Letters to the Editor 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://www4.ncsu.edu/~rwchabay/mi) 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(12), 1045-1050, Dec. 1999 and R. Chabay and B. Sherwood, "Modern Mechanics," American Journal of Physics 72(4), 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 actingthrough 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 equalto 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'ssecond law in the relativistically correct form dp_vector/dt = Fnet_vector. 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 gravitional 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 bythe 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 EnergyI 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, 21402-5040email: mungan@usna.edu
Voodoo ScienceFor three years I've taught the second semester of algebra-based introductory physics at Emory University. Late in the semester I have them read Chapter 7 from the book Voodoo Science by Robert Park (Oxford University Press, 2000). The chapter discusses the controversy surrounding whether or not electric power lines cause cancer. The chapter includes many of the scientific (and non-scientific) studies done, and makes the emphatic point that all the evidence shows there is no link, that electric power lines do not cause cancer. Furthermore, it discusses in detail much of the bad science and bad journalism surrounding the issue. The reading blends two topics: electric power lines, related to the physics they're learning, and health issues. The class is predominantly pre-medical students, and they find the article quite interesting. After the students have done the reading as homework, I lead a class discussion. I try hard to only ask questions and make very few statements myself -- in general, the reading has already done an excellent job defending the views I personally hold. The students always enthusiastically participate in the discussion; in groups of up to 50, over half of them offer a comment at least once during the discussion. Usually the article gets them quite excited, in fact; many of the students are passionate about the health issues the reading raises. One year, the class included more than two hundred students, so I broke them up into separate smaller groups over two days, which resulted in reasonable discussions despite the large class size. Later, as a homework assignment, the students write a two paragraph essay responding to the reading, either on one of the questions I suggest, or on a topic of their own. These essays are mostly quite good. Student feedback, both informal and on the anonymous end-of-semester evaluations, indicates that they really enjoy the reading. I have also taught the calculus-based introductory class, and assigned supplemental readings, covering topics such as nanotechnology, general relativity, and pseudoscience. Related homework assignments included a brief essay response to the reading, or locating a current article about the topic and explaining it in their own words. The essays again are very good and students indicate they enjoy the assignment, and the break from the traditional physics curriculum. The _Voodoo Science_ reading seems to be the favorite, though, for both the algebra-based and calculus-based classes. You can get permission to distribute copies of Chapter 7 from Voodoo Science from www.copyright.com for a fee. Alternatively, your campus bookstore may be able to handle this for you by letting you distribute a coursepack with the copies, with the copyright fee as part of the price. Interested teachers can contact me by email for a list of discussion questions I've used with the Voodoo Science reading. Don't worry, using email won't cause cancer either! Eric R. Weeks, Physics Department, Emory University, Atlanta, GA 30322weeks@physics.emory.edu
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