4: Introduction to Quantum Physics

Slides:



Advertisements
Similar presentations
Happyphysics.com Physics Lecture Resources Prof. Mineesh Gulati Head-Physics Wing Happy Model Hr. Sec. School, Udhampur, J&K Website: happyphysics.com.
Advertisements

1 My Chapter 27 Lecture. 2 Chapter 27: Early Quantum Physics and the Photon Blackbody Radiation The Photoelectric Effect Compton Scattering Early Models.
Quantum Physics ISAT 241 Analytical Methods III Fall 2003 David J. Lawrence.
APHY201 4/29/ The Electron   Cathode rays are light waves or particles?
Physics 2 Chapter 27 Sections 1-3.
1 Light as a Particle The photoelectric effect. In 1888, Heinrich Hertz discovered that electrons could be ejected from a sample by shining light on it.
Wave-Particle Duality 1: The Beginnings of Quantum Mechanics.
Blackbody Radiation Photoelectric Effect Wave-Particle Duality sections 30-1 – 30-4 Physics 1161: Lecture 28.
2. The Particle-like Properties Of Electromagnetic Radiation
Chapter 27 Quantum Physics.  Understand the relationship between wavelength and intensity for blackbody radiation  Understand how Planck’s Hypothesis.
Electromagnetic Radiation
The dual nature of light l wave theory of light explains most phenomena involving light: propagation in straight line reflection refraction superposition,
Quantum Theory of Light A TimeLine. Light as an EM Wave.
1 Light as a Particle In 1888, Heinrich Hertz discovered that electrons could be ejected from a sample by shining light on it. This is known as the photoelectric.
Physics at the end of XIX Century Major Discoveries of XX Century
Vacuum tube - V, only for shorter than certain wavelength Current V VoVo Fixed wavelength Varying intensity I2I 3I Maximum electron energy 0.
Introduction to Quantum Physics
Classical vs Quantum Mechanics Rutherford’s model of the atom: electrons orbiting around a dense, massive positive nucleus Expected to be able to use classical.
Light: oscillating electric and magnetic fields - electromagnetic (EM) radiation - travelling wave Characterize a wave by its wavelength,, or frequency,
The Photoelectric Effect
Quantum Physics. Black Body Radiation Intensity of blackbody radiation Classical Rayleigh-Jeans law for radiation emission Planck’s expression h =
Early Quantum Theory and Models of the Atom
Electron Configurations & the Periodic Table Chapter 7.
Ch 9 pages ; Lecture 19 – The Hydrogen atom.
Chapter 39 Particles Behaving as Waves
Young/Freeman University Physics 11e. Ch 38 Photons, Electrons, and Atoms © 2005 Pearson Education.
Midterm results will be posted downstairs (by the labs) this afternoon No office hours today.
CHAPTER 40 : INTRODUCTION TO QUANTUM PHYSICS 40.2) The Photoelectric Effect Light incident on certain metal surfaces caused electrons to be emitted from.
Early Quantum Theory AP Physics Chapter 27. Early Quantum Theory 27.1 Discovery and Properties of the Electron.
Quantum Mechanics. Planck’s Law A blackbody is a hypothetical body which absorbs radiation perfectly for every wave length. The radiation law of Rayleigh-Jeans.
As an object gets hot, it gives Off energy in the form of Electromagnetic radiation.
Quantum Physics. Quantum Theory Max Planck, examining heat radiation (ir light) proposes energy is quantized, or occurring in discrete small packets with.
Wave-Particle Duality: The Beginnings of Quantum Mechanics.
Thompson’s experiment (discovery of electron) + - V + - Physics at the end of XIX Century and Major Discoveries of XX Century.
Baby-Quiz 1.Why are diffraction effects of your eyes more important during the day than at night? 2.Will the converging lens focus blue light or red light.
Chemistry 330 Chapter 11 Quantum Mechanics – The Concepts.
Blackbody A black body is an ideal system that absorbs all radiation incident on it The electromagnetic radiation emitted by a black body is called blackbody.
Physics 1C Lecture 28A. Blackbody Radiation Any object emits EM radiation (thermal radiation). A blackbody is any body that is a perfect absorber or emitter.
Chapter 27- Atomic/Quantum Physics
Quantum Physics Chapter 27!.
Photons, Electrons, and Atoms. Visible and non-visable light Frequencies around Hz Much higher than electric circuits Theory was about vibrating.
28.3 THE BOHR THEORY OF HYDROGEN At the beginning of the 20th century, scientists were puzzled by the failure of classical physics to explain the characteristics.
Quantum Theory & the History of Light
1 Electromagnetic Radiation c=  How many wavelengths pass through point P in one second? Frequency! P.
Quantum Theory Waves Behave Like Particles Maxwell’s Wave Theory (1860) Maxwell postulated that changing electric fields produce changing magnetic fields:
Classical Physics Newton’s laws: Newton’s laws: allow prediction of precise trajectory for particles, with precise locations and precise energy at every.
Rutherford’s Model: Conclusion Massive nucleus of diameter m and combined proton mass equal to half of the nuclear mass Planetary model: Electrons.
Origin of Quantum Theory
1 2. Atoms and Electrons How to describe a new physical phenomenon? New natural phenomenon Previously existing theory Not explained Explained New theoryPredicts.
Need for Quantum Physics Problems remained from classical mechanics that relativity didn’t explain Problems remained from classical mechanics that relativity.
Introduction to Modern Physics A (mainly) historical perspective on - atomic physics  - nuclear physics - particle physics.
Ch2 Bohr’s atomic model Four puzzles –Blackbody radiation –The photoelectric effect –Compton effect –Atomic spectra Balmer formula Bohr’s model Frank-Hertz.
Unit 12: Part 2 Quantum Physics. Overview Quantization: Planck’s Hypothesis Quanta of Light: Photons and the Photoelectric Effect Quantum “Particles”:
Chapter 33 Early Quantum Theory and Models of Atom.
Physics 213 General Physics Lecture Exam 3 Results Average = 141 points.
QUANTUM AND NUCLEAR PHYSICS. Wave Particle Duality In some situations light exhibits properties that are wave-like or particle like. Light does not show.
3.1 Discovery of the X-Ray and the Electron 3.2Determination of Electron Charge 3.3Line Spectra 3.4Quantization 3.5Blackbody Radiation 3.6Photoelectric.
1© Manhattan Press (H.K.) Ltd Continuous spectra Spectra Sun’s spectrum and Fraunhofer lines.
The Atomic Models of Thomson and Rutherford Rutherford Scattering The Classic Atomic Model The Bohr Model of the Hydrogen Atom Successes & Failures of.
3.1 Discovery of the X-Ray and the Electron 3.2Determination of Electron Charge 3.3Line Spectra 3.4Quantization 3.5Blackbody Radiation 3.6Photoelectric.
Chapter 39 Particles Behaving as Waves
Chapter 6 Electronic Structure of Atoms
Origin of Quantum Theory
General Physics (PHY 2140) Lecture 33 Modern Physics Atomic Physics
Chapter 39 Particles Behaving as Waves
Blackbody Radiation All bodies at a temperature T emit and absorb thermal electromagnetic radiation Blackbody radiation In thermal equilibrium, the power.
Early Quantum Theory AP Physics Chapter 27.
Chapter 29 Photoelectric Effect
Chapter 39 Particles Behaving as Waves
Photoelectric Effect And Quantum Mechanics.
Presentation transcript:

