damorbel (talk) 20:20, 30 April 2012 (UTC)

When no work flows between systems (one system doesn't do work on the other) then conservatin of energy demands conservation of heat. So it's easy to fall into the trap of simplifying things, by picking systems in which no work done is done between systems. The problem (as noted below) is that in advection of heat, there is advection of mass, which amounts to bouncing some masses off another system, much like throwing rocks or shooting bullets at it. That's kind of like work. This is transfer of energy by the kinetic energy of the bulk flow (bulk current of advection), but there is not necessarily any entropy involved at the input, so it's a funny type of energy transfer. My example is a river of liquid helium at 0 K (helium is a liquid even at 0 K) which I can run into a test system (like it strike and splash off the face of your perfect cube, which is at some temp) at the speed of sound, or however fast you like, and thereby "heat up" (or add energy to) the second system (the cube), up to any temperature you like (until its atoms have the same kinetic energy as the helium atoms, energy will flow into the system I'm "heating"). But is it art? Is this process "heating"? Does it "count"? It's advective. It's kinetic. But where is my input temperature? S B H arris 00:19, 1 May 2012 (UTC)

User:Sbharris, you write "...where is my input temperature?" In France they use an anal thermometer, which gives an 'output temperature'. English speakers tend to look down on the French so they use a mouth thermometer, would this meet the requirements for your 'input temperature'? --Damorbel (talk) 07:26, 1 May 2012 (UTC)

In the U.S. we often use auricular temperatures. So our ears are good for something, which is good, since we don't usually use them for listening. I take that the same is true of wherever you are? S B H arris 18:31, 1 May 2012 (UTC)

muddles arising from Reifism

Count Iblis writes above at 00:05, 16 April 2012: "Work is always well defined (being the change in internal energy due to the change in external parameters), heat transfer is thus also always well defined."

Count Iblis makes a fundamental mistake of simple physics in writing that comment.

In physics, change in internal energy is not well defined by change in external parameters alone. Also needed for a calculation of the change of internal energy is information about a non-deformation variable, such as pressure, or, dare I say it, temperature. The distinction between work and heat needs even more. For its calculation, it needs also the record of the course of the values of the conjugate generalized forces belonging to the external parameters.

Probably Count Iblis has made this fundamental mistake in simple physics because he has been muddled by reading the angel of muddle, Reif 1965. Count Iblis' mind is full of stories about quantum mechanical Hamiltonians, told by Reif 1965, that get him into muddles like this. Reif 1965 is full of hubris about how clever he is with his better way of teaching physics, but look at the result in this case!

Count Iblis writes also above at 15:32, 16 April 2012: "If you can't define heat in general (i.e. during non-equilibrium conditions), then you have a huge problem, because we all know that heat flows during non-equilibrium conditions."

One cannot be sure exactly what Count Iblis means by this loosely worded comment, but it looks hard to separate it from a statement that heat is convected during non-equilibrium conditions. Synthesizing this comment of his with his above fundamental mistake in simple physics, it seems likely that Count Iblis is making the fundamental mistake of thinking that because a body has a high density of internal energy, and some of its components are moving, that this constitutes heat transfer. With all respect to engineering terminology, physicists do not say that heat is convected. They say that internal energy is convected, but the above comment by Count Iblis looks hard to distinguish from a statement that heat is convected, though his comment is loosely worded and its precise meaning is indeterminate. More muddle probably arising from an overdose of Reif 1965.Chjoaygame (talk) 08:28, 30 April 2012 (UTC)

You wrote: "In physics, change in internal energy is not well defined by change in external parameters alone. Also needed for a calculation of the change of internal energy is information about a non-deformation variable, such as pressure, or, dare I say it, temperature."

This is wrong, when you specify the initial state and the external parameters as a function of time, then the final state is fixed (in classical physics, as well as in quantum physics). If it is unknowable in practice without doing an actual experiment to measure it, because a lack of a (local) thermodynamic description makes computations a tour de force, then so be it.

And nothing would prevent you from doing such experiments, measure the thermodynamic variables in the final state (where things have settled down and you have thermal equilibrium again), and go from the initial state to the final statre via an alternative quasistatic path and determine the change in internal energy that way. Internal energy being a thermodynamic state variable is path independent, it's a unique function of the thermodynamic variables, so it is well defined. Count Iblis (talk) 16:26, 30 April 2012 (UTC)

In thermodynamics, where heat is allowed, the initial state is specified by the external variables and by one other variable, Carathéodory's non-deformation variable. That might be the internal energy, the entropy, the pressure, or the temperature, or indeed the refractive index. To specify the final state, one needs the same or an equivalent set of variables. It is not enough to state only the changes in the external variables: the change in the non-deformation variable must also be stated. The system does not evolve from the thermodynamic initial state deterministically solely on the basis of the initial state and the external variables; the change in the external non-deformation variable conditions also affects the evolution from the initial to the final state, e.g. whether the change is or is not adiabatic.

In classical mechanics, as contrasted with thermodynamics, there is no heat allowed; the evolution is deterministic as you say. But we are not talking about classical mechanics. And so also is it for quantum mechanics. For quantum mechanics, people can talk about "adiabatic" changes, but they mean by the what in thermodynamics are called "quasi-static, reversible adiabatic changes". For example, Fermi in his 1936 Thermodynamics, is so habituated to quantum mechanical thinking that he forgets which subject he is talking about and actually defines "adiabatic" on page 25: "A transformation of a thermodynamical system is said to be adiabatic if it is reversible and if the system is thermally insulated so that no heat can be exchanged between it and its environment during the transformation." Dear Count Iblis, you are in good company in your mistake, but still it is a mistake. And I blame Reif for muddling you like this; perhaps Fermi is also an accessory before the fact.Chjoaygame (talk) 19:57, 30 April 2012 (UTC)

You stated: "It is not enough to state only the changes in the external variables: the change in the non-deformation variable must also be stated" But that's only the case if you assume a thermodynamic description of the whole process from start to end and that will force you to only consider quasistatic processes.

I agree that in an exact description, be it according to classical mechanics, or quantum mechanics, everything is work. That's why Reif uses the work "macroscopic work", which is the change in internal energy averaged over the ensemble due to the change in the external parameters. The choice of the external parameters is arbitrary; in principle you could have chosen all the degrees of freedom of the system, and then everything counts as work. Obviously, if two molecules collide with each other, you can say that one molecule has performed work on the other.

The whole point of thermodynamics is to describre a macroscopic system in terms of a few variables that characterize the macroscopic state of the system. In general, this is not possible, but in thermal equilibrium, one can do this. But one needs an extra variable, the entropy, which originates from the statistical nature of what one is actually doing. But there is no escape from the fact that you cannot describe the general state of a system consisting of 10^23 molecules in terms of only a few variables. That the thermodynamic description will break down away from equilibrium, is thus no surprise.

One may try to cover that up by introducing more variables to capture what goes on if one is slightly away from thermal equilibrium (e.g. assuming local thermal equilibrium with a slowly varying temperature and pressure), but fundamentally, this doesn't work in an exact sense. Because, you are then also forced to introduce additional transport coefficients like viscosity, heat conduction coefficient, that you need to treat as fundamental. The more precise you want to be, the more such phenomenological coefficients you need to introduce.

E.g. in case of pure thermal conduction, the current density for heat flow is not simply proportional to temperature gradiemnt, there are also terms involving powers of the gradient, higher derivatives etc. etc. The reason why local thermal equilibrium works in practice, is because one still limits the number of independent variables to describe the system compared to the full "10^23" independent variables. So, even though one intoroduces a continuous temperature function, which formally needs to be described by an uncountable number of degrees of freedom, one makes the (hidden) assumption that this function is slowly varying, gradients are small, and the higher order derivatives can be ingored.

So, basically, what you are arguing for is a pipe dream from the outset, it cannot possibly work. The only correct definition of heat and work must be based on fundamental physics, contain how one (in principe) averages over ensembles, so it will have to make clear what information about the exact state of the system is thrown away to divide energy change into heat and work. Count Iblis (talk) 16:02, 1 May 2012 (UTC)

You write: "The only correct definition of heat and work must be based on fundamental physics ... " Your idea of "the only correct", your "fundamental" physics, seen from the point of view of your reading of your 1965 textbook. You say that I am "arguing for a pipe dream"; I would say that you believe I am arguing for something that I am not arguing for. I am arguing for the admission of several points of view into this article; perhaps that is indeed a pipe dream.

