We are much struck with the desirable results that follow from introducing brief discussions of modern physics topics during the course of ordinary, relatively routine first courses in physics for college freshmen and sophomores. We propose in this paper that teachers of secondary physics courses might well also consider the incorporation of such discussions into their courses

Flansburg, L. D.

Kasper, J. E.

University of Northern Iowa

Iowa Science Teachers Journal Volume 5 Number 1 Article 17 1967 Some Trends in College Physics and Their Reflections on High School Physics J. E. Kasper Coe College L. D. Flansburg Coe College Follow this and additional works at: https://scholarworks.uni.edu/istj  Part of the Science and Mathematics Education Commons Let us know how access to this document benefits you Copyright © Copyright 1967 by the Iowa Academy of Science Recommended Citation Kasper, J. E. and Flansburg, L. D. (1967) "Some Trends in College Physics and Their Reflections on High School Physics," Iowa Science Teachers Journal: Vol. 5 : No. 1 , Article 17. Available at: https://scholarworks.uni.edu/istj/vol5/iss1/17 This Article is brought to you for free and open access by the Iowa Academy of Science at UNI ScholarWorks. It has been accepted for inclusion in Iowa Science Teachers Journal by an authorized editor of UNI ScholarWorks. For more information, please contact scholarworks@uni.edu. Some Trends in College Physics and Their Reflections on High School Physics J. E. KASPER and L. D. FLANSBURG Coe College This paper was presented at the Iowa Science Teachers' Association Physics Section, at Des Moines, Oc~ tober 21, 1966. We are much struck with the desir-able results that follow from intro-ducing brief discussions of modern physics topics during the course of ordinary, relatively routine first courses in physics for college fresh-men and sophomores. We propose in this paper that teachers of secondary physics courses might well also con-sider the incorporation of such dis-cussions into their courses. · We claim that generally one can achieve considerable effects by spend-ing only a few minutes of a class peri-od at this, though some topics would require more time for their discus-sion. In either case, some of the ad-vantages are : ( 1) These topics represent physics as it is today. ( 2) They tend to motivate students by showing use of classical ideas in more exciting modern contexts or by showing the breakdown of classical ideas. ( 3) They tend to cause students to look more critically at classical ideas, and to take them less as rigidly and certainly estab-lished. ( 4) Sometimes students are led to investigate these topics more on their own, and so to extend their areas of awareness. In the following we attempt to be concrete, rather than philosophical, by showing in some detail how a few se-lected topics can be dealt with. We also append a suggested list of other topics by title only. Early in a course in physics, a stu-dent ought to realize the kinds of physical theory which exist and some-thing of their interrelations and limi-tations. For example, Newtonian me-chanics is usually the first branch of physics studied, and both overt and subtle effects ought to follow if a stu-dent realizes that it is applicable only in a certain domain, and is actually a limiting case of a larger, more general theory. In a few minutes time, one could draw on a blackboard the chart shown in Figure 1. SPE ED 22 RELATIVISTIC QUANTUM M ECHA NIC S QUANT U M ME C HANI C S RE L AT I VITY NEWTONI AN ME CHANI CS. SIZE Figure 1. Relationships of the various physical theories to the speed and size of objects. Among the things to be gained by such a brief discussion are that the student learns at the outset that New-tonian mechanics is a limited theory, and not a cornerstone of a fixed kind of theory, that he at least learns that there is a radically different kind of theory called Quantum Theory and something about when it is important, and that neither relativity nor quan-tum mechanics is complete, codified , and settled. Students seem to find it exciting that theories are only tentative, in-complete, and subject to modifica-tion. Newtonian mechanics seen in the more general realm of discourse looks better. At many points in mechanics, one can take a few minutes to say some-thing about how the particular topic being discussed looks in relativistic theory. An example is the second law F = ma. Point out that this apparently axiomatically true law, which students tend to swallow whole, is not really true. This might be the thing which would cause the student to consider much more critically all the rest of Newtonian mechanics. It might even lead him to buy a paperback book on relativity theory, and to read it. Many books still say that mass is a measure of quantity of matter, though good ones point out that this is not really an acceptable definition. One then has an ideal reason for discussing E = mc2 briefly, and then explaining how masses may change. It would not take long even to write out the masses for a set of protons and neutrons, and then to show that the mass of these particles when combined into a nu-cleus