Teaching Modern Physics using Selected Nobel Lectures

A. Stinner

Introduction

Some years ago I realized that what I can do as a University educator preparing students who are planning to become physics teachers is to build on their undergraduate knowledge of modern physics using an unconventional approach. I decided to give them some enthusiasm and self confidence for the teaching of the ideas and the concepts of modern physics, using a selected number of appropriate Nobel lectures. Based on my prior experience, I was convinced that the conventional approach revisiting the main ideas of modern physics using a textbook would only lead to boredom.

Using seminal papers of the great physicists of the past to teach physics is notoriously difficult. Papers by the Nobel laureates chosen that contributed to the work on which the Nobel Prize was awarded are generally inaccessible to students. However, there are many Nobel lectures that are accessible and can be fruitfully studied by students.

What follows is a brief description and a rationale of the course I present to physics teacher candidates at the University of Manitoba. The paper also containes a shortened version of a handout produced by one of my students (in consultation with the instructor) based on the work of J.J. Thomson, as reported in his Nobel lecture.

Description of the presentation

I always begin my classes with a quotation by G.P. Thomson, the son of J.J. Thomson, taken from his Nobel lecture:

The goddess of learning is fabled to have sprung full-grown from the brain of Zeus, but it is seldom that a scientific conception is born in its final form, or owns a single parent. More often it is a product of a series of minds, each in turn modifying the ideas of those that came before, and providing material for those that came after. The electron is no exception.

I then emphasize that the Nobel lectures chosen must illustrate the interconnectedness of ideas and the dependence of new work on earlier achievements, as described in the statement. (Nobel lectures chosen, with a shortened version of the citation, are listed below.)

A note of explanation must be added here. Roentgen did not give an acceptance speech and Einstein’s Nobel lecture (given a year later) was not based on the work for which he was awarded the prize (photoelectric effect). For Roentgen, my students read relevant articles taken from the special edition of "History of Physics", an AAPT publication. The Einstein acceptance speech is based on his two theories of relativity, and is generally inaccessible to students. Here I made an exception, and I ask my students to read the first part of his ultimately revolutionary 1905 paper on relativity. Finally, Rutherford received his Nobel Prize in chemistry, much to his annoyance, and the second Nobel Prize of Madame Curie was also in chemistry.

The following is a shortened version of a student’s summary of the work of J.J. Thomson. This report is handed out after the PPT (PowerPoint) presentation by the student-presenter, to be discussed in detail in the following session. Of course, appropriate diagrams and pictures are contained in the PPT presentation, which are also handed out to the students.

Carriers of negative electricity

Thomson begins his lecture by reviewing the experiments by Crookes to show that cathode rays travel in straight lines. These "rays" were found to be absorbed by a thin plate of mica. Two views were prevalent in 1897: one, held by English physicists, that the rays are negatively electrified bodies, shot off the cathode with great velocity, and the other, supported by German physicists, that these rays are vibrations in the ether.

The arguments in favor of the rays being negatively charged particles were: they are deflected by electric and magnetic fields, as we expect moving charges to behave, and they can be confined in a vessel to give up their negative charges.

If the electric field E and the magnetic field B are so arranged that the forces cancel we have:

Bev = Ee

Therefore:

v = E / B

where B is the magnetic field, e the charge on the negative particle, v is the velocity of the particle (in the horizontal direction) and e the electric charge of the particle.

We can now determine the velocity of the particles. It turns out that the velocity can be as high as 1/3 the velocity of light, or about 60,000 miles per second.

Having found the velocity of the rays, we can determine the e/m ratio of the particle. When the particles find themselves in a constant electric field they experience a constant force. The physics here is like that of a bullet projected horizontally with a velocity v and being acted upon by a gravitational force. It is easy to show that the displacement of the particle will be given by

d = ½ Ee l² / mv²

where l is the horizontal length, m the mass of the particle.

We can now find the displacement d and then calculate the e/m ratio of the particle:

e/m = V / B² l²

(Thomson expressed this as)

e/m = V θ / B² l d

where

θ = d/l

This ratio seems to be independent of the velocity as well as the kind of electrodes we use!

The value for e/m found was about 1.7x10⁷ as measured in the cgs system of units. The value of this ratio found for atoms of hydrogen was only about 10⁴. Therefore, this ratio for the corpuscle associated with cathode rays is about 1700 times larger. The conclusion Thomson reached was that the mass of the corpuscle was about 1/1700 that of the hydrogen atom.

There are many sources of cathode rays: metals heated to a high temperature and any substance when heated gives out corpuscles to some degree; sodium and potassium give off negative corpuscles even when cold and exposed to light. Radioactive materials (uranium and radium) emit them continuously and at very high velocities

Thomson goes on to describe how the newly discovered Wilson cloud chamber has assisted physicists to show those properties described above. He also discusses a first attempt to find the charge on these particles using Stokes’ law. He then estimates the charge on a particle to be about 3.0 x10^-10 electrostatic units, or about 10^-20 electromagnetic units.