What you say just above contains some things that I think are right, and which I am surprised that you introduce now but have not acknowledged long ago. But you also make a basic methodological error in your misreading of Reif. You say: "The choice of the external parameters is arbitrary." This is an invention of yours, an erroneous figment of your own mind. There is indeed some choice, but it is governed by empirical facts, not by the purely theoretical and extreme arbitrariness that you feel entitled to. This error utterly invalidates your overall conclusions.

You offer your interpretation of why Reif uses the term "macroscopic work", an interpretation that I say is an erroneous figment of your own mind, not imagined by Reif 1965 as to how he might be misread. When you encounter a physical problem that cannot be described effectively with the usual thermodynamic macroscopic variables, instead of saying so, you shift the goalposts. You say "Oh, I will just increase the number of external variables to cover the problem." That is forbidden by the methodological rules of thermodynamics. You wrap this up for yourself in fancy language about quantum mechanical Hamiltonians, so that the mistaken nature of your move is not so obvious to you. The essence of the usual thermodynamic external variables is that they have already been recognized on empirical grounds as the adequate or most detailed feasible macroscopic description. You just say "Oh, in principle I will have a new description with more detail." That is not thermodynamics. It is a mistaken interpretation of Reif.

Some time ago I recognized that your approach was governed by an excessive and mistaken enthusiasm, but it is not till now that I have been able to specifically identify its core dogma. It is that you are happy to ask the definitions of heat and work to extend themselves to be constituted from up to 10²³ "macroscopic" external variables. That idea is so far from what is valid in thermodynamics that it did not occur to me till now that you could build your "fundamental physics" on it. For example, Prigogine & Defay (trans. 1954) write on page 1: "It is a matter of experience that when we have specified a certain number of macroscopic properties of a system, then all the other properties are fixed. For a given system under certain circumstances there will be a definite number of properties or variables to be fixed before the state of the system is completely defined. ... We have a free choice of which particular variables to select, but once they are chosen all other variables are fixed. ... It is important to remember that initially we have a free choice of the independent variables, but once made we cannot change our choice arbitrarily in the course of a problem. All changes of variable must be made in accordance with the correct mathematical procedure." I mentioned this once above, with a comment about fingers and toes, but you dismissed it, thinking how clever you were to do so. The problem is that unlike real teachers such as Prigogine and Defay, Reif thinks he is such a sophisticated and modern pedagogue that he does not need to emphasize basic thermodynamic methodology such as this, but can instead dazzle his students with fancy talk about quantum mechanical Hamiltonians; you and the Wikipedia article are the victims of his hubristic would-be cleverness.

Following your misreading of Reif, you say to yourself that while heat and work are always definable in principle by allowing up to 10²³ "macroscopic" external variables, thermodynamics does not follow by using the same 10²³ "macroscopic" external variables; then you can say to yourself that thermodynamics breaks down for violent processes even while heat and work continue to be defined; at the same time you ignore that you are now defining heat and work in terms of the 10²³ "macroscopic" external variables variables while not following that definition for thermodynamics. I would call that changing horses in mid-stream, or moving the goal posts. Contrary to this move of yours, the ordinary notion of heat and work in thermodynamics is that they are defined coherently with the thermodynamical external variables and the non-deformation variable.

The realistic physical alternative to your un-thermodynamic re-definition of heat and work is to admit that there are some physical problems, more or less your "violent processes", the detailed time courses of which cannot be dealt with by thermodynamics and its ordinary definitions of heat and work. In particular, for such physical problems, the thermdoynamical concepts of heat and work break down. A fair indicator of the likelihood of this is the breakdown of the definability of temperature for the detailed time course of a process.

I will not offer further criticism of, or epithets for, your argumentation. I no longer have time or heart for it. This leaves your violently imposed point of view standing in the article. I trust this will make you feel good.Chjoaygame (talk) 18:27, 1 May 2012 (UTC)Chjoaygame (talk) 08:09, 2 May 2012 (UTC)

logical structure of thermodynamics

In physics, heat is dealt with mostly by thermodynamics.

Thermodynamics is built on the distinction between heat and work. If work can be defined for a process, then heat is also defined for it. The modern approach to thermodynamics describes processes as passages between states, and for the states, it defines internal energy and entropy. Temperature is a necessary consequence. No temperature means that there must be something wrong with either energy or entropy; entropy is the weak point, because it needs the definability of work and heat. No temperature therefore logically requires no distinction between heat and work; that means no thermodynamic macroscopic description.

What about the statistical definition of entropy? It is the amount of information needed to take you from the macroscopic thermodynamic description to the microscopic description. You need the macroscopic description in order to define the entropy by the statistical definition. No macroscopic thermodynamic description implies no entropy by the statistical definition.

There are various axiomatic schemes for the macroscopic thermodynamic description. The Carathéodory one is considered by many to be the most mathematically elegant. But it contains the same physics as the other schemes; it differs only in the mathematical structure. The insistence on excluding temperature from the definition of heat is for mathematical elegance, not for any sound physical reason. Actually Carathéodory himself in his own scheme defines temperature but does not define heat at all. One wonders then why it is so burningly important for some that heat should be defined without reference to temperature.

On the other hand, admitting empirical temperature and constructing the other concepts from it has a good intuitive as well as physical basis. There are those who hate that thought. That's a good part of why they like to insist on the exclusive dominance of the Carathédory–Born approach that, in some form or other, dominates many, but not all, modern texts.Chjoaygame (talk) 06:06, 4 May 2012 (UTC)

The flow heat implies a breakdown of thermodynamic equilibrium, therefore the state of the system(s) during the heat flow cannot (in general) be described within a thermodynamic framework. At most (for the general case) you can look at initial and final states of thermodynamic equilibrium and then define how much heat was transferred between the systems.

Only in the special case of quasistatic changes from the initial to the final state can a thermodynamic description of the change itself be given. Now, since work and heat are path dependent, this means that for the general case, you need to be able to define work in a way that doesn't depend on the assumption of thermodynamic equilibrium. This is done by defining work the change in internal energy due to the change in the external parameters, what then remains is the heat abosorbed by the system.

Entropy is defined completely independend from heat, work and temperature. It requires a specification of an ensemble of systems with identical macroscopic specification in terms of external parameters (which in general do not involve non-mechanical thermodynamic quantities like pressure, temperature), from which one assumes the system can be considered to be randomly drawn.

Then with heat, work, temperature and entropy already defined independent of each other, one can prove all the thermodynamical relations among them. Count Iblis (talk) 15:39, 4 May 2012 (UTC)

Count Iblis writes: "The flow heat implies a breakdown of thermodynamic equilibrium, therefore the state of the system(s) during the heat flow cannot (in general) be described within a thermodynamic framework. ... Only in the special case of quasistatic changes from the initial to the final state can a thermodynamic description of the change itself be given." He means that the continuum of states during the heat flow cannot in general be described within a classical equilibrium thermodynamic framework.

But thermodynamics is not limited to the classical equilibrium framework. Temperature can be well defined in the presence of flow. The thermometer and the body of interest are put in thermal communication. This can be by thermal conduction or by radiation. Thermal equilibrium by conduction is present when the thermometer reading is steady (Eu, B.C. (2002), Generalized Thermodynamics. The Thermodynamics of Irreversible Processes and Generalized Hydrodynamics, Kluwer Academic Publishers, Dordrecht, ISBN 1–4020–0788–4, page 13). This gives a valid non-equilibrium temperature if the body of interest is well enough behaved. There is a significant literature, much of it written since 1965, about when this is the case; this is sometimes referred to as classical irreversible thermodynamics. There are plenty of cases when the body of interest is not well enough behaved, and for them, thermodynamics may not be salvagable.

Count Iblis writes: "Entropy is defined completely independend from heat, work and temperature. It requires a specification of an ensemble of systems with identical macroscopic specification in terms of external parameters (which in general do not involve non-mechanical thermodynamic quantities like pressure, temperature), from which one assumes the system can be considered to be randomly drawn."

