FEd April 1999 Newsletter - Why I teach my students things that are incorrect

April 1999



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Why I teach my students things that are incorrect

S.L. Haan

We physicists can be sticklers for correctness. It grates on us when we see scientific errors in books, movies, and so forth. It especially grates on us when we see errors in educational materials. I remember my first encounters as an assistant professor with elementary school science materials. I was appalled at the errors, and I wrote letters to publishers to try to get the errors corrected. I also made a very conscious effort in my first years of teaching to avoid teaching anything that I knew to be wrong. If a concept was too complex to be completely understood by beginning students, but the material was unavoidable, I'd teach my students just part of the concept.

Now I regularly teach things that I know are incorrect. Without bashfulness. I'm even admitting publicly that I do it.

What are some examples of things I teach that are incorrect? Among other things, I teach that planets travel in elliptical orbits around the sun. I teach that mass is conserved. I teach that force equals mass times acceleration. I teach that warm air holds more moisture than cold air. I even teach that electrons orbit atomic nuclei somewhat like miniature planets orbiting a sun.

Planetary orbits around the sun are not perfectly elliptical. The interactions between the planets (and occasional asteroids or meteors) perturb them; variations in the orbit of Mercury provided important tests of general relativity; and so forth. Each of the things listed is incorrect--but in some ways is "almost correct" and even useful. In fact, I submit that each person should learn that these things are true before he or she can appreciate that they are not true. The equivalence of mass and energy, for example, becomes significant only if one already understands the classical idea that mass and energy are conserved separately.

Am I dishonest to my students when I teach them ideas that I know are incorrect? If I am teaching an introductory course in classical mechanics, do I have a responsibility to go beyond teaching "mass is conserved" or "F=ma" to include a disclaimer that it really isn't so? I've tried including disclaimers in the past, but feedback from student journals and exams has led me to conclude that more often than not my disclaimers just confuse the students.

Yet I do admit that I find it somewhat unsettling to hammer on a concept that I know is incorrect, or at best is only a partial truth. Happily, I have found a way that I can teach incorrect ideas with integrity. What I do is to teach my students models, and I tell my students what I'm doing. I don't teach that electrons orbit nuclei like little miniature planets orbiting the sun as if it were the final word on atomic structure--instead I teach a model of the atom, and I tell my students that I'm teaching them about a model. My goal is that when I've finished, they will understand the model and also recognize that what they have learned is a model and not the "final word."

Of course, for this approach to be effective students need to understand what models are. Consequently, I spend some class time specifically discussing models. We discuss that some models can be physical constructions--e.g., a globe as a model of the earth--while other models are sets of analogies or ideas that are intended to help us understand intrinsically complex systems. For example, the "particle model" of matter postulates that all matter is composed of indestructible particles called atoms, whose masses are conserved, and which can be rearranged in a host of ways to form different molecules.

So I'm actually quite selective about what I teach that's "incorrect." I'm still very careful to try to get the facts --i.e., the data itself--correct. It's only in the explanatory principles that I allow for "mistruths" to arise. I don't feel obligated to try to tell students everything I know about a topic.

In some ways, my teaching hasn't changed much. But one very important change is that in the past I rarely made a conscious effort to tell my students when I was teaching a model or theory, so I don't think they were as prepared for learning refinements or revisions as my present students. When students know that what they have learned is a model, they will be naturally inquisitive about the limitations and shortcomings of the model.

Since students apparently must construct a scaffold of understanding in order to learn physics, I have come to the conclusion that it is very dangerous to try to avoid teaching something that is wrong by distilling out only certain concepts from a complex system. For example, if we want to teach about atomic structure and we only teach that electrons are bound to atomic nuclei and that various atomic states can be described in terms of energy, then as far as I know we haven't said anything that is wrong. (Note that I avoided saying that electrons could only have certain prescribed energies, since I know that would be incorrect--states which are coherent superpositions of energy levels are quite possible.) But what have the students learned? Perhaps empty verbalism. Or perhaps they have constructed their own model of an atom and have somehow grafted our teaching onto it. I think they'd be a lot better off learning the Bohr model of the atom in its entirety (including its popular extension into multi-electron atoms that makes my rigorous side cringe). Then, when students are ready, we can refine the model or teach them a whole new model.

In many ways using models in our teaching is consistent with the historical development of science. Our scientific theories are themselves in many ways just models that have grown in complexity and depth with time, or perhaps have been replaced by superior models. Our theories may themselves need additional refinement in the future. A hundred years ago someone could teach F= ma and believe it to be fully correct. I believe we should still teach it today without obfuscating it in a cloud of relativity; we just need to let our students know that what we're teaching--and all our scientific theories--should not be considered the "final word" on a subject.

By the way, I still try my hardest to stamp out factual errors. There's no excuse for teaching facts that aren't facts--such as that the Coriolis effect causes water in sinks to spin counterclockwise in the Northern Hemisphere. Or making movies that have noises on the moon or in space. Or ...


Stanley L. Haan is Professor and Chair of the Department of Physics and Astronomy at Calvin College in Grand Rapids, MI