Abstract
“What is heat?” was the title of a 1954 article by Freeman J. Dyson, published in Scientific American. Apparently, it was appropriate to ask this question at that time. The answer is given in the very first sentence of the article: heat is disordered energy. We will ask the same question again, but with a different expectation for its answer. Let us imagine that all the thermodynamic knowledge is already available: both the theory of phenomenological thermodynamics and that of statistical thermodynamics, including quantum statistics, but that the term “heat” has not yet been attributed to any of the variables of the theory. With the question “What is heat?” we now mean: which of the physical quantities deserves this name? There are several candidates: the quantities , , and . We can then formulate a desideratum, or a profile: What properties should such a measure of the quantity or amount of heat ideally have? Then, we evaluate all the candidates for their suitability. It turns out that the winner is the quantity , which we know by the name of entropy. In the second part of the paper, we examine why entropy has not succeeded in establishing itself as a measure for the amount of heat, and we show that there is a real chance today to make up for what was missed.
Highlights
Which Physical Quantity DeservesToday we have a comprehensive knowledge about how states and processes which we consider to belong to thermodynamics can be described macroscopically as well as microscopically
A more modern, enlightened and perhaps somewhat disappointing answer to the question would be: heat is the name given to a physical quantity, i.e., a variable in a well-working theory
The question of what is the nature of heat was not important to him. The two terms he uses, namely “Chaleur” and “Calorique”, do not express any essential difference for him, as one can see in the following statement [15] (p. 15): “We find it useless to explain here what is the quantity of caloric or quantity of heat, nor to describe how these quantities are measured by the calorimeter
Summary
Today we have a comprehensive knowledge about how states and processes which we consider to belong to thermodynamics can be described macroscopically as well as microscopically. A more modern, enlightened and perhaps somewhat disappointing answer to the question would be: heat is the name given to a physical quantity, i.e., a variable in a well-working theory. Among them is the quantity Q (which received the term after all), and the quantities H and S (today called “enthalpy” and “entropy”) Another candidate is a quantity which does not have a symbol of its own, and which is called “thermal energy”. All of these quantities have in common that in some context one deals with them as with measures of an amount of heat.
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