Count Iblis is utterly mistaken here. The ensemble is only fully specified for thermodynamics when also one non-mechanical, or in more Carathéodoric language, one non-deformation, quantity is specified. That is why it is called 'thermo'dynamics. That Count Iblis thinks otherwise shows that he has not learnt thermodynamics from a real thermodynamics text, but is making it up for himself on the basis of his private misreading of the angel of confusion Reif. The 'random' drawing is determined by the non-deformation quantity. The non-deformation quantity might be the internal energy, the entropy, the temperature, the pressure, the viscosity, or the refractive index, or something else suitable (Prigogine, I., Defay, R. (1954), translation Chemical Thermodynamics, Longmans Green and Co, London, page 1; Pippard, A.B. (1957/1966), Elements of Classical Thermodynamics for Advanced Students of Physics, Cambridge University Press, page 8; Bailyn, M. (1994), A Survey of Thermodynamics, American Institute of Physics Press, New York, ISBN 0–88318–797–3, page 144).

Thus Count Iblis' version is his own homespun invention, not thermodynamics as usual. This is because Count Iblis as a student worked from the angel of confusion Reif, and has not yet moved on from there.Chjoaygame (talk) 22:52, 4 May 2012 (UTC)

Thinking more about this.

Count Iblis starts his comment: "The flow of heat implies a breakdown of thermodynamic equilibrium." He seems to be following the idea of the header of this section, logical structure of thermodynamics. Count Iblis continues: "This is done by defining work the change in internal energy due to the change in the external parameters." That still sounds like a thermodynamical calculation. Perhaps I might be forgiven for thinking he is talking about thermoydnamics.

But no, that's not how it is. He is really talking about statistical mechanics, though largely using language of thermodynamics.

The calculation of the work cannot be done without knowing the path, and that means knowing not only the external variables, but also the path values of the non-deformation variable. Count Iblis thinks he can forget about the non-deformation variable, I guess because he has said to himself the magic words "quantum mechanical Hamiltonian". But the physics doesn't forget about it. It is just the path that makes the distinction between heat and work. In a suitably well-behaved system, the work is an integral of the generalized force such as the pressure

P

with respect to the increment

d V

of the external variable. The generalized force can be had only by knowing the path values of the non-deformation variable. Count Iblis thinks he can forget about that because he has uttered the magic words. But for that he would still need to know the time dependence of the quantum mechanical Hamiltonian, from which to calculate the time course of the generalized force that is needed for the calculation of the work.

So now I can see more clearly where Count Iblis is coming from. He is taking the viewpoint that the statistical ensemble is the basis from which arise the macroscopic experimental observations that the thermodynamics describes, and conveniently forgetting about the need for the path calculation to get the work, because he has uttered the magic words. I see it the other way, that the experimenatal observations are the basic facts which are to be explained by the construction of a suitable statistical ensemble, and I don't forget the need for the path information to get the work. As I see it, the ensemble is a theoretical construct developed solely in order to explain the experimental observations as described in thermodynamic terms. There is no prospect of actually making the ensemble as a fact in the laboratory. The experiment can be repeated many times, and each time will most likely be realized by a different member of the theoretical ensemble, but no one has the slightest prospect of doing the experiment often enough to realize every member of the ensemble, and no one has the slightest prospect of knowing which member of the ensemble corresponds with which repetition of the experiment. So I regard the ensemble as a figment of the theorist's imagination; an entity that is good, admirable, suitable, clever, excellent, worthy, interesting, remarkable, insightful, enlightening, and generally meritorious, but still a figment of the imagination of the statistical mechanical theorist. The ensemble has to be constructed in the theorist's mind in just the right way to make it predict the experimental facts that are described by thermodynamics. The experimental facts are primarily described by a particular kind of macroscopic variable, as considered in thermodynamics, and the ensemble is tailored and trimmed to make it fit, not the other way round.

In a nutshell, Count Iblis often seems as if he wants to make the article about statistical mechanics. I don't think he would deny that he thinks that outranks, and is "deeper" than, and generally superior to, the mere facts of observation of the thermodynamical account. I say that this article is about thermodynamics primarily, and only secondarily about statistical mechanics, in so far as it may explain the thermodynamical account in terms of the theoretical adventures of molecules. The thermodynamic notion of heat is a macroscopic notion, and the statistical mechanical explanation is a theoretical explanation, a most admirable and excellent explanation, but not a primarily thermodynamical account. Excessively exclusive attention to Reif makes people forget that.Chjoaygame (talk) 06:50, 5 May 2012 (UTC)

point of view of Landsberg 1961

I refer to Landsberg, P.T. (1961), Thermodynamics with Quantum Statistical Illustrations, Interscience, New York, volume 2 of the series edited by I. Prigogine, Monographs in Statistical Physics and Thermodynamics. This is one of the ten general references named by Buchdahl 1966. Landsberg also wrote another book (1978) listed by Callen 1985 in his bibliography for statistical mechanics, entitled Thermodynamics and Statistical Mechanics, Oxford University Press, Oxford UK, ISBN 0–19–851142–6, which has Chapter 1, pages 1–4, copied with hardly any changes from §1 of Chapter 1 of the 1961 book.

Landsberg is a Carathéodory man to his finger tips, with much attention to finer mathematical matters.

In Chapter 2, on page 6 of the 1978 book he allows two systems, specified by sets of independent variables ${x 1$ } and ${x 2$ }, to interact thermally till they reach thermal equilibrium. Then he writes: "Thus thermal equilibrium is specified by a relation $f 12 ({x 1},{x 2}) = 0$ . This relation is assumed to be unique in the sense that, if all but one variable is known, $f = 0$ determines the remaining variable uniquely. This remaining variable is in fact a measure of the temperature."

Landsberg is recognizing that Carathéodory's non-deformation variable "is in fact a measure of [empirical] temperature". Carathédory does not mention this in so many words, but just goes on to develop absolute thermodynamic temperature before introducing the word temperature. But his systems already have empirical temperatures existing in their presupposed physical constitution.

From this I infer that Landsberg explicitly recognizes the existence of empirical temperature as presupposed for the definition of thermal interaction leading to thermal equilibrium. This is in agreement with the view of Kittel & Kroemer 1980.

I claim that it is also in agreement with a natural reading of Reif 1965. For example, Reif 1965 on page 94 writes: "Let us now discuss in greater detail the thermal interaction between two macroscopic systems $A$ and $A'$ . We shall denote the respective energies of these systems by $E$ and $E'$ ." I observe that $E$ and $E'$ are non-deformation variables of Carathéodory and thus qualify as empirical temperatures. It is evident that, in their very essence, Reif's macroscopic systems are such that temperature is definable for them. The issue is not whether Reif has mentioned this at any particular stage of his presentation. The issue is as to the physical character of his systems, whether or not temperature is definable for them. It is.Chjoaygame (talk) 07:15, 6 May 2012 (UTC)

Internal energy and enthalpy

I think that this section can and must be improved:

The introduction of the enthalpy by dH = dU + pdV is incorrect since a term Vdp is missing;
The relation pdV = δW_boundary follows from elementary mechanics, so it can be introduced much simpler;
Also the relation TdS = δQ_rev can be introduced simpler.

Please agree or disagree with this point of view.

(Adwaele (talk) 09:40, 18 May 2012 (UTC)).

The introduction of the enthalpy by dH = dU + pdV is incorrect since a term Vdp is missing

I agree with your criticism. There is no notice given there in the article that it is intended there that the process shall be at constant pressure.Chjoaygame (talk) 10:26, 20 May 2012 (UTC)

The relation pdV = δW_boundary follows from elementary mechanics, so it can be introduced much simpler

I agree that the notation δW_boundary and δW_other needs clarification, as well as a reliable source.Chjoaygame (talk) 10:29, 20 May 2012 (UTC)

Also the relation TdS = δQ_rev can be introduced simpler

I don't find the present statements about this to be objectionable.Chjoaygame (talk) 10:31, 20 May 2012 (UTC)

How can....

It be possible for a statement like this to appear in the article:-

"The SI unit of heat is the Joule. Heat can be measured with a calorimeter, or determined indirectly by calculations based on other quantities, relying for instance on the first law of thermodynamics"?

It is a statement of what heat is, devoid of any reference to temperature!

