The nature of heat (noun or verb?)

Question: Explain the nature of heat.

There is no consensual definition of heat. For example, Baierlein (1994) opines that heat is an adjective, Zemansky (1970) disagrees that heat is a verb, and Romer (2001) proposes that heat is not a noun. Essentially, heat can be distinguished as a “transfer” of energy (verb) and “energy” transferred (noun). However, it can be confusing for students when heat may mean “a form of energy” or “process of energy transfer” within a textbook.

In the nineteenth century, there were two competing concepts of heat: “caloric (or material) theory” and “kinetic (or mechanical) theory” (Chang 2004). In the early twentieth century, the term heat was even more confusing because there were at least three different definitions of heat: (1) energy in transition from a hot to a cold body, with the usual symbol Q; (2) internal energy or energy stored in a body, with the symbols E or U; and (3) enthalpy and it can be represented by the function U + PV, with the symbol H (Stuart, 1938). Currently, you may find different definitions of heat in biology, chemistry, and physics. Importantly, there are differences in opinion whether it is appropriate to use words such as flow or transfer in definitions of heat because they have the connotations that heat is a form of fluid or substance.

How would Feynman answer?

       It is possible that Feynman would explain heat as a noun and a verb (or a process) and discuss problems of defining heat as shown below.

1. Heat is a noun:

During a British Broadcasting Corporation (BBC) television interview, Feynman (1994) explains that “you can either have the idea that heat is some kind of a fluid which flows from a hot thing, and leaks into the cold thing; or you can have a deeper understanding, which is closer to the way it is – that the atoms are jiggling, and their jiggling passes their motion on to the others (p. 127).” However, the notion of heat as a form of fluid can be attributed to the caloric theory of heat in the nineteen century or earlier. In addition, de Berg (2008) clarifies that “[t]he terms, heat flow, or energy flow, are remnants of the old caloric theory of heat in which heat was considered as a material fluid that had the capacity to flow. Identifying heat as motion is also a remnant of the early kinetic ideas of the 19th century (p. 80).” Thus, some scientists and educational researchers might consider Feynman to be misleading the public.

Similarly, in The Feynman Lectures on Physics, Feynman mentions that “the jiggling motion is what we represent as heat: when we increase the temperature, we increase the motion (Feynman et al., 1963, section 1–2 Matter is made of atoms).” Furthermore, he elaborates that “we can change the amount of heat. What is the heat in the case of ice? The atoms are not standing still. They are jiggling and vibrating (Feynman et al., 1963, section 1–2 Matter is made of atoms).” Feynman considers heat to be due to the kinetic energy of atoms or atomic vibrations. Essentially, the amount of heat is dependent on the temperature, and thus, heat is a noun.

Feynman elaborates that “[t]he heat is ordinarily in the form of the molecular motion of the hot gas (Feynman et al., 1963, section 1–4 Chemical reactions).” This would suggest that heat is the internal energy of a system. This description of heat is commonly found in biology textbooks (Doige & Day, 2012). Therefore, students may find it confusing because heat may mean “internal energy” and “energy in transit.” More importantly, when we define the First Law of Thermodynamics in terms of ΔU = Q + W, it becomes necessary to distinguish the internal energy U and heat Q as the energy in transit due to a temperature difference. In other words, we should not define heat as internal energy (U) and energy in transit (Q) that can be found in the same equation.

2. Heat is a verb:

In Feynman’s own words, “[i]f we heat the water, the jiggling increases and the volume between the atoms increases, and if the heating continues there comes a time when the pull between the molecules is not enough to hold them together and they do fly apart and become separated from one another (Feynman et al., 1963, section 1–2 Matter is made of atoms).” Feynman also uses the term heat as a verb. However, this is different from some physicists and physics educator who only use heat as a verb or define heat as a process of energy transfer. The use of heat as a process also clearly means that heat is not a form of substance or fluid.

In formulating the first law of thermodynamics, Feynman mentions that “[l]et us begin by stating the first law, the conservation of energy: if one has a system and puts heat into it, and does work on it, then its energy is increased by the heat put in and the work done. We can write this as follows: The heat Q put into the system, plus the W done on the system, is the increase in the energy U of the system; the latter energy is sometimes called the internal energy: Change in U = Q + W (Feynman et al., 1963, section 44–1 Heat engines; the first law).” In short, the term heat may be used when there is a non-mechanical transfer of energy into a system. Nevertheless, we can be more precise by defining heat as a method of energy transfer due to a temperature difference.

