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vol I chap 2 sect 2

Volume I: Motion

Previous: 2.1. Regions for doing experiments.

2.2. Physics Nobel Lectures by Thomson, Millikan, Franck, Hertz, and Compton.

In what follows, we consider two kinds of publications that are available in the web page of the Nobel Foundation: a document called WORK that explains in a very synthetic form the reasons for awarding the Physics Nobel Prize to Thomson, Millikan, Franck, Hertz and Condon, and their corresponding Nobel Lectures. The first document is fully quoted; the text is indicated in quotation marks. The second document is divided in two parts and inserted as boxes: the first box is called Accepted knowledge or questions under discussion in laureate´s time and the second box refers to Laureate´s contributions or explanations. All the documents include their corresponding references. When the Nobel Lecture contains subtitles, they are mentioned after the title of the Lecture.

1906 Physics Nobel Prize awarded to Thomson.

WORK: "The idea that electricity is transmitted by a tiny particle related to the atom was first forwarded in the 1830s. In the 1890s, J.J. Thomson (1856-1940) managed to estimate its magnitude by performing experiments with charged particles in gases. In 1897 he showed that cathode rays (radiation emitted when a voltage is applied between two metal plates inside a glass tube filled with low-pressure gas) consist of particles— electrons—that conduct electricity. Thomson also concluded that electrons are part of atoms."

MLA style: J.J. Thomson – Facts. NobelPrize.org. Nobel Prize Outreach AB 2023. Wed. 1 Mar 2023. https://www.nobelprize.org/prizes/physics/1906/thomson/facts/

NOBEL LECTURE: Carriers of Negative Electricity.

  • Introductory
  • Electric deflection of the rays
  • Determination of e/m
  • Corpuscles very widely distributed
  • Magnitude of the electric charge carried by the corpuscle

MLA style: J.J. Thomson – Nobel Lecture. NobelPrize.org. Nobel Prize Outreach AB 2024. Sat. 13 Apr 2024. https://www.nobelprize.org/prizes/physics/1906/thomson/lecture/

BOX 2.1: Accepted knowledge or questions under discussion in Thomson´s times.

  • When an electric discharge is sent through a highly exhausted tube, a current of corpuscles of negative electricity comes out in straight lines from the cathode; these are the cathode rays which can be modified by external actions of magnetic and electric forces.
  • The charged corpuscles obtained from cathode rays were produced in other circumstances; for instances, when alkali metals were exposed to light and where radioactive salts were put into flames. These corpuscles seemed to be part of all type of atoms.

BOX 2.2: Thomson´s contributions or explanations.

  • When a particle of charge e moves with velocity \(v\) across the lines of force of a magnetic field of intensity \(H\), it experiments a force \(Hev\). If an extra electric field of intensity \(X\) is applied, a force \(Xe\) will act upon the cathode rays. If these two forces are balanced (\(Hev = Xe\)) the rays are not deflected but they move with a velocity \(v = H/X\). By measuring both intensities this velocity can be determined. In a very highly exhausted tube \(v\) may be ⅓ the velocity of light.
  • "For the corpuscle in the cathode rays the value for the ratio \(e/m\) was \(1.7 \times 10^7\); however, the corresponding value for a charged atom of hydrogen was of the order of \(10^4\). Therefore, assuming that the electric charge was practically the same in the two cases, the mass of the corpuscle is only about 1/1,700 of that of the hydrogen atom."
  • "The very large value of \(e/m\) for the charged corpuscles, as compared with that for the atom of hydrogen, is due to the smallness of \(m\) the mass, and not to the greatness of e the charge."
  • "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."

1923 Physics Nobel Prize awarded to Millikan.

WORK: "During the 1890s the theory that electricity was conveyed by a miniscule unit, the electron, gained acceptance. In 1910 Robert Millikan (1868-1953) succeeded in precisely determining the magnitude of the electron’s charge. Small electrically charged drops of oil were suspended between two metal plates where they were subjected to the downward force of gravity and the upward attraction of an electrical field. By measuring how the various drops of oil moved about, Millikan showed that their charge always was a multiple of a precisely determined charge—the electron’s charge."

MLA style: Robert A. Millikan – Facts. NobelPrize.org. Nobel Prize Outreach AB 2023. Thu. 2 Mar 2023. https://www.nobelprize.org/prizes/physics/1923/millikan/facts/

NOBEL LECTURE: The Electron and the Light-Quant from the Experimental Point of View.

