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Carl E. Mungan, Professor
Carl E. Mungan, Professor

Letters to the Editor

Energy 

I enjoyed reading Art Hobson’s article1 in the Summer 2004 Newsletter and wish to comment on two points dealing with energy.

1. Hobson favors the term “thermal energy transfer” but in the next sentence notes that this should not be confused with “thermal energy.” Unfortunately, I think this is confusing. Let’s consider a different kind of example to clarify the issue. Suppose that two systems A and B interact (in isolation) and we observe that system B has gained charge. The correct conclusion is that system A lost charge and that there was a charge transfer from A to B. Now take this example and substitute “thermal energy” everywhere that the word “charge” occurs. Our correct conclusion is now manifestly wrong (unless we restrict ourselves to calorimetry experiments!). As Art correctly points out, I can change thermal energy by, say, doing work.

In my mind, this is exactly why we should use a term based on “heat” and not on “thermal energy transfer.” I don’t want students to confuse the end result (change in thermal energy of an object) with the process (heat, work, particle transfer, radiative transfer, etc).

Incidentally, I think there’s also a big problem with simply turning heat into the verb “heating.” In common textbook parlance,“heating/cooling” refers to any temperature or phase change, even if done by work. Other slight changes in wording such as “does heating” and “heats” don’t fully resolve this issue. Some educators attempt to impute a distinct meaning to “warming” as a solution, but students generally don’t appreciate such fine shades of meaning.

2. Hobson proposes dropping the term “potential energy” but fails to address the issue of conservative forces. It is because the adjectives “potential” and “conservative” are so closely related that an umbrella term is desired for these forms of energy.

Instead Art suggests using “more descriptive terms” such as elastic energy. But consider say a set of iron atoms joined together with metallic bonds to make a small helical spiral. Should we call the interaction energy between these atoms elastic energy, chemical energy, or electrostatic energy? As others have noted,2 many of these descriptions of “forms” of energy are not well differentiated. In particular, I myself am not clear about the meaning of 3 out of the 5 “common” forms of potential energy listed by Hobson. What exactly is “electromagnetic energy”? If chemical is “microscopic electromagnetic,” why list it separately from “electromagnetic”? As for “nuclear,” does that include rest energy? the electromagnetic repulsion between protons in a nucleus? strong and weak force interactions? only the energy released in fusion/fission reactions?

Energy is a central concept in our introductory courses and clarifying the terminology here is especially important. So I applaud Art’s efforts but am not sure we should start changing the textbook treatments of thermal and potential energy just yet.

1. A. Hobson, “Words matter,” APS Forum on Education Summer 2004 Newsletter, pp. 2-4.

2. E. McIldowie, “A trial of two energies,” Phys. Educ. 39, 212-214 (Mar. 2004). Also see G. Falk, G. Herrmann, and G. B. Schmid, “Energy forms or energy carriers?” Am. J. Phys. 51, 1074-1077 (Dec. 1983).

Carl Mungan, Physics Dept., U.S. Naval Academy, Annapolis, MD, 21402-5040, email: mungan@usna.edu

Comments on “Words Matter”

I enjoyed the Summer 2004 FEd newsletter article "Words Matter" by Art Hobson, and I share many of his concerns. I offer some comments on these issues.

In our calculus-based intro textbook "Matter & Interactions" (http://matterandinteractions.org/), Ruth Chabay and I consistently use the term "thermal energy transfer" and do not use the word "heat." We condition the name by saying this is "energy transfer associated with a temperature difference." Alas, we find experimentally in the classroom that there are serious problems with this too! The students parse the phrase as "the transfer of thermal energy" rather than as "the thermal transfer of energy." As a result they continue to confuse Q with the change in thermal energy.

Chabay has started using the phrase "thermal transfer of energy" in her teaching, which is surely better and may fix the problem. Another possibility is to call Q "microscopic work." This is particularly appropriate in our curriculum, where we deliberately integrate mechanics and thermal physics as one integrated subject rather than as two isolated subjects, and where we emphasize the atomic nature of matter and continually make macro/micro connections (see R. Chabay and B. Sherwood, "Bringing atoms into first-year physics," American Journal of Physics 67, 1045-1050, Dec. 1999 and R. Chabay and B. Sherwood, "Modern Mechanics," American Journal of Physics 72, 439-445, April 2004). The term "microscopic work" is also physically correct and descriptive, in that the energy flow is associated with collisions of (on average) higher-speed molecules with (on average) lower-speed molecules, with interatomic forces acting through distances.

Another approach I've tried is to couch early exercises in terms of situations where there were both W and Q, not just Q. If Q = 50 J flows into water, raising the internal energy of the water by 50 J, the numerical equivalence encourages students to consider these two very different concepts to be the same thing. But if Q = 50 J and there is also W = 20 J of mechanical work done (say by stirring the water very vigorously), then the thermal energy rise is 70 J and is not numerically equal to Q. This seems to help quite a bit.

Using the naming conventions of Fred Reif, we call Newton's second law "the momentum principle" and instead of the third law we speak of the "reciprocity" of gravitational and electric forces, including electric interatomic forces. We introduce Newton's second law in the relativistically correct form dp/dt = Fnet. As for the third law, we downgrade it from the rank of a law since it is not true for magnetic interactions (consider two protons, one at the origin moving in the +x direction and the other on the x-axis moving in the +y direction; you'll find that the magnetic forces are not equal and opposite, and the sum of the particle momenta changes with time, with corresponding change in the field momentum). Rather we focus on the form of the force law and point out the "reciprocity" inherent in the gravitational force law (interchange m1 and m2) and the electric force law (interchange q1 and q2). This change of emphasis helps understand why the (interatomic, electric) forces exerted by the small car on the big truck are just as big as those exerted by the big truck on the small car.

I don't agree that chemical energy is a kind of potential energy, because the molecular energy levels include kinetic energy as well as potential energy. Similarly with nuclear energy, where one can speak of a nuclear potential energy. But the energy levels also involve kinetic energy as well as potential energy. I do agree that "potential energy" is not a very good name. In our course we often speak rather of "pair-wise interaction energy".

Bruce Sherwood, Dept. of Physics, North Carolina State University