Some Physics teaching whispered fallacies
C. H. Wörner
Instituto de Física, Pontificia
Universidad Católica de Valparaíso,
Av. Brasil 2950, Valparaíso
02,
E-mail: cworner@ucv.
Abstract
Some erroneous common facts that appear in a current informal learning context are discussed. In this note, it is shown several examples that illustrate this practice.
I. INTRODUCTION
Informal learning (sometimes called experiential learning) has been recently recognized as a useful method widely used for learning purposes. Some people asset that the use of botanical gardens, science fairs or exhibitions, museums, etc. may be used -and in fact they had been used- for the enhancement of science knowledge. Other learning situations described by this term refer to the side-by-side knowledge obtained for fellows in the same environmental situation, i.e. by high school or undergraduate students. A third situation came today in mind: it is not really necessary to have a physical neighborhood to access the information. The World Wide Web network is a source of an almost infinite quantity of informal data and facts. Furthermore, they are advocates for the thesis that, because all the information is located in the web, the main purpose of instruction is to learn to use it and not to discuss the content of this information. Last but not least, I guess that almost all you (and me) know about PCs arises from informal learning.
Appealing as it is, it is not the
purpose of this note to discuss the utility and pertinence of these learning
resources. I simply stress the fact that we, Physics teachers, are submitted to
this type of influence since the pre-net times. Also, caution must be exerted
as that this type of learning must be severely tested before acceptation. In order
to discuss this fact, I will present some examples of commonly believed facts,
that, although have been shown false in the literature, continue to be taught
by a kind of fellow-to-fellow heritage. I will call these cases myths, in a
different sense that other well know (proper) myths such as the
A recent nice account by Sawiki [2] on myths about gravity and tides triggered the present note.
II. EXAMPLES
A. The coincidence of
To be in tune with the last paragraph let us consider the
common statement that the year Galileo dies,
The fallacy stems from the fact
that both dates belong to different calendars. The
B. The flow of cathedral window glasses
It is a well-known fact that atoms in glasses and liquids present a disordered state or short-range order as opposite to crystalline materials that present long-range order. Therefore, solid glasses (as the ones belonging to cathedral windows) must possess liquid properties, that is, they must flow. Surely, solid glass viscosity must be greater than ordinary liquids, but for long exposure times, the consequent deformation can be noted. Medieval glass windows seems to be the perfect target for testing purposes and it is asserted that they are wider in its lower than in its upper edges.
In fact, it has been shown [4] that this effect is measurable at room temperature during historical times (say, 5000 years), and therefore the thickness difference, if it exist, can be attributed to manufacturing defects. Furthermore, it is curious that older glass objects (ancient glass vases) do not seem to show this effect and do not appear in this myth.
C. The different rotation sense of water into a washbasin exhaust in the North and South hemispheres
Vortex movements are well known phenomena in domestic bathroom environments. Claims had been whispered that due Coriolis non-inertial forces liquid spins in opposite senses in different hemispheres, in due account for the earth rotation around its North-South axis.
This argument has been labeled an "old wives' tale" by Crane [5], showing that this effect would be unnoticeable if the liquid is initially quiet. What the observer visually perceives is the increase in rotation (due to angular momentum conservation) of the liquid due to the fact that its initial angular momentum is different from zero. If fact, it is possible to whirl the liquid in either sense, by simply selecting the initial angular moment of the system in a "clockwise" or "anti-clockwise" state.
D. Römer’s "measurement" of the velocity of light
Galileo -in his book "Two New Sciences"- proposes an experiment to measure the velocity of light. In the words of Salviati (Galileo himself) the unsuccessful experiment was not due to the instantaneous propagation of light, instead "(light propagation) if not instantaneous, ...is very swift" [6]. In 1676, Olef Römer [7] communicates his explanation on the apparent anomaly in the measurement of the period of the Jupiter’s moons. This fact is usually mentioned as the first measurement of the velocity of light. In fact, Römer does not communicate this measurement, noting only that "le retardement de la lumiere" (that is, the finite velocity of light) is a necessary condition to explain the astronomical phenomenon. Based in Römer data, Huygens indeed published the first known figure for the velocity of light (for further comments, see the papers of Wrobleski [8] and Saito [9]).