MLA style: Robert A. Millikan – Nobel Lecture. NobelPrize.org. Nobel Prize Outreach AB 2023. Thu. 2 Mar 2023. https://www.nobelprize.org/prizes/physics/1923/millikan/lecture/

BOX 2.3: Accepted knowledge or questions under discussion in Millikan´s times.

  • The conception of electrical particles or atoms of electricity goes back to Benjamin Franklin about 1750. To test this conception, it is necessary to change charges on the smallest possible amount and by the most minute possible steps.
  • It has been observed that bodies can receive electric positive or negative charges; however, there was the question of why and how charge changes must occur. For some people the apparent unitary character of electricity was but a statistical phenomenon.
  • Some questions are still under discussion: is the charge of the electron divisible or not? will ever have another unit of elementary charge?

BOX 2.4: Millikan´s contributions or explanations.

  • Doing experiments with charged pith balls required round and homogeneous bodies. Due to this difficulty the pith ball was replaced by individual oil droplets a thousandth of a millimeter in diameter.
  • The experimental setting was tested and improved while the experiments were performed from 1909 to 1923. The motion of the drop depended on the balance among three forces: the weight of the drop, its friction with air inside the container and the force of an external battery whose intensity and polarity could be changed.
  • The first direct experimental proof of the exact validity of Einstein´s equation for the photoelectric effect and the first direct determination of Planck’s constant were made in 1914. Einstein’s equation is one of exact validity (always within the present small limits of experimental error) and of very general applicability. Einstein’s light-quanta may be considered as experimentally established.

1925 Physics Nobel Prize awarded to Franck.

WORK: "After the publication of Niels Bohr’s theory on the structure of the atom, James Franck (1882-1964) and Gustav Hertz conducted an experiment in 1913 to verify it. A potential difference was applied to a tube containing a low-pressure gas. When the potential difference was increased, the current flowing through the tube also increased until it reached a certain voltage, when it suddenly declined. The result supported Bohr’s theory, in which electrons can only have specific, discrete energies. The potential difference increased the free electrons’ mobility until, at a certain energy level, bound electrons jumped to a higher-energy orbit instead."

MLA style: James Franck – Facts. NobelPrize.org. Nobel Prize Outreach AB 2023. Thu. 2 Mar 2023. https://www.nobelprize.org/prizes/physics/1925/franck/facts/

NOBEL LECTURE: Transformation of Kinetic Energy of Free Electrons into Excitation Energy of Atoms by Impacts.

MLA style: James Franck – Nobel Lecture. NobelPrize.org. Nobel Prize Outreach AB 2023. Fri. 3 Mar 2023. https://www.nobelprize.org/prizes/physics/1925/franck/lecture/

BOX 2.5: Accepted knowledge or questions under discussion in Franck’s times.

  • Discharges of electricity through gases are due to collisions between slow electrons and atoms. Two key concepts in a kinetic theory of electrons in gases are the free path-length and the ionization energy.
  • The free path-length is the average distance that an electron travels in a straight line in between two successive collisions with atoms.
  • Due to the small mass of the electron, the transfer of momentum from the slow electron to the atom at rest of the inert gases is very small and the collision is an elastic one. However, by accumulation of collisions, the energy loss can be significant and measurable.

BOX 2.6: Franck´s contributions or explanations.