is less than the original total mass. As soon as the idea of force is intro-duced, it seems desirable to state ex-actly what forces exist in nature. There are these: electric, magnetic, strong nuclear, weak nuclear, and gra-vitational forces. Their relative strengths should be realized. The fundamental puzzle about why these forces and these only exist is enticing. Also, a student can be kd easily ( by merely mentioning that the problem exists) to tum his thoughts to the true natures of some forces which his book mentions, but which are not in the list. Such a force is the friction-al force. Another is air drag. Another is the force by which a table supports an object. At the forefront of physics today, one is left with little that is reliable to cling to. Forces and potential t'Jlergy functions begin to fa il one. Tlw weak nuclear force is not understood at all , and the strong nuclear force simply cannot be represented o~ thought of in a way which compares with the way in which one deals with, say, the gravitational force. 23 Instead, conservation laws come to the fore. We suggest that wh~n the first classical conservation law comes up in class, that one could well de-vote a little time to discussing the cur-rent basic importance of conservation laws. It might be desirable to list the conservation laws currently known. If an instructor thinks the effort is really worthwhile, then he could well go on to point out relations between sym-metries and the conservation laws. Again, the rem<;>val of the idea of physical ideas as being set for all time in the eighteenth or nineteenth cen-turies, and the insertion of the idea that we have at best an approximate and tentative notion of how the uni-verse hangs together, is important. A student might actually begin to think that he could help mankind learn more, since so little is really settled .. . the door is open to all. Fundamental Particles So much of the subject of physics in this mid-twentieth century era is bound up with the mysteries of the so-called fundamental particles, that stu-dents should at the very least have an idea of what particles exist, what their leading properties are, and why the questions concerning them are of transcendent importance. A second-ary-school course may incorporate these matters in its latter stages, but we suggest that there is advantage in opening up the subject very early in a course. Whenever in the course the structure of matter arises as a topic, as it so frequently does, one can make use of a prior introduction of a list of the fundamental particles. This would tend to give the entire course a much more modern flavor, and to provoke real interest. It would make it possible for a student to read or at least to ex-amine some of the literature more easily-Scientific American articles, for example. As a concrete suggestion, we men-tion that straightforward collision problems could be made to seem morP-exciting by having them deal with in-teractions between neutrons or other particles. Again, centripetal force ap-plied to the · simple hydrogen atom would be a welcome addition to rocks b eing spun at the ends of strings, as has been the study of simple questions about satellite motions. When the sub-ject of electricity is entered upon, hosts of occasions for referring to fun-damental particles arise. What would happen if one replaced the proton and electron in a hydrogen atom by a posi-tron and an electron, respectively? By a proton and a negative or meson? Would a pi meson behave according to the principles which explain why an electron can move in a circular path in a uniform magnetic field? Of the many particular topics in solid state physics which could be used, let me select electrical conduc-tion and discuss how it might be pre-sented to secondary physics students. Certainly in a development such as this, I make use of concepts that most secondary students have not encoun-tered. Part of the point of this discus-sion is to conyince you that these con-cepts should be part of a secondary course. After all, these concepts are part of our most basic understanding of nature. For an electron, moving in the metal lattice, the dominate character-istics will, of course, be of the de Broglie wavelength, = h/ p, where h = Planck's constant and p = momentum of the electron. So if we consider what will happen in a peri-odic lattice, we see that the electron will act very much like a wave travel-ing through a periodic lattice ( X-ray through a crystal for instance) . The 8 mm cartridge film gives students a quick and reliable picture of what happens. 