UNIVERSITY PHYSICS II CHAPTER 26 An Aperitif of Modern physics 826. 1 Some important discoveries at the end of the 20th century I. The discovery of the electron Spot of Filament To vacuum pump Measure the charge to mass ratio of electron q 2 VE BL2
1 1. The discovery of the electron Measure the charge to mass ratio of electron §26.1 Some important discoveries at the end of the 20th century 2 2 2 B L yE m q =
8 26.1 Some important discoveries at the end of the 20th century JJ. Thomson(1856--1940) 826. 1 Some important discoveries at the end of the 20th century J J. Thomsons original tube
2 J. J. Thomson (1856--1940) §26.1 Some important discoveries at the end of the 20th century J. J. Thomson’s original tube §26.1 Some important discoveries at the end of the 20th century
8 26.1 Some important discoveries at the end of the 20th century a Measured the charge of the cathode rays; b Make a static electric deflection of the cathode rays c Measured the charge to mass ratio of the cathode rays d. prove the universal existence of the electron The charge to mass ratio of electron =17588×10C/kg TThe mass of electron m=9.11×10kg 826. 1 Some important discoveries at the end of the 20th century The charge of electron q=-e=-1.602×10-9C The charge of electron is quantized. The discovery of electron was the first clear evidence that the indivisible atoms of nature had structure
3 a. Measured the charge of the cathode rays; b. Make a static electric deflection of the cathode rays; c. Measured the charge to mass ratio of the cathode rays; d. Prove the universal existence of the electron. §26.1 Some important discoveries at the end of the 20th century The charge to mass ratio of electron 1.7588 10 C/kg 11 = × m q The mass of electron 9.11 10 kg −31 m = × The charge of electron 1.602 10 C −19 q = −e = − × The discovery of electron was the first clear evidence that the indivisible atoms of nature had structure. §26.1 Some important discoveries at the end of the 20th century The charge of electron is quantized
826. 1 Some important discoveries at the end of the 20th century 2. The discovery of X-rays Pyrex glass envelope Electron Tungsten target X 8 26.1 Some important discoveries at the end of the 20th century The characteristics of X-rays: a. It is generated whenever high-energy cathode rays strike solid naterials b. Matter is more or less transparent to X-rays X-ray photograph of a human
4 2. The discovery of X-rays §26.1 Some important discoveries at the end of the 20th century The characteristics of X-rays: a. It is generated whenever high-energy cathode rays strike solid materials. b. Matter is more or less transparent to X-rays. §26.1 Some important discoveries at the end of the 20th century
826. 1 Some important discoveries at the end of the 20th century c. Photographic film is affected by X-rays, so its use as a detector was assured from the beginning d. The rays is not deviated by electric and magnetic fields. and so are uncharged The applications of X-rays: The wave nature of X-rays makes them useful tools for the study of the structure of crystals and molecules, where the atoms and molecules acts as three-dimensional diffraction gratings 8 26.1 Some important discoveries at the end of the 20th century 屏 晶体 2
5 c. Photographic film is affected by X-rays, so its use as a detector was assured from the beginning. d. The rays is not deviated by electric and magnetic fields, and so are uncharged. The wave nature of X-rays makes them useful tools for the study of the structure of crystals and molecules, where the atoms and molecules acts as three-dimensional diffraction gratings. The applications of X-rays: §26.1 Some important discoveries at the end of the 20th century §26.1 Some important discoveries at the end of the 20th century
826. 1 Some important discoveries at the end of the 20th century 3. The discovery of radioactivity Radioactivity occur naturally and have with us on the earth from the very beginning Henri Becquerel discovered Uranium, Marie curie discovered polonium and radium Ernest rutherford found that the substances emit several distinct types of radiations. One is a penetrating radiation, dubbed a that propagates through several centimeters in air and can even penetrate very thin metal foils. Another less penetrating radiation, dubbed B, is easily stopped by even a sheet of paper Another type, called r, was discovered in 1900 and is much more penetrating than even the a radiation 526.2 The appearance of Plank' s constant h 1. Blackbody Radiation 心 Thermal radiation 800K l000K 1200K 1400K Rb cu The color emitted by atoms after they have exited by heat is characteristic of the particular element they comprise 6
6 3. The discovery of radioactivity Radioactivity occur naturally and have with us on the earth from the very beginning. Henri Becquerel discovered Uranium, Marie Curie discovered Polonium and Radium Ernest Rutherford found that the substances emit several distinct types of radiations. One is a penetrating radiation, dubbed α, that propagates through several centimeters in air and can even penetrate very thin metal foils. Another less penetrating radiation, dubbed β, is easily stopped by even a sheet of paper. Another type, called γ, was discovered in 1900 and is much more penetrating than even the α radiation. §26.1 Some important discoveries at the end of the 20th century §26.2 The appearance of Plank’s constant h 1. Blackbody Radiation 800K 1000K 1200K 1400K Thermal radiation Sr Rb Cu The color emitted by atoms after they have exited by heat is characteristic of the particular element they comprise