Interestingly, Feynman explains that “when we stretch a rubber band it heats, and when we release the tension of the band it cools. Now our instincts might suggest that if we heated a band, it might pull: that the fact that pulling a band heats it might imply that heating a band should cause it to contract (Feynman et al., 1963, section 44–1 Heat engines; the first law).” Conversely, Feynman states that “[w]hen we stretch a rubber band, we find that its temperature falls (Feynman et al., 1963, section 45–2 Applications).” That is, Feynman contradicts himself in the previous chapter by saying that the temperature falls. However, this is likely a careless mistake because the temperature should increase when the rubber band is stretched. It can be simply explained by the first law of thermodynamics, ΔU = ΔQ + FΔL. Feynman’s mistake could be related to the use of a mathematical expression for work done by the rubber band, −FΔL.

3. Problems of defining heat: 

Some physicists prefer to define heat in terms of the first law of thermodynamics. Nevertheless, Feynman explains that “[i]f we have a hot thing and a cold thing, the heat goes from hot to cold. So the law of entropy is one such law. But we expect to understand the law of entropy from the point of view of mechanics. In fact, we have just been successful in obtaining all the consequences of the argument that heat cannot flow backwards by itself from just mechanical arguments, and we thereby obtained an understanding of the Second Law. Apparently, we can get irreversibility from reversible equations… Since our question has to do with the entropy, our problem is to try to find a microscopic description of entropy (Feynman et al., 1963, section 46–4 Irreversibility).” Thus, it is possible that Feynman would relate a problem of defining heat to the law of entropy.

On the other hand, Canagaratna (1969) argues that another problem of defining heat is a problem of defining a measure of thermal interactions. He explains that “the ice-calorimetric method and the heat capacity method are unable to define q for irreversible processes taking place between any two bodies. Since the concept of heat has no necessary connection with reversible processes, it must be concluded that q can be defined in all its generality only through the use of the first law (Canagaranta, 1969, p. 683).” In essence, he proposes operational definitions of heat by using an ice-calorimetric method and heat capacity method. Moreover, Canagaranta (1969) opines that heat should be defined only by using the first law of thermodynamics, but it may involve irreversible mechanical work experimentally.

Lastly, and ideally, a scientific term should have only a definition such that there is no confusion when the term is used. Currently, there are daily definitions of heat that are not related to science. In addition, the term heat has a variety of definitions that can be used differently in biology, chemistry, and physics. For instance, definitions of heat may mean internal energy in biology and include terms such as “in contact” in chemistry. There are also different opinions how heat should be defined in physics.

       To conclude, Feynman would use the word heat as a noun or a verb. Importantly, some scientists and educational researchers disagree with him in explaining heat as a form of fluid, and defining heat as the internal energy. However, Feynman might discuss problems of defining heat or how heat could be used differently depending on the context.

Note: 

1. During the Messenger Lectures, Feynman (1965) mentions that “[h]eat is supposed to be jiggling, and the word for a hot thing is just the word for a mass of atoms which are jiggling. But for a while, if we are talking about heat, we sometimes forget about the atoms jiggling (p. 124).” 

2. In the words of Feynman, “[w]e call this form of energy heat energy, but we know that it is not really a new form, it is just kinetic energy — internal motion (Feynman et al., 1963, section 4–4 Other forms of energy).”

References:

1. Baierlein, R. (1994). Entropy and the second law: A pedagogical alternative. American Journal of Physics, 62(1), 15–26.

2. Canagaratna, S. G. (1969). Critique of the definitions of heat. American Journal of Physics, 37(7), 679–683. 

3. Chang, H. (2004). Inventing temperature. Oxford, United Kingdom: Oxford University Press. 

4. De Berg, K. C. (2008). The Concepts of Heat and Temperature: The Problem of Determining the Content for the Construction of an Historical Case Study which is Sensitive to Nature of Science Issues and Teaching–Learning Issues. Science & Education, 17(1), 75–114. 

5. Doige, C. A. & Day, T. (2012). A typology of undergraduate textbook definitions of ‘heat’ across science disciplines. International Journal of Science Education, 34(5), 677–700.

6. Feynman, R. P. (1965). The character of physical law. Cambridge: MIT Press. 
7. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley. 

8. Feynman, R. P. (1994). No Ordinary Genius: The Illustrated Richard Feynman. New York: W. W. Norton & Company.

9. Romer, R. H. (2001). Heat is not a noun. American Journal of Physics, 69(2), 107–109. 

10. Stuart, M. C. (1938). Use and Meaning of the Term Heat. American Journal of Physics, 6(1), 40.

11. Zemansky, M. W. (1970). The use and misuse of the word “heat” in physics teaching. The Physics Teacher, 8(6), 295–300.