24 Ask the students to note that: 1 ) a wave is generally reflected, 2 ) wave is transmitted when wave-length has proper relationship to the crystal lattice dimension and the angle. Then an interpretation in terms of electron motion in periodic lattice might point out that: 1 ) in a perfect crystal, an electron would generally not be transmit-ted and 2 ) electrons with particular mo-ments , p = h/ , where the wavelength satisfied the Bragg condition would be transmitted. The significance in this is that in a perfect lattice, an electron of the proper momenta ( and consequently of the proper energy) would be trans-mitted without resistance. But every student knows that electrons do ex-perience resistance, so an explanation is in order and even your student might quickly spot that the copper wire is not a perfect crystal lattice. You might suggest their looking at it this way. If you consider a more rea-listic picture of the lattice, it would be composed of many separate crys-tals joined along their surfaces . .rc,,yif .. l IJ.w., ,Ja.,,y I. • I I •/ ( ,. \ \ . \ \ \ \ . ,· Figure 2. Transmission of electrons in an irregular crystal. Then based on the ripple tank model, what would they expect to happen to a wave propagating through the crys-tal on the left ( Fig. 2) in the direction indicated? The answer at which they should arrive is, of course, that the wave will probably not satisfy the Bragg condition for the crystal on the 25 right ( Fig. 2 ) and, consequent]~,, will be refl ected ( or sca ttered ) off of the crys tal. In the scattering events, the electron would generally give up some energy to the lattice and this re-peated scattering is the phenomena of resistance ( the scattering centers arc not completely fixed so some motion will result and consequently energy will be transferred ), Going back to the interpretation of the film loop, we can get still more mileage from this simple idea of elec-tron motion through a crystal. The permitted wavelengths , you will re-call , were fractional multiples of the fundamental wavelength and the wavelengths were related to the mo-mentum by = h/ p or = h/ mv, where m = mass of the electron and v = vdocity of the electron. So then the permitted wavelengths determine a set of permitted energies ( Fig. 3 ). E-1 E, Figure 3. Permitted energies of trans-mitted waves E1, E2, E3-. Then point out that this suggests the band structure of solids which is im-portant in the explanation of semicon-ductor phenomena. This simple model doesn't say any-thing about the width of the bands. If you wish to pursue this with your c:lass . you might argue that in a crys-tal the energy levels have widths by pointing out that for an isolated atom, we expect exact energy levels, but for two closely spaced atoms the exclu-sion principle ( Paulie exclusion prin-ciple) prohibits the energy levels hav-ing exactly the same value, so two very closely spaced levels appear ( Fig. 4) . Then it follows that when the order of 1022 · atoms are put to-gether in a crystal there are the or-der of 1022 very closely spaced levels and these form an energy band with, as with any electron energy levels, two electrons permitted per level. Carrying this description even fur-ther, you could give a quite accurate distinction between insulators and conductors based on the level to which the energy bands are filled by GL va...c.a.nt bo. .,J b Figure 4. Energy bands that may be oc-cupied by electrons. available electrons ( Fig. 5). Then by considering the energy re-quired to move an electron from a filled level to a vacant ( or conduc-tion ) level, it 'will be obvious that this model properly describes the low resistance of conductors and the high resistance of insulators. The descrip-tion of semiconductors, of course, fol-C Figure 5. Energy Spectra for (a) Insulators, ( b) Conductors and ( c) Semi-conductors 26 lows directly by making the energy gap ( from filled level to vacant band) less than for an insulator, but still not a conductor ( Fig. 5). and then a discussion of semiconduc-tor diodes and transistors. Other topics reflecting a more cur-rent point of view could be developed in areas such as: 1) specific heats, 2) emission of electrons from metals, 3) magnetism, and 4) elementary parti-cles, to list a few. This could easily be followed by a discussion of the distinction between intrinsic and doped semiconductors FOR THOSE INTERESTED The following copies of special issues of The Oregon Science Teacher are available in the quantities and at the costs indicated : Issue F eb. 66 Apr. 66 Oct. 66 Dec. 66 Feb. 67 Jan. 66 No. of pages Content 24 QUICKIES: 202 easy ideas for class-room demonstration and "discussion starters" ( with index ) 18 12 A volume of science humor ( parodies, puns, cartoons, etc. ) Cost of Conversion to the Metric System ( an economist's report on some little-known aspects of this problem) Price $0.'25 .25 .25 20 Polarized Light ( carefully selected .50 26 52 articles on the basic principles; ideas for classroom use; references and available materials) A SCIENCE Reader ( a quartet of spe- .50 cially selected articles of particular interest -and adaptability-to secondary science teaching) 1966 Science Teachers calendar ( cal- .25 endar no good, but included data still valid: 65 per cent of dates list at least one event of interest to science teachers and/ or students) Orders will be filled on a "first-come-first-served" basis Order from: Gene Doty 27 Hillsboro High School Hillsboro, Oregon 97123 

Some Trends in College Physics and Their Reflections on High School Physics

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Some Trends in College Physics and Their Reflections on High School Physics

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