4: Introduction to Quantum Physics Blackbody Radiation and Planck’s Hypothesis The Photoelectric Effect Compton Effect Atomic Spectra The Bohr Quantum Model of the Atom 1

Material objects obey Newtons Laws of Motion Classical Physics Material objects obey Newtons Laws of Motion Electricity and Magnetism obey Maxwells Equations Position and momentum are defined at all times Initial Position and momentum plus knowledge of all forces acting on system predict with certainty the position and momentum at all later times. Could not explain Black Body Radiation Photo Electric Effect Discrete Spectral Lines 2

Blackbody Radiation and Planck’s Hypothesis Classical explanation Any object with a temperature T>0 K radiates away thermal energy through the emission of electromagnetic radiation Classical explanation heat causes accelerated charges (Maxwell like distribution of accelerations) that emit radiation of various frequencies 3

Incandescent Spectra produced from Thermal Radiation intensity Animation frequency 4

Wiens Displacement Law = 2 . 898 mK max Rayleigh - Jeans Law 2 p ckT I ( l , T ) = l 4 Intensity of radiation of wavelength l at temp T However this only agrees with experiment at long l Lim I ( l , T ) = ¥ Ultraviolet Catastrophe l ¯ ( Þ ¥ total energy density ) 5

6

En = energy of quantum state n of molecule Planck’s Assumptions Oscillating molecules that emit the radiation only have discrete energies En = nhn n = quantum number En = energy of quantum state n of molecule Molecules emit or absorb energy in discrete units of light called QUANTA 7

E2 hn hn=E E1 E = E2-E1 8

Animation The Photoelectric Effect Light A Electron G V A is maintained at a positive potential by battery. IG = 0 until monochromatic light of certain l is incident G V Animation 9

plate A has negative potential high intensity light low intensity light -V0 V plate A has negative potential Stopping Potential When A is negative only electrons having K.E. > eV0 will reach A, independent of light intensity Maximum K.E. of ejected electrons Kmax= eV0 10