Is there a reliable source for it? Otherwise it should go. --Damorbel (talk) 16:01, 23 May 2012 (UTC)

Usually it is assumed for calorimetry that temperature will be measured.Chjoaygame (talk) 20:39, 23 May 2012 (UTC)

"assumed"? I have never seen this, is it original research? Without a reliable source, it will go. --Damorbel (talk) 05:46, 24 May 2012 (UTC)

Maxwell recommended the use of temperature measurement in calorimetry. Reference provided.Chjoaygame (talk) 06:55, 24 May 2012 (UTC)

Doesn't this contradict itself?

The opening section has:-

Because it is by definition a transfer of energy, heat is always associated with a process of some kind, and "heat" is used interchangeably with "heat flow" and "heat transfer".

And it goes on to say :-

In common usage, the noun heat has a broader meaning, and can refer to temperature or to the sensation felt when touching or being close to a high-temperature object.[9]

The second section refers to 'a broader meaning'? Um - does this 'broader meaning' have the same definition as the first one then? There is already an extensive disambiguation link Heat

This is confused and confusing, not suitable for an encyclopedia. --Damorbel (talk) 19:10, 23 May 2012 (UTC)

I made an edit to make explicit that a diversity of meanings is intended.Chjoaygame (talk) 20:54, 23 May 2012 (UTC)

"a diversity of meanings is intended" This could work if you repeated the link to the disambiguation but why? I suggest it needs revision to remove the confusion. In the absence of someone else's revision I shall do it.--Damorbel (talk) 05:53, 24 May 2012 (UTC)

Re - Overview

Where it has:- they exchange thermal energy

Now going to the article Thermal energy; the very first line has:-

Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system temperature

Which is different from the definition in the Heat section which defines heat as 'heat flow', or 'heat transfer' so making a logical impossibility. How can something (heat) be defined by a a qualified version of itself, ('heat flow' etc.) that is a version of itself that is restricted somehow in its meaning? --Damorbel (talk) 19:47, 23 May 2012 (UTC)

The Wikipedia article on thermal energy is about a private conception belonging to some specialists and it is not profitable to try to connect it with the Wikipedia article on heat which primarily takes the viewpoint of physicists.Chjoaygame (talk) 20:58, 23 May 2012 (UTC)

They are not the same anyway. Heat is not the same as thermal energy. Heat is the energy that flows spontaneously across a boundary into a system, that is not work, and is associated with an entropy increase. Heat is a flow of energy, but it's not thermal energy until it stops flowing. S B H arris 23:34, 23 May 2012 (UTC)

"but it's not thermal energy until it stops flowing". So what is your measure of 'heat' when it stops flowing, Joules or Kelvins? --Damorbel (talk) 06:03, 24 May 2012 (UTC)

Heat is always measured in joules (note lower-case) as is all energy, including thermal energy. Thermal energy is loosely "potential heat content" of an object-- that is, what we call the heat energy after it flowed into an object and has stopped. If we do no work, this part of the internal energy is guaranteed also to be extractable, again as heat (hence the idea of a POTENTIAL heat CONTENT). Thermal energy is that part of the internal energy that one can extract from an object, using only a pure temperature difference, and no other tricks. This causes the energy to be extracted as heat (a flow of thermal energy). Thermal energy is heat capacity times temperature: dU(therm) = CdT. For all the arguing about these terms, this part is not that complicated. S B H arris 20:51, 24 May 2012 (UTC)

What devilish tricky rogues these physicists are! No matter how uncomplicated a thing might naturally be, they will make it complicated, just because they like to make trouble. For example, they distinguish various heat capacities, such as those at constant pressure and at constant volume.Chjoaygame (talk) 07:19, 26 May 2012 (UTC)

Of course, but the heat capacity at contant volume is the one where no mechanical work is done, so we have no tranformation of heat into work or out of work. Thus, work is not involved and heat capacity involves only heat going in or out. In such circumstances heat is conserved, and acts like ye olde caloric. It can be treated as a conserved fluid (the thermal energy) and indeed the heat equation does just that, having as its only constant the thermal diffusivity, which is a ratio of thermal conductivity to the volume-specific heat capacity (which itself is the product of the density and the mass-specific heat capacity). These are all assumed to be constant-volume heat capacities. This works very well in approximation for solids and liquids where volumes change relatively little during heating (heat flow) and temperature change. When I say "little," I mean too little to do much P*V work.

Of course the heat equation breaks down in gases except where they are held at constant volume. All this is merely a way of saying that you can integrate heat to get internal energy change, when no work is done (this is just conservation of energy), and we call such internal energy changes "thermal energy changes" since they are due to nothing more than heat input, or outflow. With that qualification, which is always understood, what's the big problem? S B H arris 22:41, 26 May 2012 (UTC)

physicists don't say that heat is convected?!

Physicists don't say that heat is convected?! If true, that would explain a lot. More specifically, you would say that heat cannot be advected (like my my examples of bullets and gas streams with no heat-like velocity distributions). Since the diffusion part of convection (whether free or force convection) would presumably still be kosher, leaving simple thermal diffusion left as the only game in town when it comes to heat (radiation is sort of thermal diffusion with photons). It comes to me that perhaps the advective part of "heat transfer" (internal energy transfer) in rheids (fluids) is what is giving us all the problems. Heat transfer by diffusion always proceeds with a nice regular thermal gradient where every little region has nice a definable temperature (that happens in solids, and rheids with no mass-transfer of thermal energy look thermally like solids). In radiation you can talk about an ideal pair of reservoirs with a temperature difference, but that never actually happens in nature. In nature, what actually happens is the skins of all objects immediately equilibrate to the various temperatures that are defined by the various view-factors and conductivities in the system, and then old fashioned solid conduction as per the heat equation happens, after that. So, without advection you can't even set up the problems that are vexing us where heat supposedly "flows" but temperature cannot be defined. So perhaps that is the answer. We've been trying to talk the advection part of convection-heat transfer, and if thermodynamically and in physics that's verbotten, then that is our answer. You can always break up convection into a diffusive part, and an advected part that can be gotten rid of by changing reference frames, and transfers energy between systems by simple kinetic energy without any entropy. That's why it doesn't act like "heat" and why it sometimes has no "temperature." Does a river of liquid helium at 0 K have a temperature? No. But it can transfer energy into a system, simply by striking it and bouncing off. Tell me how much energy it must transfer, and I'll tell you how fast is has to run, in order to strike and heat the second system. Do we call that transfered energy, "advected heat"? No. Or, yes? It has no temperature. There's our problem. Is it heat? S B H arris 00:08, 1 May 2012 (UTC)

Physicists will say that internal energy can be advected.

An example where temperature cannot be defined is in a very dilute gas, when the Knudsen number is greater than 1, and there is significant non-uniform flow, and the molecular velocity distribution is far from Maxwell-Boltzmann.Chjoaygame (talk) 06:27, 1 May 2012 (UTC)

And what will happen when I stick a thermometer in such a system? Are you saying the temperature it measures is not the "true" temperature, or that it gives me false number since there is no defined temperature? What, then, does the thermometer give me?

BTW, very small low pressure systems as you describe are subject to experimentation all the time (by "small" I mean system dimension < Knudsen 1, else what do you mean by Knudsen number?). In such systems, physicists cheerfully go on measuring heat convection and advection. [1] and temperature also. Are they deluding themselves?

In my helium example, if you stick a thermometer in the flow, it will heat up from something friction-like if the flow is a gas flow at 0 K, although I don't know if there will be frictional heating with actual superfluid helium since there is no friction. How the hell does that work? The molecules have to get around objects, and do they not sometimes strike them and transfer momentum? And if they are moving very rapidly, is this not significant momentum?

