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Chapter 24

Chapter 24. The Electromagnetic Wave. 0) Summary of electromagnetism to Maxwell (1860). • Kelvin: lines-of-force to replace action-at-a-distance (Coulomb’s law) • Faraday extended idea of fields • Maxwell rationalized E&M as stresses and strains in a medium, and tied up the whole field.

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Chapter 24

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  1. Chapter 24 The Electromagnetic Wave

  2. 0) Summary of electromagnetism to Maxwell (1860) • Kelvin: lines-of-force to replace action-at-a-distance (Coulomb’s law) • Faraday extended idea of fields • Maxwell rationalized E&M as stresses and strains in a medium, and tied up the whole field.

  3. a) Gauss’s Law A line that doesn’t start or finish inside must leave if it enters.

  4. (in free space) Gauss’s law expresses Coulomb’s law, but valid for moving charges

  5. b) Gauss’s Law for magnetism (since there are no magnetic monopoles)

  6. (in free space) c) Ampere’s Law

  7. Recall so around a loop, d) Faraday’s Law

  8. c) b) d) In free space a)

  9. loop Displacement Current I + + + + + + + + A1 A2 e) Maxwell’s displacement current Current through A1 is I current through A2 is zero both are bounded by the loop Ampere’s law needs a term for the growing electric field

  10. Recall Faraday’s law: Maxwell looked for a term proportional to (in free space)

  11. f) Maxwell’s Equations in free space

  12. g) Solutions to Maxwell’s equations Solution to F=ma for constant F is x(t): Solution to Maxwell’s equations give

  13. The concept: Oscillating source -> E changes -> B changes -> E changes…

  14. The solution:

  15. The solution: where Maxwell’s equations predict a disturbance that propogates through empty space at a speed:

  16. The conclusion was inescapable: light is “an electromagnetic disturbance in the form of waves” propogated in the ether.

  17. 1) Nature of electromagnetic waves • Radiation field propogates indefinitely • Transverse wave • E and B perpendicular to each other and to v • No medium needed (speed lower in medium)

  18. Reception

  19. Reception

  20. 2) The electromagnetic spectrum • c = fl • Range of f and l is more than 20 orders of magnitude

  21. 3) The speed of light • Definition: cº 299 792 458 m/s exactly Meter º distance light travels in 1/299792458 s (1 second defined by oscillations in Cs)

  22. Measurements: • 100 BC, Hero of Alexandria, eye blinking: c infinite • Descartes, geometry of eclipse: c infinite

  23. 1600, Galileo, lanterns: c > mach 10

  24. 1676, Roemer, eclipse of IO: c = 2 x 108 m/s

  25. 1728, Bradley, stellar aberration: c=3.01x108m/s

  26. 1849, Fizeau, toothed wheel: c = 3.13x108 m/s

  27. 1875, Foucault, rotating mirror: c = 2.99796x108m/s

  28. 1878 - 1926, Michelson, rotating mirror: c=2.99796x108m/s

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