  • In the calculations of the free path-length, the electron behaves like a gaseous electrically charged impurity with a vanishingly small impact radius.
  • The experimental setting to measure the energy loss during accumulated collisions consists of three electrodes aligned inside a tube: an electron source consisting in a tungsten wire that is heated (G), a wire-screen electrode (N) that initially is charged positive with respect to G and a third electrode (P) where the electrons arrive.
  • A potential difference V between G and N accelerates electrons. When there is no gas between G and N the electrons acquire a kinetic energy \(½mv^2 = eV\), where \(e\) is the charge and m the mass of the electron. Those electrons that arrive at P produce a current registered in a galvanometer. When a decelerating potential energy is introduced between N and P a distribution of energy of the arriving electrons is detected. If the arriving electrons are repelled at P the current diminishes and even it is null because the electrons have lost energy by collision with the atoms. For high pressures of the gas the number of collisions electron-atom increases considerably.
  • In the case of polyatomic gases, the greater average energy loss could be due to the formation of negative ions or to a transfer of the kinetic energy of the electrons into vibrational and rotational degrees of freedom of the molecules.
  • When the impacting electrons have higher velocities, their energies are transferred into the internal energy of the impacted atoms. The collisions are now inelastic, and the atoms are excited to luminescence or become ionized: this will correspond to the ionization energy (ionization voltage).
  • "It can be seen that in Hg vapour this partial electron current increases with increasing acceleration, similar to the characteristic glow-electron current in vacuum, until the critical energy stage is reached when the current falls suddenly to almost zero. Since the electrons cannot lose more or less than the critical amount of energy, the cycle begins anew with further increase of voltage" …
  • "The process repeats itself periodically as soon as the accelerating voltage overreaches a multiple of the critical voltage. The distance between the succeeding maxima gives an exact value of the critical voltage. This is 4.9 V for mercury vapour." …
  • "If the conjectured conversion of kinetic energy into light on impact should take place, ‘then on bombardment with 4.9 eV electrons, the line 2,537 A, and only this line out of the complete line spectrum of mercury, should appear." …
  • "The first works of Niels Bohr on his atomic theory appeared half a year before the completion of this work." …
  • "The elastic collisions at low electron velocities show that for these impacts no energy is taken up as inner energy, and the first critical energy step results in just that amount of energy required for the excitation of the longest wave absorption line of Hg."

1925 Physics Nobel Prize awarded to Hertz.

WORK: "After the publication of Niels Bohr’s theory on the structure of the atom, Gustav Hertz (1987-1975) and James Franck conducted an experiment in 1913 to verify it. A potential difference was applied to a tube containing a low-pressure gas. When the potential difference was increased, the current flowing through the tube also increased until it reached a certain voltage, when it suddenly declined. The result supported Bohr’s theory, in which electrons can only have specific, discrete energies. The potential difference increased the free electrons’ mobility until, at a certain energy level, bound electrons jumped to a higher-energy orbit instead."

MLA style: Gustav Hertz – Facts. NobelPrize.org. Nobel Prize Outreach AB 2023. Thu. 2 Mar 2023. https://www.nobelprize.org/prizes/physics/1925/hertz/facts/

NOBEL LECTURE: The Results of the Electron-Impact Tests in the Light of Bohr’s Theory of Atoms.

MLA style: Gustav Hertz – Nobel Lecture. NobelPrize.org. Nobel Prize Outreach AB 2023. Thu. 2 Mar 2023. https://www.nobelprize.org/prizes/physics/1925/hertz/lecture/

BOX 2.7: Accepted knowledge or questions under discussion in Hertz´s times.

  • Atoms contains electrons: negative electrical charges in quantized energy levels. Atoms exchange energy with electromagnetic radiation producing observed spectral lines representing the absorption or emission of photons of frequency \(ν\). These photons have quantized energies (\(E_n = nhν\)) that correspond to the differences in the electronic energy levels participating in each transition: \(E_1 – E_2 = nhν\).

  • Bohr´s atomic theory is based on the following assumptions:

(1) "For every atom there is an infinite number of discrete stationary states, which are characterized by given internal energy levels in which the atom can exist without emitting radiation.

(2) Emission and absorption of radiation are always connected with a transition of the atom from one stationary state to another, emission involving transition to a state of lower energy, and absorption involving transition to a state of higher energy.

(3) The frequency of the radiation emitted or absorbed respectively during such a transition is given by the equation where h is Planck’s constant and EI and E2 denote the energy of the atom in the two stationary states."

BOX 2.8: Hertz´s contributions or explanations.

  • "The significance of investigations on the ionization of atoms by electron impact is due to the fact that they have provided a direct experimental proof of the basic assumptions of Bohr’s theory of atoms. This lecture will summarize the most important results, and show that they agree in every detail, so far as can be observed at present, with what we should expect on the basis of Bohr’s theory."
  • "In the experimental investigation of these processes a given energy is usually imparted to the electrons by accelerating them by a given voltage. The energy of an electron after the collision is studied by determining the retarding potential which it can still overcome. Therefore, the excitation energy of a given state corresponds to the potential difference through which an electron with zero initial velocity has to fall in order to make its energy equal to the excitation energy of the atom. This excitation potential is thus equal to the excitation energy divided by the charge of the electron. The ionization potential is associated with the ionization energy in the same way."…
  • "Summarizing therefore, it can be stated that all the results so far attained with the electron-impact method agree very closely with Bohr’s theory and in particular that they verify experimentally Bohr’s interpretation of the series terms as a measure of the energy of the atom in its various stationary states."