Wave theory of light does not predict such properties Observed Properties 1 . No electrons ejected if n £ n (cut off frequency ) c 2 . If n ³ n the number of photo electrons µ light intensity c 3 . K is independent of light intensity max 4. K ­ as n ­ max 5 . Electrons are emitted instantaneously even at low light intensities Wave theory of light does not predict such properties 11

Einstein explained this by the hypothesis that light is quantized in energy packets = QUANTA with energy E = h n he called such quanta PHOTONS . The intensity of the light is proportional to the number of such quanta i . e . I µ nh n In order for electrons to be emitted they must pass through surface . \ use f amount of energy to overcome surface barrier º Ionization Potential º Work Function K = h n - f = h n - h n max c 12

Einsteins Theory Predicts 1 . K = h n - f ; so K depends on n max max 2 . h n ³ f ; for emission of electrons 3 . h n - f only depends on n not on intensity 4. K ­ as n ­ max 5 . single electrons are excited by light (not many gradually) Þ instantaneous emission Kmax = hn-f slope = h Kmax nc 13

More Evidence that light is composed of particles Compton Effect q f scattered photon scattered electron 14

[ ] ( ) ( ) Observed scattering intensity I I = I l , q ; incident l ¹ scattered l - this contradicts classical theory D l = l - l Compton ( 1923 ) suggested treating photon as particle hc E = h n = l The Special Theory of Relativity gives E = pc [ p is the magnitude of the momentum of the photon ] hc h \ pc = Þ p = l l D E = D p = tot tot h Þ D l = ( 1 - cos q ) m c e Þ l ­ ; n ¯ ; E ¯ during collision photon h Compton Wavelength of electron = m c e 15

What is Light?

Youngs Double Slit Experiment Light is composed of waves Photo Electric Effect Light is composed of particles Compton Effect Paradox? Wave Particle Duality 16

Atomic Spectra 17

Absorption Spectra gas gas Emission Spectra 18

19

20

æ 1 1 1 ö ç = R - ÷ ; n = n + 1 , n + 2 , K l è n n ø R = 1 . 0973732 H è n 2 n 2 ø 2 1 1 1 2 7 R = 1 . 0973732 ´ 10 m-1 º Rydberg Constant H n = 1 Û Lyman 1 n = 2 Û Balmer 1 n = 3 Û Paschen 1 n = 4 Û Brackett 1 21

Bohr Model

Animation Angular Momentum Quantization 1 . Electron moves in circular orbit about nucleus 2 . Electron can only exist in specific orbits determined by Angular Momentum Quantization h L = m v r = I w = n = n h ; n = 1 , 2 , K e 2 p é v ù I = mr ; w = 2 ë r û 3 . Electrons in such orbits DO NOT radiate energy although they are accelerating. Such orbits are thus called STATIONARY STATES 4. Atoms radiate only when electron jumps from higher energy (large radius ) to lower energy (smaller radius ) orbits . The frequency of light they radiate is given by E - E Animation n = h l h 22

- ( ) ( ) ( ) kq q e U r = = - k r r k = coulombs constant 1 e E r = K 2 ( ) U r = 1 2 = - k r r k = coulombs constant - 1 e 2 ( ) r E r = K + U = m v - k 2 2 e r + If electrons speed is constant m v 2 e 2 e 2 F = m a = e = k Þ m v 2 = k c e c r r 2 e r 1 1 e 2 \ m v 2 = k 2 e 2 r 1 e 2 Þ E ( r ) = - k 2 r 23

Quantization of Angular Momentum ß n h n h r = Û v = m v m r e e n 2 h ke 2 2 \ m v 2 = = e m r 2 r e n 2 h 2 Þ r = ; n = 1 , 2 , K m ke 2 e \ r = r i . e . r depends on n n h 2 Bohr radius is defined as r = m ke 2 e so that r = n r 2 n 24

for the energy we obtain using these values for r in the expression n for the energy we obtain m k 2 e 4 æ 1 ö E = - e ; n = 1 , 2 , K 2 è n ø n h 2 2 æ 1 ö = - 13 . 6 eV è ø n 2 thus the frequencies of emitted photons are E - E m k æ 2 e 4 1 1 ö n = 2 1 = e ç - ÷ 21 h 2h h n n 2 è 2 2 ø 1 2 1 n m k æ 2 e 4 1 1 ö = = ç e - ÷ l c 2h h 2 c è n 2 n 2 ø 1 2 Theoretical expression for Rydberg constant m k 2 e 4 R = e H 2h h 2 c which is in good agreement with experimental value 25