# Lect21a - 1 Quantum Theory • Particles and Waves •...

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Unformatted text preview: 11/16/11 1 Quantum Theory • Particles and Waves • Particle properties of light: Photons • Wavelike properties of particles: electrons – interference – Electron diffraction – X-ray diffraction • Electron spin • Wavefunction • Tunneling Physics 104, Fall 2011 1 Physics 104 Lecture 20 Nov. 14, 2011 Chapter 28 (1 of 2) 11/16/11 • Midterm Exam 3 • 29 Nov, 5:45PM-7:00PM • Exam info : • Chapters: 24,25,26,28,29 11/16/11 Physics 104, Fall 2009 2 11/16/11 2 De Broglie Waves Physics 104, Fall 2011 3 λ h p = So far only for photons, but De B postulated that it holds for any object with momentum- an electron, a nucleus, an atom, a baseball,…... Should be able to see interference and diffraction for material particles!! p h = λ p = E c = hf c = h λ (Matter waves) 11/16/11 Wavelength of Electrons Physics 104, Fall 2011 4 Photon with energy E=1eV: What about an electron with 1eV kinetic energy? Е = 1.23 nm for a 1eV electron B i g d i f f e r e n c e ! Solve for E = hc λ ⇒ λ = hc E = 1240 eV nm 1 eV = 1240 nm KE = 1 2 mv 2 and p=mv, so KE= p 2 2m p = 2 m (K.E.) λ = h 2 m (KE) = hc 2 mc 2 (KE) = 1240 eV nm 2(511,000 eV)(1 eV) Wednesday, November 16, 2011 11/16/11 3 Question Wednesday, November 16, 2011 Physics 104, Fall 2011 5 A stone is dropped from the top of a building. What happens to the de Broglie wavelength of the stone as it falls? 1. It decreases 2. It stays the same 3. It increases Speed, v, KE=mv 2 /2, and momentum, p=mv, all increase. p = h λ ⇒ λ = h p Question Compare the wavelength of a bowling ball with the wavelength of a golf ball, if each have 10 Joules of kinetic energy. 1) λ bowling > λ golf 2) λ bowling = λ golf 3) λ bowling < λ golf Physics 104, Fall 2011 6 λ = h p λ = h 2 m (KE) Bowling ball has larger mass, smaller momentum for same energy, smaller de Broglie wavelength Perform quick estimate of the de Broglie wavelength! 11/16/11 11/16/11 4 Use of interference experiments to study materials at atomic distance scale Matter waves: Electron diffraction • Wavelength of 1 eV electron: 1.2nm (slide 3) • Need more energy to form an electron beam: 50eV • Wavelength of 50eV electron (Example 28.5 in textbook): ~0.17nm Photon waves X-ray diffraction • Energy: ~ 10keV • Wavelength of 1eV photon: 1240nm (slide 3) • Wavelength of 10keV photon (X-ray): 0.124nm 11/16/11 Physics 104, Fall 2011 7 Atomic distance scale: ~0.1nm need short wavelengths, shorter than light, close to the interatomic distance scale Both methods have been used very successfully! X-Ray Diffraction (Bragg’s Law) Physics 104, Fall 2011 8 11/16/11 11/16/11 5 X-Ray Diffraction (Bragg’s Law) Physics 104, Fall 2011 9 Note: Angle is measured with respect to the horizontal 2 d sin θ = m λ ( m = 1,2, 3, ) 2 d sin θ = m λ ( m = 1,2, 3, ) 11/16/11 Structure of Biological Molecules Physics 104, Fall 2011 10 X-Ray diffraction from DNA Rosalind Franklin’s picture Discovery of DNA structure 11/16/11 11/16/11...
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## This note was uploaded on 11/30/2011 for the course PHYSICS 104 taught by Professor Dasu/karle during the Fall '11 term at University of Wisconsin.

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Lect21a - 1 Quantum Theory • Particles and Waves •...

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