1927 Physics Nobel Prize awarded to Compton.

WORK: "According to Einstein’s photoelectric effect theory, light consists of quanta, “packages” with definite energies corresponding to certain frequencies. A light quantum is called a photon. When Arthur Compton (1892-1962) directed X-ray photons onto a metal surface in 1922, electrons were emancipated and the X-rays’ wavelength increased because some of the incident photon energy was transferred to the electrons. The experiment confirmed that electromagnetic radiation could also be described as photon particles following the laws of mechanics."

MLA style: Arthur H. Compton – Facts. NobelPrize.org. Nobel Prize Outreach AB 2023. Thu. 2 Mar 2023. https://www.nobelprize.org/prizes/physics/1927/compton/facts/

NOBEL LECTURE: X-rays as a Branch of Optics.

• The refraction and reflection of X-rays

• The diffraction of X-rays

• The scattering of X-rays and light

MLA style: Arthur H. Compton – Nobel Lecture. NobelPrize.org. Nobel Prize Outreach AB 2023. Thu. 2 Mar 2023. https://www.nobelprize.org/prizes/physics/1927/compton/lecture/

BOX 2.9: Accepted knowledge or questions under discussion in Compton´s times.

  • In what sense and to what extend X-rays have shown the same characteristics as light? Such extension brought more knowledge about the laws of optics, the structure of matter and the nature of radiation.
  • Experiments on refraction and reflection of X-rays can be understood by assuming that X rays consist of electromagnetic waves much shorter than those of light.
  • Extending the optical phenomenon of diffraction into the region of X-rays had two important consequences: crystals are appropriate gratings for diffracting X-rays and X-rays can be used for spectroscopical purposes.

BOX 2.10: Compton´s contributions or explanations.

  • Experiments performed in Compton´s laboratory indicate that the spectrum of the secondary X-rays shows that the primary beam has been split into two parts, one of the same wavelength and the other of increased wavelength.*
  • "Thus in the wavelength of secondary radiation we have a gradually increasing departure from the classical electron theory of scattering as we go from the optical region to the region of X-rays and γ-rays. The question arises, are these secondary X-rays of increased wavelength to be classed as scattered X-rays or as fluorescent?*
  • "According to the classical theory, an electromagnetic wave is scattered when it sets the electrons which it traverses into forced oscillations, and these oscillating electrons reradiate the energy which they receive. In order to account for the change in wavelength of the scattered rays, however, we have had to adopt a wholly different picture of the scattering process, as shown in Fig. 9. Here we do not think of the X-rays as waves but as light corpuscles, quanta, or, as we may call them, photons. Moreover, there is nothing here of the forced oscillation pictured on the classical view, but a sort of elastic collision, in which the energy and momentum are conserved." [Figure 2.9 shows that for each recoil electron there is a scattered photon, and that the energy and momentum of the system photon plus electron are conserved in the scattering process.]
  • "The evidence for the existence of directed quanta of radiation afforded by this experiment is very direct. The experiment shows that associated with each recoil electron there is scattered X-ray energy enough to produce a secondary ray, and that this energy proceeds in a direction determined at the moment of ejection of the recoil electron."
  • "Thus we see that as a study of the scattering of radiation is extended into the very high frequencies of X-rays, the manner of scattering changes. For the lower frequencies the phenomena could be accounted for in terms of waves. For these higher frequencies we can find no interpretation of the scattering except in terms of the deflection of corpuscles or photons of radiation. Yet it is certain that the two types of radiation, light and X-rays, are essentially the same kind of thing. We are thus confronted with the dilemma of having before us a convincing evidence that radiation consists of waves, and at the same time that it consists of corpuscles."
  • "We have thus seen how the essentially optical properties of radiation have been recognized and studied in the realm of X-rays. A study of the refraction and specular reflection of X-rays has given an important confirmation of the electron theory of dispersion, and has enabled us to count with high precision the number of electrons in the atom. The diffraction of X-rays by crystals has given wonderfully exact information regarding the structure of crystals, and has greatly extended our knowledge of spectra. When X-rays were diffracted by ruled gratings, it made possible the study of the complete spectrum from the longest to the shortest waves. In the diffuse scattering of radiation, we have found a gradual change from the scattering of waves to the scattering of corpuscles."

Next: 2.3. Knowledge domains for understanding.