Anyway, leaving aside such problems, an engineer would say that the advected portion of heat is simply "thermal energy" that is transported across a boundary by virtue of a macroscopic current, and is no different than tranporting heat by taking a hot solid cube HERE and moving it over into the target system to heat it, THERE. In that case, the "heat" that counts is the one that is left over after you adjust your reference frame, and the kinetic energy of the fluid or the heat-carry solid wouldn't count itself as "heat", but rather energy input by some other form (work, perhaps). A thermometer doesn't see it until it has been transformed in the other system into thermal energy (thermalized) by random impacts there. Again my example of heating a system by firing O K crystal bullets at it and watching them disintegrate and transfer vibrations that end up as temperature increases. If you DEFINE this kinetic energy input as "work" then I'm not heating the system, by definition. But if you don't define it as "work" then I AM heating it, by definition. But doing so without any temperature gradient. Count Iblis above suggests that the idea of temperature gradients in the real world as heat transfer is happening, is a convenient fiction, and isn't true, as it demands a non-continuum picture in which there are little differential volumes, each with a little temperature (a temperature field in time and space) and that this sort of thing has a reality. Whereas, when heat is actually flowing in a solid (the best defined system we have), there actually is no such physical thing. "Temperatures" as we define and idealze them, only "appear" after heat flow has STOPPED (since only then do equilibrated systems where we define T appear-- the best example being a gas where perfect M-B distributions are not seen while heat is flowing through the gas). So that leads to a conundrum of what causes heat to flow while it is actually flowing. I believe I see this point. S B H arris 18:16, 1 May 2012 (UTC)

You response makes sense, but I would differ from it in some ways.

You ask: "And what will happen when I stick a thermometer in such a system?"

That is a question that I agree is most important and relevant and valid. I have discussed it in general terms above, with literature references.

For the system I mention, my answer is that it will depend very critically on the exact mode of operation and size and design of your thermometer, with different results for different thermometers. Any one thermometer that you use will return answers that change probably faster than your thermometer will record, and will depend on time and position in a wildly erratic way, different for each time you repeat the experiment.

And under those conditions I think you and I will agree that temperature is not definable. It is under those conditions that I think it will in general not make sense to try to define work transfer, or to distinguish heat transfer from work transfer.

I referred to systems where the mean free path is long compared with the distances between significant non-uniformities within the system.

You write: "physicists cheerfully go on measuring heat convection and advection". I would say that physicists cheerfully go on measuring internal energy advection, do not think of measuring heat advection, and do not try to measure heat conduction because they have no access to temperature gradients. I think engineers would perhaps speak differently, and I would not speak for them.

What makes heat flow? Quite a few things, but temperature gradient is one of them, when it is definable. I think we agree about that.Chjoaygame (talk) 18:54, 1 May 2012 (UTC)

Chjoaygame, if heat is to 'flow' it might well be considered as mass transfer, a well established concept in physics, do you agree about this? If yes, should we then be considering 'heat' as having mass equivalent (E = mc²)? --Damorbel (talk) 06:58, 18 May 2012 (UTC)

I see that the major contributors to the thermal physics articles are still happily in the nonsense area! Why has nobody taken the trouble to identify that convection arises from gravitational forces, forces for which a gravitational field is required? Even the Wiki article on convection does not clearly distinguish 'natural' (gravitational) convection from 'forced' (um - 'unnatural'?) convection. Using the Wiki article on convecton as a guide one could feel free to explain to students that stirrng your tea was a example of 'convection'. Happy days! --Damorbel (talk) 06:58, 18 May 2012 (UTC)

You might try reading the thing: [2] S B H arris 23:03, 23 May 2012 (UTC)

The heading "physicists don't say that heat is convected?" (NB"?") They would be nuts if they did, they would be right back in the good (old?) (caloric) days where 'heat' was thought to be a fluid and it 'flowed'. Convection, advection, are both forms of mass transport by fluid (gas, liquid) flow; the fluids involved have a temperature thus associated energy also, but it is the mass that flows, not the energy. --Damorbel (talk) 08:16, 9 June 2012 (UTC)

Chjoaygame, if you introduce the Knudsen number into a discussion about heat you should give a good explanation why. The Knudsen number is based on a very generally described dimension called "a representative physical length scale". The Knudsen number is the ratio of the mean free path to this "representative physical length scale" which can be the dimension of an orifice through which gas is flowing. One might whish to discuss this, but why in the article about Heat? What is there about Heat that has anything to do with linear dimensions?

You write "An example where temperature cannot be defined is in a very dilute gas" This a rather controversial statement since, if a collection of molecules is sufficiently dense to be called a gas, then it must also be able to have a defined temperature. Further, if the 'pressure' is reduced to the point where the molecular interaction is effectively absent then the individual molecules still have an energy thus they must also still have a temperature (see 'Boltzmann constant').--Damorbel (talk) 09:03, 9 June 2012 (UTC)

revised good faith edit

It is going too far to try to say what is the everyday use of the word heat. The attempt that was made was faulty, and gave no suitable dictionary reference. It is enough to say what is covered in the present article.Chjoaygame (talk) 07:09, 26 May 2012 (UTC)

I think we should make clear to the reader – who may not have a background in thermodynamics – that the meanings of the term "heat" in physics and in everyday use are quite different. A dictionary reference is easy to find and give, such as "Entry for heat. Dictionary.com Unabridged (v 1.1). Random House, Inc.".

In my opinion, the definition presently given in the section Overview is strange: "heat is defined as energy transferred to the system by thermal interactions". Which system? The following sentences then suggest that these "thermal interactions" are microscopic particle interactions, entirely disregarding convection and radiation. If it is meant to cover all forms of heat transfer, then is this not essentially circular, telling the reader that heat is that which is transferred in heat transfer? (By the way, Heat transfer is, according to that article, not the transfer of heat, as one might naively suppose.)

A final question. Is there some reason not to link the term "thermal energy" to its article? --Lambiam 13:44, 26 May 2012 (UTC)

Yes, the distinction, between the ordinary language use of the word heat, and the present use, as a technical term of physics, should be made more up front.

To say, as does the lead, that heat is energy transferred from one system to another by thermal transfer is hardly informative. Thermal interaction is just interaction essentially involving heat. The use of the term system here is presumptive of a context that has not yet been stated. The use of the definite article, "the system", in the overview section seems ungrammatical, I agree. The use of the phrase thermal energy in this article is of very dubious merit, and probably a better course than linking it to the article on thermal energy would, I think, be to change the present text. There are other flaws throughout the article.Chjoaygame (talk) 21:09, 26 May 2012 (UTC)Chjoaygame (talk) 21:13, 26 May 2012 (UTC)

Proposed changes, June 1, 2012

Dear colleagues:

I am considering three things:

Modify the order of the sections in the Article. E.g. the concept of heat capacity should be introduced before internal energy and enthalpy.
Remove the overlap between the sections on entropy and on internal energy and enthalpy.
Add a section called Heat or heat flow?. You can find a draft version below. Personally I think this could be very clarifying, but it may be controversial. Adding this section may imply rephrasing existing sections.

I would like to have consenses before I modify the Article as I want to avoid that we will edit our texts in the Article over and over again, thus confusing the readers.

I would greatly appreciate your opinion.

The proposed new section would have the title Heat or heat flow? and read as follows:

In thermodynamics heat is not a function of state but a process parameter. In order to understand the implications of this statement let us look at two bodies a and b at temperatures T_a and T_b and with internal energies U_a and U_b. Suppose T_a > T_b. We connect them for some time by a heat-conducting wire with negligible heat capacity. During this time heat flows from the warm body to the cold body given by

{\dot {Q}}={\frac {\mathrm {d} U_{b}}{\mathrm {d} t}}=-{\frac {\mathrm {d} U_{a}}{\mathrm {d} t}}.

Initially the temperatures are T_ai and T_bi and the corresponding internal energies are U_ai and U_bi. (The final state gets lower index f.) The total amount of heat transferred from a to b is Q = U_bf - U_bi = -(U_af - U_ai). The net result of the process is that the internal energy of a has decreased and the internal energy of b has increased.

The fact that heat is not a function of state means that the heat Q was nowhere before the process and it is nowhere after the process. Heat is only a meaningful concept when it flows. In other words: heat flow is the basic concept and not heat. Of course one can integrate the heat flow over time and obtain a certain quantity of heat, but there is no region in space where the heat is “stored”. In daily life one would say that heat is stored in body b, but thermodynamically this is incorrect. No object is capable of storing heat. In this sence the term heat capacity, which plays a central role in thermodynamics, is misleading.

Also in other fields of thermodynamics the term heat is used in a confusing if not incorrect way. Look at the famous experiment of Joule in which he determined the so-called mechanical equivalent of heat. He used a falling weight to spin a paddle-wheel in an insulated barrel of water. Through the paddle the water is brought into turbulent motion and comes to rest after some time. Joule observed that the temperature of the water has increased, but the potential energy of the weight is not converted into heat. It has increased the internal energy of the water. Thermodynamically never in this experiment heat is involved. The water with the paddle form an adiabatic system.