Since we know the charge to mass ratio, we can now estimate the mass of the negatively charged particle. This mass turns out to be about 6x10^-28 g.

The conclusion then is that "in all known cases in which negative electricity occurs in gases at very low pressures, it occurs in the form of corpuscles, small bodies with an invariable charge and mass.

Questions based on the Nobel lecture by J.J. Thomson:

1. In what year did J.J. Thomson discover his "negatively charged corpuscle", we now call the electron?
2. What were the two hypotheses about what cathode rays are initially?
3. What were the main arguments in favor of the particle theory of cathode rays?
4. What were the two main conclusions about the "particle" that was discovered?
5. What physical arrangement allowed the calculation of the velocity of the particle?
6. About how fast did these particles move?
7. What are some of the sources of these particles?
8. How was the Wilson cloud chamber used to find the charge of the particles?
9. How did Thomson estimate the mass of the particle?

Thomson concludes his lecture by stating that: "In all known cases in which electricity occurs in gases at very low pressures, it occurs in the form of corpuscles, small bodies with an invariable charge and mass. The case is entirely different with positive electricity." What did he mean?

Main concepts: Electric field, magnetic field, electric charge, force, potential difference, kinetic energy.

Questions and Problems:

1. How do physicists produce a constant electric field? A constant magnetic field? Explain.
2. Who first suggested the name of electron for Thomson’s electric corpuscle? When was this suggested?
3. What were the arguments and evidence for believing that cathode rays are negatively charged particles?
4. Describe how Thomson set up his apparatus and explain how he found the e/m ratio of the electron.
5. How did Thomson estimate the charge on the electron?
6. In our experiment, we used a 2000 V potential difference for both the plate voltage and the anode voltage. The coil had 320 turns, and its diameter was 15 cm. The plate separation was 5.0 cm, and the length of the plate 7.0 cm. The ammeter reading of the current was 1.0 Amps.
   Using the method of Thomson, calculate the e/m ratio, based on these figures.

Comparison with a "typical" contemporary textbook presentation:

1. Read the textbook presentation of J.J. Thomson’s experiment and compare the content with the historical description, taken directly from the Nobel lecture. Comment.
2. Here is one of the questions in the text: "Electrons move through a
   6.0x10-2 T magnetic field balanced by a 3.0x103 N/C electric field. What is the speed of the electrons?"
   What assumptions does the author make about students conceptual understanding? How would you change, or extend the problem in order to go beyond just testing the students’ ability to "plug in values" and find an answer?

Relevant Articles:

"The discovery of the electron: a centenary", by Leif Gerward, Physics Education,
"J.J. Thomson, The Electron, and Atomic Structure", by Helge Kragh, The Physics Teacher, Sept. 1997.

Conclusions:

My students have generally found the reading, the studying, and the discussions of the selected Nobel lectures refreshingly different from the lecture-based and textbook-centered presentations in their undergraduate years. Revisiting the basic ideas, concepts and empirical evidence presented in textbooks using this historical approach allows students to read the summary of the work of a Nobel laureate from an accessible primary source. It is hoped that having had this background study they not only understand the basic ideas of modern physics better but also have developed confidence and enthusiasm to present them on a level accessible to their physics students in high school.

Wilhelm Roentgen, The discovery of the remarkable rays named after him. (1901)
J.J. Thomson, The experimental investigations on the conduction of electricity by gases. (1906) (Our emphasis is the discovery of the electron.)
Ernest Rutherford, The chemistry of radioactive substances. (1908)
William Henry Bragg, and William Lawrence Bragg, The analysis of crystal structure by means of x-rays. (1915)
Madame Curie, The discovery of the elements radium and polonium. (1911)
Niels Bohr, The structure of atoms and of the radiation emanating from them. (1922)
Albert Einstein, The discovery of the law of the photoelectric effect. (1921)
Robert Millikan, The elementary charge of electricity and the photoelectric effect. (1923)
Arthur Compton, For his discovery of the effect named after him. (1927)
Lois de Broglie, For his discovery of the wave nature of electrons. (1929)
James Chadwick, The discovery of the neutron. (1935)
G.P. Thomson, The experimental discovery of the diffraction of electrons. (1937)

Arthur Stinner is a professor of science education at the University of Manitoba. He specializes in physics education and history of science, and his interests are in contextual science teaching and the writing of science plays. This article is based on a paper presented at the 2008 summer AAPT meeting in Edmonton.

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This article is not peer refereed and represents solely the views of the author and not necessarily the views of APS.