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History of the Atom: Physics Chapter 27

History of the Atom: Physics Chapter 27. Early work in Electricity and Magnetism. Oersted: Current makes magnetic field Faraday/Henry: Magnetic fields moving make currents Maxwell: Electricity, magnetism and light are all parts of the electromagnetic field Hertz:

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History of the Atom: Physics Chapter 27

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  1. History of the Atom: Physics Chapter 27

  2. Early work in Electricity and Magnetism • Oersted: • Current makes magnetic field • Faraday/Henry: • Magnetic fields moving make currents • Maxwell: • Electricity, magnetism and light are all parts of the electromagnetic field • Hertz: • Experiments supported Maxwell’s work

  3. James Clerk Maxwell • 1831-1879 • Showed that electricity and magnetism were related, and were related to atoms • Predicted that accelerating charges would make waves (electromagnetic radiation)

  4. Cathode Ray Tube Experiments • Glass tube with wire at each end; as much air pumped out as possible • Charge passed across tube makes fluorescent glow • William Crookes • Tube coated with fluorescent material can be made to glow in one focused dot • Rays travel in straight lines • Ray carries negative charge

  5. Joseph John Thomson • 1856-1940 • Used a study of the cathode ray tube to determine the presence of electrons 1897 • Suggested the plum pudding model of the atom and the existance of isotopes • Won the Nobel Prize in Physics in 1906

  6. J. J. Thomson’s Experiment • Thomson used both a magnetic field and electric field to deflect the electrons • He measured the deflection of the ray and calculated the charge:mass ratio of electrons

  7. J. J. Thomson’s Experiment • Used magnetic field to show cathode rays had negative charge • Used electric field to show cathode rays were particles with negative charge • Used varying electric currents to determine charge to mass ratio • Force caused by electric field: qE • Force caused by magnetic field: Bqv • When these forces are equal Bqv=qE • Then v = E/B • When electric field removed, particles given centripetal force by magnetic field Bqv = mv2/r • Solved for mass/charge ratio: m/q = Br/v • Thomson calculated m/q as 5.686 x 10-12 kg/C • Evidence suggested particles very small and came from atom

  8. J. J. Thomson’s experiment • Thomson calculated m/q as 5.686 x 10-12 kg/C • Millikan calculated q = 1.602 x 10-19 C • Can be used to calculate m: • m=(5.686 x 10-12 kg/C) q • m calculated as 9.109 x 10-31 kg • Method can be used for any charged particle

  9. Example • A beam of electrons travels an undeflected path in a cathode ray tube. E is 7.0 x 103 N/C. B is 3.5 x 10-2 T. What is the speed of the electrons as they travel through the tube? • What we know: • E = 7.0 x 103 N/C B=3.5 x 10-2 T • Equation: • v = E/B • Substitute: • v = (7.0 x 103N/A s) / (3.5 x 10-2 N/A m) • Solve! • v = 2.0 x 105 m/s

  10. Example • An electron of mass 9.11 x 10-31 kg moves with a speed of 2.0 x 105 m/s across a magnetic field. The magnetic induction is 8.0 x 10-4 T. What is the radius of the circular path followed by the electrons while in the field? • What we know: • M = 9.11 x 10-31 kg B=8.0 x 10-4 T v=2.0 x 105 m/s • Equation: • Bqv = mv2/r so r = (mv) / (Bq) • Substitute: • R = (9.11x10-31kg)(2.0x105 m/s) • (8.0x10-4N/Am)(1.6x10-19As) • Solve! r = 1.4 x 10-3 m

  11. Robert A. Millikan • 1858-1953 • Used the 'falling drop method' to determine the charge of the electron (-1.6022 x 10-19 C) and mass of electron as 9.10 x 10-28 g • Investigated photoelectric effect and spectroscopy of elements • Won the Nobel Prize in Physics in 1923

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