826.2 The appearance of Plank's constant h c Ideal blackbody a body that absorbs all radiation incident on it is called ideal blackbody 1879 Josef Stefan found an empirical relation between the power per unit area radiated by a blackbody and the temperature RIOT 4 526.2 The appearance of Plank' s constant h Wiens displacement law .T=0.28978×10mK 4000K
7 Ideal blackbody A body that absorbs all radiation incident on it is called ideal blackbody. 1879 Josef Stefan found an empirical relation between the power per unit area radiated by a blackbody and the temperature. 4 R = σT §26.2 The appearance of Plank’s constant h §26.2 The appearance of Plank’s constant h Wien’s displacement law 0.28978 10 m K 2 max = × ⋅ − λ T
8 26.2 The appearance of Plank' s constant h Experimental law of blackbody radiation e(λ,T Ultraviolet catastrophe e(礼,T=C1 fIend Rayleigh-Jeans e(λ,T)=CT 01234 (m) 526.2 The appearance of Plank' s constant h Every attempt to explain the blackbody spectrum based on electromagnetic theory and thermodynamics failed to predict the shape of the blackbody spectrum 2.Pank’ s law and plank’ s constant (a, T)=2rhc 2(ekxr-1) 8
8 0 1 2 3 4 5 6 7 8 9 λ(µ m) ( , ) e0 λ T Experimental law of blackbody radiation 4 ( , ) − eo λ T = CTλ Wien T C o e T C e λ λ λ 2 5 1 ( , ) − − = Rayleigh-Jeans Ultraviolet catastrophe §26.2 The appearance of Plank’s constant h 2 5 1 0 ( , ) 2 ( 1) − − = − k T hc e T hc e λ λ π λ 0 λ (T,λ) 0 e 2. Plank’s law and Plank’s constant §26.2 The appearance of Plank’s constant h Every attempt to explain the blackbody spectrum based on electromagnetic theory and thermodynamics failed to predict the shape of the blackbody spectrum
8 26.2 The appearance of Plank' s constant h In 1901, Plank assumed that the energy e associated with the light inside the cavity was present only in finite packets(bundles proportional to the frequency v E=hy Where h was an unknown constant that he hoped to be able to set to zero after taking appropriate mathematical limits. h=6.626×10J.s It is a fundamental constant of nature. called by plank the quantum of action Now we called it plank’ s constant 826.3 The photoelectric effect 1. Apparatus and phenomenon a photocell is made by enclosing a metal plate and a collecting wire in an evacuated glass tube EM radiation(visible or 光电管 UVB) falls on the metal plate; some of the emitted electrons make = their way to the collecting g wire, which complete the circuit. An ammeter measures the current in the circuit 安培表
9 §26.2 The appearance of Plank’s constant h In 1901, Plank assumed that the energy E associated with the light inside the cavity was present only in finite packets (bundles ) proportional to the frequency ν. E = hν Where h was an unknown constant that he hoped to be able to set to zero after taking appropriate mathematical limits. 6.626 10 J s 34 = × ⋅ − h It is a fundamental constant of nature, called by Plank the quantum of action. Now we called it Plank’s constant. §26.3 The photoelectric effect 1. Apparatus and phenomenon A photocell is made by enclosing a metal plate and a collecting wire in an evacuated glass tube. EM radiation(visible or UVB) falls on the metal plate; some of the emitted electrons make their way to the collecting wire, which complete the circuit. An ammeter measures the current in the circuit
826.3 The photoelectric effect 2. Experimental results and the troubles of the classical theory bright light causes an increase in current but does not cause the individual electrons to gain higher energies. The maximum kinetic energy of the electrons is independent of the intensity of the light. Classically, more intense light has larger amplitude em field and thus delivers more energy. That should not only enable a larger number of electrons to escape from the metal, it should also enable the electrons emitted to have more kinetic energy. 826.3 The photoelectric effect The maximum kinetic energy of emitted electrons does depend on the frequency of the incident radiation. Thus if the incident light is very dim but high in frequency, electrons with large kinetic energies are released. Classically, there is no explanation for a frequency dependence For a given metal, there is a threshold frequency ve. If the frequency of the incident light is below the threshold, no electrons are emitted-no matter what the intensity of the incident light. Again, classical physics has no explanation of the frequency dependence. 10
10 2. Experimental results and the troubles of the classical theory Bright light causes an increase in current but does not cause the individual electrons to gain higher energies. The maximum kinetic energy of the electrons is independent of the intensity of the light. Classically, more intense light has larger amplitude EM field and thus delivers more energy. That should not only enable a larger number of electrons to escape from the metal, it should also enable the electrons emitted to have more kinetic energy. §26.3 The photoelectric effect The maximum kinetic energy of emitted electrons does depend on the frequency of the incident radiation. Thus, if the incident light is very dim but high in frequency, electrons with large kinetic energies are released. Classically, there is no explanation for a frequency dependence. For a given metal, there is a threshold frequency νc. If the frequency of the incident light is below the threshold, no electrons are emitted—no matter what the intensity of the incident light. Again, classical physics has no explanation of the frequency dependence. §26.3 The photoelectric effect
