PHY122_L22_PostingAfterLecture

# PHY122_L22_PostingAfterLecture - PHY122 Physics for the...

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PHY122 – Physics for the Life Sciences II Lecture 22 Quantum Physics, part 2 Make sure to read the “note” on slides 23, 24, 25 You should see Brian Greene’s NOVA program “Quantum Leap” (the 3 rd of a 4-part series “The Fabric of the Cosmos”) See it at http://video.pbs.org/video/2167398185 HW 13 due Sunday, 27 Nov. at 8 pm Note: Clicker Channel 21 11/22/2011 Lecture 22 1

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Quantum physics challenges the mind… 11/22/2011 Lecture 22 2 From: http://phys.wordpress.com/2006/06/09/quantum-mechanical-quotes/
More… 11/22/2011 Lecture 22 3 Albert Einstein to Otto Stern (quoted by A. Pais in Subtle is the Lord)

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Lecture 22 4 Planck’s theory from last lecture Imagine a black body with walls made of hot conductor (metal): Electromagnetic standing waves must have nodes at the boundaries (walls). • Associate the Energy of each wave with its Frequency : E = hf . – Planck’s constant: h = 6.63×10 –34 Js b Energy and Wavelength: E = h c / λ Add all possible waves and their energies: with well-established statistical thermodynamics, this gives an excellent fit! • What a radical solution! Energy must be quantized! Max Planck (1858-1947). Nobel Prize: 1918 for “his discovery of energy quanta” hc= 1240 eV·nm 11/22/2011 and wavelength must be matched to cavity dimensions !
Lecture 22 5 Einstein’s Revolution #2 from last lecture Einstein takes Planck at his word: E = hf • Special Relativity for light ( massless “photons” ) gives • And thus photons carry momentum [cf., nonrelativistic momentum: p = mv Light is (by defninition) “relativistic”] This explains the Photoelectric Effect: • light must be above a threshold frequency, i.e., above a THRESHOLD ENERGY, i.e., below a threshold wavelength, to knock electrons off a metal surface in vacuum • Einstein’s 1921 Nobel Prize! pc hf E = = 11/22/2011 g = ² = G ²/± = G ³

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Electron Diffraction from last lecture Indeed, a beam of “monochromatic” electrons “shining” on a double slit, produces an interference pattern! EVEN if they come at only 1 electron per second !! BUT: ONLY if we do not measure WHICH of the two slits they go through ! – “spooky magic” of quantum physics! – deduced from slit separation d , the momentum p=mv of the electrons, and the interference pattern: 11/22/2011 Lecture 22 6 h p λ = 1240 eV nm hc = gsinG ± = ² ³ ; ´ = 0, µ1, µ2, … the de Broglie formula Louis de Broglie (1892-1987) Ph.D. 1924 Nobel Prize 1927 “for his discovery of the wave nature of electrons”
A K = 100 eV electron has de Broglie wavelength λ = 0.123 nm. What is λ in nm for a K = 0.01 eV electron? 11/22/2011 Lecture 22 7 12.3 51 123 17 1.23 15 .0123 7 OTHER 25 Matched Acceptable Value: 51, Within Range: 52 Acceptable Value: 12.3, Acceptable Range: {11.3, 13.3} g = G ± = G ² ± ² = G ² ´ ² µ 2 2 1 2 2 p K m mv = = correct

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What is “Waving” for a Particle ?? from last lecture Physicists discussed/argued about what exactly is “waving”
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PHY122_L22_PostingAfterLecture - PHY122 Physics for the...

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