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Exam 3 Information and Schedule for PHY208

Get all the details you need for Exam 3 in PHY208, including the date, time, location, and material covered. Make sure to bring your calculator and a note sheet.

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Exam 3 Information and Schedule for PHY208

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  1. Exam 3 is Thursday Dec. 3 (after Thanksgiving) 5:30-7 pm, Birge 145 Students w / scheduled academic conflict please stay after class Tues. Nov. 24 to arrange alternate time. Covers: all material since exam 2. Bring: Calculator One (double-sided) 8 1/2 x 11 note sheet Schedule: Week14HW: assigned Thur. Nov. 19, due Fri. Dec. 4 (two weeks) Last material for exam: Lecture of Tues. Nov. 24 Exam review: Tuesday, Dec. 1, in class Phy208 Lect. 23

  2. Summary of Photoelectric effect • Light comes in photons - particles of light • h=Planck’s constant • Red photon is low frequency, low energy. • (Ultra)violet is high frequency, high energy. • Electron in metal absorbs one photon • Can escape metal if photon energy large enough • Ephoton>Work function Eo • Excess energy Ephoton-Eo shows up as kinetic energy Phy208 Lect. 23

  3. Photon properties of light • Photon of frequency f has energy hf • Red light made of ONLY red photons • The intensity of the beam can be increased by increasing the number of photons/second. • Photons/second = energy/second = power Phy208 Lect. 23

  4. E=4hf Energy E=3hf E=2hf E=hf Quantization of light Quantum mechanically, brightness can only be changed in steps, with energy differences of hf. • Possible energies for green light (=500 nm) • One quantum of energy:one photon • Two quanta of energytwo photons • etc • Think about light as a particle rather than wave. Phy208 Lect. 23

  5. Thompson’s model of atom • J.J. Thomson’s model of atom • A volume of positive charge • Electrons embedded throughout the volume • A change from Newton’s model of the atom as a tiny, hard, indestructible sphere • This model is not correct! Phy208 Lect. 23

  6. Phy208 Lect. 23

  7. Resulted in new model • Planetary model • Based on results of thin foil experiments • Positive charge is concentrated in the center of the atom, called the nucleus • Electrons orbit the nucleus like planets orbit the sun Phy208 Lect. 23

  8. Difference between atoms • Simplest is Hydrogen: • 1 electron orbiting 1 proton • Other atoms • number of orbiting negative electrons same as number of positive protons in nucleus • Different elements have different number of orbiting electrons • Helium: 2 electrons • Copper: 29 electrons • Uranium: 92 electrons! • Organized into periodic table of elements First concentrate on hydrogen atom Phy208 Lect. 23

  9. Planetary model and radiation • Circular motion of orbiting electrons: • electrons emit EM radiation at orbital frequency. • Similar to radio waves emitted by accelerating electrons in a antenna. • In an atom, emitted EM wave carries away energy • Electron predicted to continually lose energy. • The electron would eventually spiral into the nucleus • However most atoms are stable! Phy208 Lect. 23

  10. Line spectra from atoms • Atoms do emit radiation, but only at certain discrete frequencies. • Emission pattern unique to different atoms • Spectrum is an atomic ‘fingerprint’, used to identify atoms (e.g. in space). Hydrogen Mercury Wavelength (nm) Phy208 Lect. 23

  11. Einitial Photon Efinal Stable orbit Stable orbit The Bohr atom • Retained ‘planetary’ picture with circular orbits • Only certain orbits are stable • Radiation emitted only when electron jumps from one stable orbit to another. • Here, the emitted photon has an energy ofEinitial-Efinal Phy208 Lect. 23

  12. Zero energy n=4 n=3 n=2 n=1 Energy levels • Instead of drawing orbits, just indicate energy an electron would have if it were in that orbit. Energy axis Phy208 Lect. 23

  13. Zero energy n=4 n=3 n=2 Energy n=1 Hydrogen atom energies • Quantized energy levels: • Each corresponds to different • Orbit radius • Velocity • Particle wavefunction • Energy • Each described by a quantum number n Phy208 Lect. 23

  14. Emitting and absorbing light Zero energy n=4 n=4 Photon is emitted when electron drops from one quantum state to another n=3 n=3 n=2 n=2 Photon emittedhf=E2-E1 Photon absorbedhf=E2-E1 n=1 n=1 Absorbing a photon of correct energy makes electron jump to higher quantum state. Phy208 Lect. 23

  15. Hydrogen emission • This says hydrogen emits only photons of a particular wavelength, frequency • Photon energy = hf, so this means a particular energy. • Conservation of energy: • Energy carried away by photon is lost by the orbiting electron. Phy208 Lect. 23

  16. Zero energy n=4 n=3 Photon emittedhf=E2-E1 n=2 hf = hc/ = 1240 eV-nm/  n=1 Hydrogen atom An electron drops from an -1.5 eV energy level to one with energy of -3.4 eV. What is the wavelength of the photon emitted? A. 827 nm B. 653 nm C. 476 nm D. 365 nm E. 243 nm Phy208 Lect. 23

  17. Energy conservation for Bohr atom • Each orbit has a specific energy En=-13.6/n2 • Photon emitted when electron jumps from high energy to low energy orbit. Ei – Ef = h f • Photon absorption induces electron jump from low to high energy orbit. Ef – Ei = h f • Agrees with experiment Phy208 Lect. 23

  18. Hydrogen emission spectrum n=4 n=3 • Hydrogen is simplest atom • One electron orbiting around one proton. • The Balmer Series of emission lines empirically given by n = 4,  = 486.1 nm n = 3,  = 656.3 nm Hydrogen Phy208 Lect. 23

  19. Balmer series • Transitions terminate at n=2 • Each energy level has energy En=-13.6 / n2 eV • E.g. n to2 transition • Emitted photon has energy • Emitted wavelength Phy208 Lect. 23

  20. Electron velocity Electron orbit radius Integer:n=1,2,3… Why stable orbits? Bohr argued that the stable orbits are those for which the electron’s orbital angular momentumLis quantized as Bohr combined this with the Coulomb force to find allowed orbital radii and energies. Phy208 Lect. 23

  21. Including more physics Circular orbit, electron is accelerating (centripetal acceleration = v2/r = Force/mass) Force causing this accel. is Coulomb force ke2/r2 between pos. nucleus and neg. electron Alsogives a condition for angular momentum. Phy208 Lect. 23

  22. Bohr model of H-atom • Quantization: • Orbital motion: centripetal acceleration Coulomb force / mass Phy208 Lect. 23

  23. Radius of H-atom states and Orbital motion Quantization Quantized orbital radius Phy208 Lect. 23

  24. Energy of H-atom states Total Energy = kinetic + potential Quantized energy Phy208 Lect. 23

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