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27 Overview

27 Overview. Homework: 15, 17, 23, 29, 57, 59. Classical vs. Quantum. Classical Theories: real number variables Quantum” theories: integer variables, e.g. electron energies in an atom Quantum theories were discovered around 1900, all theories before ~1900 were Classical.

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27 Overview

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  1. 27 Overview • Homework: • 15, 17, 23, 29, 57, 59.

  2. Classical vs. Quantum • Classical Theories: real number variables • Quantum” theories: integer variables, e.g. electron energies in an atom • Quantum theories were discovered around 1900, all theories before ~1900 were Classical

  3. photoelectric effect • light causes electron emission (photocurrent) • Ex. garage door safety beam • !Not explained by Classical theories! • Photelectric phet

  4. Photoelectric Observations • Cutoff frequency fo • KEmax ~ f – fo • KEmax is (1.6E-19C)Vstop • Ex. (1.6E-19C)(6V) = 9.6E-19J • f – fo = 2E15 – 5.6E14 = 1.44E15 • KEmax/(f – fo) = 6.67e-34 J/(1/s)

  5. classical violations observed in photoelectric effect • max. KE of electrons not dependent on intensity • no electrons emitted for low frequency light, no matter how bright • Einstein: light is made of small bundles of wave energy that act or don’t act • Termed “photons” of light

  6. Planck’s Constant, h • constant connecting quantum # to actual energy: h = 6.63x10-34 J·s • Ex. photon energy = hf • Ex. energy of 500nm photon • f = c/l: E = hf = hc/l = hc/500nm • Where: hc = 1240 eV·nm

  7. work function of metal • photon energy breaks electron bond to metal, extra energy is distributed to electron. Bond energy = Wo = hfo • KEmax = hf - Wo Ex. Wo = 4.3 eV, KEmax = 5.0 eV: hf = 5.0eV + 4.3eV = 9.3 eV l = hc/E = 1240eVnm/9.3eV = 133 nm (uv light)

  8. Light Emission and Absorption • white light • Gas absorbtion • hydrogen spectra  quantum theory of electron energies (Bohr)

  9. Bohr’s Hydrogen Theory • stable orbits: • Photon energies are differences in energies:

  10. Hydrogen Example

  11. 27 Summary • blackbody & photoelectric data led to first quantum theories • many quantum effects violate our intuition • photon concept for light is connected to Planck’s constant • H atom gas spectra has simplest quantum fit (energy levels related to 1/n2)

  12. Blackbody Radiation • all objects radiate EM radiation • emission & peak frequency increases with temperature • Wien’s Law, lmaxT = 0.0029 m·K • radiation energy is quantized (Planck) • fits with continuous radiation spectra implied infinite radiation energy at small l.

  13. Blackbody Phet • Blackbody phet

  14. Quotes • Those who are not shocked when they first come across quantum theory cannot possibly have understood it. Niels Bohr. • If you are not completely confused by quantum mechanics, you do not understand it. John Wheeler. • It is safe to say that nobody understands quantum mechanics. Richard Feynman.

  15. More quotes • If [quantum theory] is correct, it signifies the end of physics as a science. Albert Einstein. • I do not like [quantum mechanics], and I am sorry I ever had anything to do with it. Erwin Schrödinger. • Quantum mechanics makes absolutely no sense. Roger Penrose.

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