A similar argument leads to the conclusion that power is more realistic than work. In order to emphasize that heat flow and power are the basic thermodynamic properties it is better to express the laws of thermodynamics in terms of time derivatives so, for closed systems,

{\frac {\mathrm {d} U}{\mathrm {d} t}}={\dot {Q}}+P,

where P is the power applied to the system, and

{\frac {\mathrm {d} S}{\mathrm {d} t}}={\frac {\dot {Q}}{T}}+{\dot {S}}_{i}{\text{   with  }}{\dot {S}}_{i}>0,

where ${\dot {S}}_{i}$ is the rate of entropy production.

The fact that heat flow is the basic quantity and not heat implies that we need a definition for heat flow rather than for heat. This would sound somehow like: heat flow is a flow of energy through matter driven by a temperature gradient. Microscopically the energy flow is due to the fact that the atoms, coming from a high-temperature region, pass a surface with a higher energy than the atoms coming from a low-temperature region. Based on this picture the coefficient of thermal conductivity can be derived with the kinetic theory of gases.^[1]

^ C. Kittel and H. Kroemer Thermal Physics W.H. Freeman and Company, New York, 1980, Chapter 14.

Adwaele (talk) 12:42, 1 June 2012 (UTC)

response 1

You have some solid points. "Heat" Q (in joules) is actually the time integral of heat flow dQ/dt. But the terms heat and heat flow (which really should be in units of power) are used synonymously, since heat is always flowing, so one gets away with it. But it's confusing to the student. And doing the integral makes it look as though the heat-power that has passed through the time-integrated is a quantity of heat energy that still resides someplace, whereas (as you point out) it is gone (and now is internal energy). You summed it up as it went past, but it passed into nowhere, and disappeared qua heat! Leaving only a total integrated (entropy*temperature) TdS, as its proxy. S B H arris 18:00, 1 June 2012 (UTC)

Thank you.Adwaele (talk) 15:31, 2 June 2012 (UTC)

Sbharris, what do you mean when you write ""Heat" Q (in joules) is actually the time integral of heat flow dQ/dt" My interpretation is that you mean 'Q = dQ/dt', i.e. Q is equal to the rate of change of Q. Wouldn't that make Q an Exponential function? OK? --Damorbel (talk) 16:39, 12 June 2012 (UTC)

response 2

First a trivial comment. Rather than saying that heat flow is a process parameter, I would prefer to say that it is a process variable or a process quantity. To be explicit, I think it right in general to say that heating refers to process rather than to state.

Second, a cautious whisper. I feel that it may be going overboard with zeal to insist officiously that heat only flows. It is wise and correct in a certain sense. But to insist on it to an extreme puts many respectable and established and otherwise-reliable thermodynamic sources in the wrong, when they talk about processes of heat production, meaning tranformation of other kinds of energy into internal energy in ways that can be made to increase temperature, such as friction and viscosity. This is verging on admitting something for the phrase 'thermal energy', but perhaps not too much, when the phrase is said to refer to state and not process. I am cautious about making this comment, fearing of course that it might provoke zealots of correct thinking to admonish me with severe strictures of rectitude. I think some more thought about this may be in order.Chjoaygame (talk) 21:15, 1 June 2012 (UTC)

Can heat appear without first flowing? Flowing as some kind of non-heat energy, spreading out in phase space and real space? The energy from viscosity or kinetic energy/impact/friction is non-thermalized (or not completely thermalized) energy when it is first transmitted into the system. It's my river of liquid helium at 0 K, impacting and splashing. Or it is a nuclear fission-fragment hitting atoms in the inside of a nuclear reactor fuel pellet at 3% of the speed of light, and following that, there is a cascade of collisions that ends up as reactor heat that goes to the turbine. The same happens with atoms that have just undergone a chemical reaction, or been rotated by the electric field of a microwave oven. They're all picked up energy and are moving fast at first, but that energy is not heat, yet. But the resulting flow of collisions that distributes this low(er)-entropy input-energy is needed (and this is a kind of energy flow), before this energy can at least become heat energy, still flowing. It must happen before you have heat (which is a flow of already-thermalized energy, no?), and before you can speak of "temperature." It happens before heat goes from the fuel rod, to the reactor coolent. After that, we have "heat transfer," but not before the fission fragment energy reaches a thermal kinetic energy spectrum (same with the neutrons, which must thermalize). It is the very act of flowing that allows for the entropy of a packet of input energy to increase to the maximum, so that we can finally talk of the object's "temperature" at that point, when that energy has finally been distributed (or semi-distributed, if there is a thermal-gradient that is nice and uniform). Otherwise, what we have is semi-thermalized internal energy, but not yet heat, and it does not yet have a temperature. It's a teenage pubescent sort of moving energy that is in a very awkward process of sorting-itself-out and relaxing. And it is in a state that (it seems) nobody really wants to talk about in thermodynamics. First the energy sorts itself out into high-temperature heat, and THEN it flows as heat, into lower temperature regions. You can't talk of heat as a process all the time, because technically the process must (sometimes) happen before you get the heat. And THEN when temperature is finally uniform, the heat goes away again!
This reminds me that I must fix the thermal energy article, which states that thermal energy is a state variable. Nonsense! Except in the limit that no chemical or PV work is done, so that a change in thermal energy becomes a change in internal energy, which is a state function. Otherwise the difference between constant pressure and constant volume heat capacities (which are different for every substance, even if good approximations) show clearly that temperature is path-dependent, and thus so is thermal energy or "heat content." You can get an object to the same temperature in two ways (for example), using two different total amounts of heat dumped into it, if you let it do PV work to get to the state in one case, but not in the other. So δW = 0 and no change in chemical potential and so on, must be specified before we talk of thermal energy in any such fashion. And then indeed, "thermal energy" (an idealized quantity in the limit of no other energies that are reversibly turned into heat at the same time) is that component of internal energy that can be extracted by a temperature gradient. But if you don't make δW = 0, you can, by fiddling with work or other internal heat/chemical potential sources, extract any number of differert amounts of thermal energies from the same object, in passing to a given lower temperature (or to absolute zero, for that matter). If you do so, that makes thermal energy content inherrently ridiculous. S B H arris 21:49, 1 June 2012 (UTC)

You address many topics. Allow me two comments: one of the limitations of thermodynamics is that the thermodynamic properties such as temperature and pressure of all subsystems must be well-defined. In a number of your examples this is not the case, so classical thermodynamics most do a step back. The second remark is that the question of heat-flow-or-not can depend on the choice of system. E.g. in the case of heating of water by an electrical heater: if one takes as the system only the water the increase in internal energy is due to heat supply. However, if one takes as the system the water and the heater together the internal energy increases due to the application of electrical power.Adwaele (talk) 15:31, 2 June 2012 (UTC)

You use some strong words. In the draft I tried to explain the point as clearly as possible but I had no intention to provoke. An enceclopedia article should inform not provoke. I am open for specific suggestions. Having said that: it is true that some thermodynamic sources could be a bit more careful in their formulation, but that should not keep us from doing it better!Adwaele (talk) 15:31, 2 June 2012 (UTC)

I whispered because I didn't want to sound too strongly. The idea that we can do better than reliable sources is bold and perhaps too bold. As I understand Wikipedia policy (a very dangerous thing for me to write!), we should say only what reliable sources say, and say what we think is true only when our thoughts are in strict accord with reliable sources; our mandate is not simply to inform but is to inform according to reliable sources. We could be very strict and insist that heat is only a flow word, but that makes us stricter than many otherwise reliable sources. Then we would be insisting that heat cannot be produced. It is of real physical importance that the quantity of heat obtainable from a given body in a given state depends on how the heat is extracted. I think for this reason alone it is reasonable to insist that heat is a process word, not a state word. But otherwise reliable sources often enough do not make the next move to insist that heat is only a flux. They speak at times of, for example, the production of heat by friction or viscosity, and of the evolution and absorption of heat by chemical reactions. For example, Bailyn 1994 on page 95 says "For heat will be produced ..." Kondepudi & Prigogine 1998 write on page "... the amount of heat evolved per unit ..." I hardly need say that the Wikipedia editorial body knows better, but Bailyn and Prigogine do have some claim to be reliable sources. So I am happy to see it made clear that heat is a process word, and that the obtaining of heat from a body depends on the nature of the process, because that is real physics. But personally I think it goes beyond the sources to insist officiously and at length that the only kind of heating is by flux; that is, as I see it, more about usage of words qua usage than about real physics.Chjoaygame (talk) 10:34, 5 June 2012 (UTC)

