Physics
Franck Hertz Experiment
The Franck-Hertz experiment is a fundamental physics experiment that demonstrates the quantization of energy levels in atoms. It involves bombarding a vapor of atoms with electrons and measuring the energy distribution of the electrons that pass through. The experiment provides evidence for the existence of discrete energy levels in atoms, supporting the development of quantum mechanics.
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4 Key excerpts on "Franck Hertz Experiment"
eBook - ePubExperiments and Demonstrations in Physics
Bar-Ilan Physics Laboratory
- Yaakov Kraftmakher(Author)
- 2014(Publication Date)
- WSPC(Publisher)
“…Professor Franck. Professor Hertz. Through clear thinking and painstaking experimental work in a field which is continuously being flooded by different hypotheses, you have provided a firm footing for future research.”C. W. Oseen, member of the Nobel Committee for Physics. Presentation of James Franck and Gustav Hertz awarded “for their discovery of the laws governing the impact of an electron upon an atom” (1925).Additional equipment: two Voltage sensors, DSO, Franck–Hertz’s tube from PHYWE, DC supply, DC amplifier, capacitor, resistors, switch, 1.5 V battery.In 1914, James Franck and Gustav Hertz reported on an experimental work concerning collisions between electrons and mercury atoms. A hot cathode emits electrons, which are accelerated by a potential applied to a grid (Fig. 1 ). The electrons undergo collisions with mercury atoms in the space between the cathode and the grid. After the grid, the electrons arrive at a metallic plate, and the current is measured by a galvanometer. The potential of the plate is kept somewhat lower than that of the grid. Therefore, if the electrons lose their energies in the collisions, their kinetic energy may become smaller than necessary to overcome the potential barrier between the grid and the plate.Fig. 1. Scheme of Franck–Hertz’s experiment for studying collisions between electrons and mercury atoms. G––galvanometer.When measuring the plate current versus the accelerating voltage, periodic maxima and minima are observed. If the kinetic energy of electrons is less than 4.9 eV, the collisions are elastic, that is, the electron can change the direction but not the velocity. When the energy reaches 4.9 eV, many collisions become completely inelastic, the electron gives up its entire kinetic energy to an atom. A bit above 4.9 eV many electrons still give 4.9 eV to the atoms, then continue with an energy that is lower by that amount. According to Bohr’s theory, this energy corresponds to the first discrete excited state. The I–V characteristic shows a minimum in the vicinity of the accelerating voltage equal to the energy level of the mercury atom. The electrons can undergo such collisions several times, so that several minima are observable when the accelerating voltage is changed over a wide range. The excited atom drops back to its original state and emits a photon of energy hf
eBook - PDFPhysics 1922 – 1941
Including Presentation Speeches and Laureates' Biographies
- Sam Stuart(Author)
- 2013(Publication Date)
- Elsevier(Publisher)
The fact that Bohr's hypotheses of 1913 have succeeded in estab-lishing this, is because they are no longer mere hypotheses but experimen-tally proved facts. The methods of verifying these hypotheses are the work of James Franck and Gustav Hertz, for which they have been awarded the Physics Nobel Prize for 1925. Franck and Hertz have opened up a new chapter in physics, viz., the the-ory of collisions of electrons on the one hand, and of atoms, ions, molecules or groups of molecules on the other. This should not be interpreted as meaning that Franck and Hertz were the first to ask what happens when an electron collides with an atom or a molecule, or that they were the orig-inators of the general method which paved the way for their discoveries and which consists of the study of the passage of a stream of electrons through a gas. The pioneer in this field is Lenard. But Franck and Hertz have devel-oped and refined Lenard's method so that it has become a tool for studying the structure of atoms, ions, molecules and groups of molecules. By means of this method and not least through the work of Franck and Hertz them-selves, a great deal of material has been obtained concerning collisions be-tween electrons and matter of different types. Although this material is im-portant, even more important at the present time is the general finding that Bohr's hypotheses concerning the different states of the atom and the con-nexion between these states and radiation, have been shown to agree com-pletely with reality. P R E S E N T A T I O N 97 Professor Franck. Professor Hertz. Through clear thinking and pain-staking experimental work in a field which is continuously being flooded by different hypotheses, you have provided a firm footing for future research. In gratitude for your work and with sincere good wishes I request you to receive the Physics Nobel Prize for 1925 from the hands of our King.