E. The colored rainbow
"The traditional description of the rainbow is that it
is made up of seven colors - red, orange, yellow,
green, blue, indigo, and violet. Actually, the rainbow is a
whole continuum of colors from red to violet and even
beyond the colors that the eye can see" [10].
Written in this quote is the correct description of the colors
observed in a rainbow as appears in the cited web page. The common belief in
the quantization of colors is obviously untrue.
Perhaps the equivoque follows from a misquote of
F. The Franck and Hertz experiment
It is usually assumed that the well known Franck and Hertz experiment is a consequent confirmation of the quantum hypothesis first proposed by Bohr. As remarked by Franck himself, "It might interest you to know that when we made the experiments we did not know Bohr's theory. We had neither read nor heard about it" (quoted by Holton [12]). In fact, they pretend to measure the ionization energy of mercury and misinterpreted its own results [13].
G. Kinetics at T=0oK
Dialog from the web [14] between teacher (T) and pupil (P):
- Question (T): "Well, look what happens if you set the temperature of the atoms in the box as low as it can go."
- Answer (P): "The atoms are stopped."
- Conclusion (T): "So that is as cold as the atoms can be. We call that Absolute Zero."
This dialog between teacher and student shows one expression for the fallacy that at 0K, there is no movement at all. The origin of this error perhaps stems from the (spurious) extrapolation of the ideal gas laws to low temperatures. On the contrary, it is a standard topic in Physics texts to study the Fermi-Dirac distribution showing that even at T=0oK, a free electron gas has a lot of (kinetic) energy.
III. FINAL REMARKS
A good pedagogical advice is "not always believe written arguments" (by the only merit of being written). A better would be "not always believe whispered arguments" (although written on the net). Informal learning is an inevitable and useful way of learning, but extreme care must be exerted on the examination of the offering evidence. Recourse to authoritative sources (although old fashioned) or better, original papers, must be considered.
Finally, the author does not assume to know all these myths and therefore the reader would add his/her experiences on true/untrue explanations for known/unknown phenomena.
ACKNOWLEDGMENTS
The author acknowledges helpful discussions with G. Iommi Amunátegui and A. Romero.
REFERENCES
[1] Quotations are untranslatable, but in a free version: "if it is not true, it seems to be"; or in a more literal mode, "if it is not true, it is worth to be discovered".
[2] Sawiki, M., Myths about Gravity and Tides, Phys. Teach. 37, 438-441 (1999).
[3] Babb, S. E., When was
[4] Dutra Zanotto, E., Do cathedral glasses flow?, Am. J. Phys. 66, 392-395 (1998).
[5] Crane, H. R., A Tornado in a Soda Bottle and Angular Momentum in the Washbasin, Phys. Teach. 25, 516-517 (1987).
[6] Galilei, G., Two New Sciences, translated by S. Drake (Wall & Emerson, Toronto, 1989) pp. 50-51.
[7] Although not directly written by Römer, this communication appeared in Journal des Savants, 233-236 (1676) and an English version in Philos. Trans. R. Society, 62, 893-894 (1677).
[8] Wroblewski, A., de Mora Luminis, A Spectacle in Two Acts with a Prologue and an Epilogue, Am. J. Phys. 53, 620-630 (1985).
[9] Saito, Y., A Discussion of Roemer Discovery concerning the Speed of Light, AAPPS Bull. 15, 9-17 (2005).
[10] http://www.eo.ucar.edu/rainbows/, Oct. 15, 2007.
[11]
Am. J. Phys. 61,108-112 (1993).
[12] Holton, G., On the Recent Past of Physics, Am. J. Phys. 29, 805-810 (1961).
[13] Frank, J. and Hertz, G., Collisions between Electron and Mercury Vapors Molecules and the Ionization Potential of such Molecules (English translation of Verhand. Deut. Physik 16, 457-467 (1914)) in The World of the Atom, Vol. 1. (Henry A. Boorse and Lloyd Motz, Eds., Basic Books, New York, 1966).
[14]http://www.colorado.edu/physics/2000/bec/temperature.html, Oct. 15, 2007.
A version of this article appeared in the Latin American Journal of Physics Education, Vol. 2, No. 1, January 2008.