Your comment stimulated me to go to my little private library and look up what the various sources say about the nature of heat. Here is the result:

C. Kittel and H. Kroemer "Thermal Physics" 2nd edition, p.227, “Heat is the transfer of energy to a system by thermal contact with a reservoir”. Of course it is not relevant that the contact is with a reservoir and it is incorrect to call heat the transfer of energy. Heat is not the transfer of something. But the basic idea is that heat is only heat when it is transferred.
H.D. Young and R.A. Freedman “University Physics” 11th edition, section 17.5: "Energy transfer that takes place solely because of a temperature difference is called heat flow or heat transfer, and energy transferred in this way is called heat." Under the heading “CAUTION” p.653: …. "In physics the term “heat” always refers to energy in transit from one body or system to another"….
S.R. de Groot and P. Mazur “Non-equilibrium thermodynamics” 1969, write the first law in terms of the time derivative of the internal energy and define from that the heat flow (p.18, Eq.(II-33)).
Landau and Lifshitz, Statistical Physics 3rd edition, part I, p.45, Eq.(13.2): They define the quantity of heat through the time derivative of the internal energy, hence via the heat flow. By the way: it is incorrect to write a heat flow as dQ/dt.
K. Denbigh “Chemical Equilibrium” 3rd edition, p.18: discusses the experiment of Joule in the same way as I do. The definition of heat is, unfortunately, rather poor as it calls heat a “mode of heat transfer”. But still, the basic statement is that heat is only heat when it flows.
E.A. Guggenheim “Thermodynamics, an advanced treatment for chemists and physicists” 4th edition, section 1.12: this section talks about heat and flow of heat, but is not very clear one way or the other.
M.W. Zemansky “Heat and thermodynamics” 3rd edition, 1951, p.63, section 4.5: "Heat is energy in transit. It flows from one point to another. When the flow has ceased, there is no longer any occasion to use the word heat. It would be just as incorrect to refer to the “heat in a body” as it would be to speak about the “work in a body”. The performance of work and the flow of heat are methods whereby the internal energy of a system is changed. It is impossible to separate or differentiate the internal energy into a mechanical and a thermal part." I agree with every word.

I went to the library of my university and ran quickly through a number of other books on thermodynamics checking the discussion about heat. The general picture was as described above: some discussions are good, some are good enough. I also looked at D. Kondepudi and I. Prigogine “Modern Thermodynamics” Section 2.1 on “The Nature of Heat”. On page 34 there is a paragraph that starts as follows: "But still, what is heat? In the classical picture of particle motion, it is a disordered form of kinetic energy…." Then it goes on giving the expression for the kinetic energy of a monatomic ideal gas (which they call molecules), suggesting that this is the expression for the heat. They end the paragraph with a remark about phase transitions, leaving the reader in the dark what this means for their definition of heat. The Section goes on talking about matter and antimatter, fields, and the Big Bang. After reading this Section one has more questions than answers. Alas, frankly speaking this is, by far, the worst discussion of the concept of heat that I have ever seen.

In conclusion: there are many examples in the literature that express that heat flow is the fundamental property. In fact, I found no sources (except the book by Kondepudi and Prigogine) that do not support this point of view. I like very much the formulation by Zemansky. (I think a reference to his book is in order.) Second best is Young and Freedman. Implicit but strong support comes from De Groot and Mazur and Landau and Lifshitz. There are also some careless definitions but still they have the intention to express that heat flow is the central issue. So, my quick literature research showed that the proposed Section contains common knowledge and that there is no reason to withhold the information from the reader.

If you still have objections against my draft, I invite you, once again, to propose a better text.(Adwaele (talk) 15:37, 8 June 2012 (UTC))

Many thanks for your generous invitation to propose a better text. I feel privileged and honoured to receive your invitation. I can, however, see that you know all about it and that no better text can possibly exist, and I would be foolish to try to suggest one. Wikipedia is fortunate to have such omniscient and infallible editors as you, and we will all be grateful to read your valuable contribution. I have been in touch with the Nobel Prize people in Stockholm and in response to your criticism of Kondepudi and Prigogine, they are withdrawing Prigogine's Nobel Prize and will shortly contact you to let you know that they are awarding it instead to you.Chjoaygame (talk) 21:02, 8 June 2012 (UTC)

Thank you for your opposition. You stimulated me to find the best references to go with the Article. This is of great value. In the coming week I will go ahead and change the Article as proposed together with the relevant references.(Adwaele (talk) 10:20, 9 June 2012 (UTC))

Your process is clear. You choose a point of view from your personal prejudice and then when stimulated to do the previously unthinkable (look at the sources), you select the parts of the ones that reflect your prejudice, which you call "the best", and dismiss all else. The Wikipedia is likely to be damaged by your actions, but why would I waste my time trying to stand in the way of a Nobel laureate like you?Chjoaygame (talk) 17:15, 9 June 2012 (UTC)

Not all sources are equal; if Adwaele is quoting Prigogine correctly, then I would say that Prigogine is not a reliable source for this topic. Some authors will start to speculate and give their own ideosyncratic opinion. If we are to include such opinions, we need to be very sure we are then quoting this in the correct context which is then likely not the standard context. Count Iblis (talk) 01:27, 10 June 2012 (UTC)

It is good to see you considering the relative reliability of sources.Chjoaygame (talk) 02:36, 10 June 2012 (UTC)

What should I do to convince Chjoaygame? The suggestion that Kondepudi and Prigogine must be right since it is has a Nobel-prize winner as the second author is certainly not convincing. This is not how science works. Science is based on arguments, not on authority. My proposed text is simply a reformulation of the statement that heat is not a function of state. It says nothing new. No wonder that one can find similar descriptions in many books, as I showed in a previous comment. I would like to add to the list of good descriptions on the nature of heat:

S.J. Blundell and K.M. Blundell, Concepts in Thermal Physics, Section 2.1
D.R. Olander, General Thermodynamics, Section 1.1.2
W.C. Reynolds, Thermodynamics, Section 2.9
R.E. Sonntag, C. Borgnakke, and G.J. Van Wylen, Fundamentals of Thermodynamics, Section 4.6

(Adwaele (talk) 09:13, 12 June 2012 (UTC))

Dear Adwaele, I am surprised that you want to convince me, and so I feel obliged to respond. The question as I see it is not as to facts of physics but is as to usage of language. We all agree that heat is a process word not a state word. This is already stated countless times in the article. It seems you are not satisfied with that many times and want to expand on it even further. The further expansion you want to drive home is that it is forbidden to speak of any other kind of heating process than the transfer kind. This is also already said many times in the article. We all (or nearly all) agree that there is no unique definite quantity of heat in a body and that heat is not a uniquely definite part of the internal energy; that is one of the most basic lessons of thermodynamics; the amount of heat that comes from a body depends on the process of extraction.

You want to dictate, at great length and with extreme and almost fanatical dogmatism, how people should use language.

Language usage is mostly thought of in terms of more or less customary usage and more or less respectability of authorship; language usage is in this sense not best thought of as "correct" or "incorrect", unlike facts of physics for which "correct" and "incorrect" are, especially for some Wikipedia editors, dearly beloved forms of dogmatic or doctrinaire or even fanatical judgment about principles and facts of physics. I hardly dare say again that you are defining a "good" source as one that echoes your unblinking prejudice that heat only flows, and you are selecting your sources accordingly. Dare I say it, if you look about you will find that respectable physicists, and more often chemists, also sometimes say that heat is produced, evolved, or absorbed. This is how the language is sometimes used. I am not here asking you to include this agreedly less universal usage, but I am not keen on an unnecessary and officious pronunciamento that more or less implies that it is strictly illegal and "incorrect". I feel already the heat of your righteous indignation or anger that I have the temerity to say this, when I feel fairly confident that you have the fixed idea that in physics and chemistry heat only flows. Dare I say it, this is not something to be settled by appeal to principles and facts of physics. Rather, it is a matter of how language is used, and, dare I say it, that is usually considered on a basis of authority and custom. So I am not keen on a further extensive repetition, in an extremely dogmatic and officiously forbidding form, of what is already repeatedly said in the article.Chjoaygame (talk) 14:49, 12 June 2012 (UTC)

Kinetic theory

I am having difficulty with the arguments presented in this article. The problem arises because of the idea that heat is the motion fof particles (see kinetic theory) which describes exactly how the temperature of (and energy in) materials is related to the motion of the particles that make up the materials contradicts most of what is described in this article. Additionally kinetic theory explains how heat is transferred between samples of material, something this article does not explain.