- J. Thorsen(Author)
- 2013(Publication Date)
- North Holland(Publisher)
Using similar considerations Bohr was able to concludes0: “Franck and Hertz assume that 4.9 volts corresponds to the energy necessary to remove an electron from the mercury atom, but it seems that their experiments may possibly be consistent with the assumption that this voltage corresponds only to the transition from the normal state to some other stationary state of the neutral atom. . . , If the above considerations are correct it will be seen that Franck and Hertz’s measurements give very strong support to the theory considered in this paper.” Bohr also explained that what Franck and Hertz had believed to be ionization was in fact a result of a photoelectric effect in the apparatus. This was experimentally verified by Bergen Davis and F.S. Goucher in August 1917 51. 48N. Bohr, On the Quantum Theory ofRadiation and the Structure of the Atom, Phil. Mag. 30 (1915) 394415. Reproduced in Vol. 2, p. [389]. 49 El. Rau, Uber die Lichterregung durch langsame Kathodenstrahlen, Sitzungsber. d. phys.-med. Ges. Wiirzburg, 1914, pp. 2G27. 50Ref. 48, p. 410. 51 B. Davis and F.S. Goucher, Ionization and Excitation of Radiation by Electron Impact in Mercury Vapor and Hydrogen, Phys. Rev. 10 (1917) 101-1 15; Ionization and Excitation ofRadiation by Electron Impact in Nitrogen, Phys. Rev. 13 (1919) 1-5. P A R T 1: T H E C L A S S I C A L T H E O R Y Franck and Hertz continued their investigations and in a paper of 1916 s2 they gave a summary of their earlier results, but they could not accept Bohr’s explanation s3, and, as mentioned above, it was not until 1919 that they finally accepted the Bohr theorys4. Bohr intended to make his own investigations in Manchester along the lines of Franck and Hertz, but they soon came to an end.
eBook - PDF- Don Shillady(Author)
- 2011(Publication Date)
- CRC Press(Publisher)
10 Early Experiments in Quantum Physics INTRODUCTION We tried to introduce the idea of quantization in Chapter 9 as a completion of a one semester course. However, we skipped over some really interesting events in the history of Science between 1900 and 1913 when Bohr derived the quantized energy of the H atom. First we want to carry out the 1901 Planck derivation of the formula for blackbody radiation [1]. Many texts just show the curve, write e ¼ h n , and move on. As a student, this author found that limited explanation very frustrating since energy quantization is a fundamental concept. Even among graduate texts in quantum mechanics, we are aware of only one that does the complete treatment which we will draw upon for the mathematics [2] but supplement with a narrative that we have found helpful to students over the years. Then in 1905, Albert Einstein (1879 – 1955), one of the most in fl uential scientists of all time, gave an explanation of the photoelectric effect [3] for which he received the Nobel Prize in 1921. Even reducing our list to essential topics, we need to discuss the Davisson – Germer experiment [4]. The photoelectric effect introduces the idea that light waves can act as particles while the Davisson – Germer experiment showed that particles can act like waves and con fi rmed the De Broglie equation [5]. However, you can be assured that it will not be as dif fi cult as you might have anticipated and if you can absorb the meaning of just these three key experiments you should be able to begin thinking in terms of quantum mechanics! Despite our slow historical development, this is 2010 and we have to get to the twenty-fi rst century somehow! STEFAN – BOLTZMANN LAW: RELATING HEAT AND LIGHT — PART I We do not want you to forget the thermodynamics you learned in earlier chapters but historically there was a shift in science with the idea of energy quantization in 1900.
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