Can anybody please explain the difference? --Damorbel (talk) 20:30, 12 June 2012 (UTC)

I will make just this one comment and will not enter into further discussion on this matter. Kinetic theory provides an explanation of heat for gases, not for materials in general. It does not work like that for heat for solids. Heat is not the motion of particles, but is in some respects explained by such motion. There is a big logical difference between a phenomenon and its explanation, and permanent failure to recognize this is a sure way to permanent muddle.Chjoaygame (talk) 03:25, 13 June 2012 (UTC)

1/ "....and will not enter into further discussion on this matter." I hope not, Wiki talk pages are about the content, not your POV.

2/ What do you mean by "... heat for gases, not for materials in general"? If you really mean this it is indeed your POV and it is entirely incorrect I suggest you read Phonons (Thermodynamics) which explains how heat vibrations propagate in solids.

3/ What do you mean by "Heat is not the motion of particles, but is in some respects explained by such motion."? Particularly - 'some respects'. It is precisely in 'some respects' that the article lacks. --Damorbel (talk) 06:41, 13 June 2012 (UTC)

Maybe I can help. I think it is important to realize that the concepts of heat in daily life and in thermodynamics differ. In thermodynamics heat is a process variable. Any attempt to understand the concept of heat from the state of the matter is bound to fail. Many people are struggling with this. Sometimes people try to identify the heat in a body as the kinetic-energy part of the internal energy. For an ideal gas that would mean that this “heat” is equal to the internal energy and proportional to the temperature. But, thermodynamically, this is “illegal”. I also have problems with the Article on heat in its present form. Please have a look at the Section “Proposed changes, June 1, 2012”. No need to read the whole discussion, but I would appreciate your opinion on my proposed text. If it works as intended it should answer your question.(Adwaele (talk) 10:15, 13 June 2012 (UTC))

"the concepts of heat in daily life and in thermodynamics differ". They do? How? Care to explain? Rather like electricty and electrodynamics? One would not expect the detail of either to be necessary for use in daily life but I am quite certain that the little the general public know abut these two maatters in daily life does not impose any real restriction on the extension to thermodynamic and electrodynamic knowledge that is needed for a thorough understaning of both. --Damorbel (talk) 19:02, 13 June 2012 (UTC)

What I mean is that, in daily life, we have the feeling that heat can be stored somewhere. A hot object contains a lot of heat. When we say that an object has a large heat capacity we seem to say that a lot of heat can be stored in the object, just like in electricity where a large capacitor can store a lot of electrical charge. We say that in certain energy-saving projects heat is stored (see the article on heat storage and e.g. Energy storage where it is said that “Thermal storage is the temporary storage or removal of heat for later use.”). Thermodynamically heat cannot be stored as it is not a function of state. It is only real when it flows. When we talk about a certain quantity of heat in thermodynamics what we really mean is the time integral of the heat flow. Compare the storage of heat with the storage of oil. In the case of heat heat flows towards the storing unit during hot periods and is extracted during cold periods. In the case of oil oil flows to the tank in time of a surplus and is extracted during time of shortage. If one asks: “Where is the oil in between supply and extraction?” the answer is obvious: the oil is in the tank. However, if one would ask “where is the heat in the period between the supply and extraction?”, in daily we would say that it is stored in the storing unit. But, thermodynamically, this is not correct. Heat is not stored, but the internal energy of the storing unit has increased. One can only speak about heat, or rather heat flow, when heat is supplied or extracted.(Adwaele (talk) 21:42, 13 June 2012 (UTC))

Adwaele, you write:- "One can only ..., when heat is supplied or extracted." In your argument, when heat is supplied or extracted are there (measurable) changes in the material which has heat "supplied or extracted"? If so, can you say what are these changes are? --Damorbel (talk) 08:20, 15 June 2012 (UTC)

"heat can be stored somewhere" I take it you mean an object (gas, liquid or solid) can have heat i.e. a temperature >0K. NB an object's temperature is not proportional to its size. This is what most people understand by heat.

"A hot object contains a lot of heat." Not true, a glowing spark may well contain less heat than an ice cube

Concepts such as heat storage and Energy storage are derived from basic theory in the sense that they rely on particular confgurations (somewhere to 'store' something).

"Thermodynamically heat cannot be stored as it is not a function of state". I suggest there is no conflict between 'storing heat' and a 'function of state'; 'stored heat'is just energy in an insulated volume (you can 'store cold' e.g. ice in a thermos flask).

I don't see any conflict between any 'daily life' experience of 'heat' and thermodynamics, after all the science of thermodynamics emerged from scientific experiments; which is another name for 'daily life' experiences writen down. --Damorbel (talk) 07:53, 14 June 2012 (UTC)

The whole point of thermodynamics is to describe a macroscopic object in terms of a few variables. Suppose e.g. that you want to give a complete description of the center of mass motion of balls that can collide with each other. You are not interested how the atoms inthe balls behave, just how the balls will move. Then what is the most minimalist description that is still correct? The equations you get from conservation of momentum involve inly the center of mass motion. But the conservation of energy equation contains both the center of mass energy and the internal energy. If you ignore the fact that center of mass energy can get transferred to internal energy, you are not going to see the balls come at rest ever. It is this transfer which is heat.

My question is quite simple, what do you call the centre-of-mass energy when the balls are not at rest? --Damorbel (talk) 07:53, 14 June 2012 (UTC)

So, the very reason heat appears, is precisely because you chose not to keep a certain number of variables explicitely in your equations, you don't want to bother about the zillions of microscopic variables. But because energy can leak into those varables and you need to work with conservation of energy, you need to account for that. The moment you add more variables so that you can keep track of smaller details of the system, heat becomes the energy transfer to the other degrees of freedom that are not captured by your variables. And if you describe the system in terms of all the 10^23 degrees of freedom, there is no heat anymore. All enegy transfer is then described by work. The thermodynamic description then gives an exact description of the physical system, the entropy of the system is then always identical to zero. Count Iblis (talk) 20:26, 13 June 2012 (UTC)

Heat or heat transfer?

This article must decide on the definition of heat. Is heat to be defined as energy (Joules) or energy transfer (Watts - Joules per second)?

Heat is primarily defined as energy transfer by conduction or thermal radiation between two closed systems, as a total amount for a complete process, defined not only by the initial and final states of the two systems, but also by the nature of the process. For some studies in classical irreversible thermodynamics with closed systems, it is useful to consider the process as having a definite time course over a path, with a rate of transfer of energy as heat. For a heat engine with a cyclic process in the working body, the average (over many cycles) rate of transfer of energy as heat is also sometimes useful, the average rate of transfer of heat from the hot reservoir to the cold reservoir, complemented by the rate of transfer of energy as work to the task body. For open systems, it is not quite so easy.Chjoaygame (talk) 13:26, 1 July 2012 (UTC)

"Heat is primarily defined as energy transfer by conduction or...". Thus Joules per second; so nothing to do with the energy (Joules) in the motion of particles? Are you sure? --Damorbel (talk) 08:21, 2 July 2012 (UTC)

Chjoaygame, your note (on your last contribution) says "Heat or heat transfer?: no need to decide; both are useful" sums up the whole article. 'Both being useful' is not an argument for treating them as being the same, it is just like treating electric charge and current as the same, do you do that? If you are not aware of the difference then I suggest you do a little personal research. --Damorbel (talk) 05:35, 5 July 2012 (UTC)

Assessment comment

The comment(s) below were originally left at Talk:Heat/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.

Very important article, but it lacks references Snailwalker | talk 00:05, 21 October 2006 (UTC)

Substituted at 20:12, 26 September 2016 (UTC)

[1] C. Kittel and H. Kroemer Thermal Physics W.H. Freeman and Company, New York, 1980, Chapter 14.

[1]