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Electromagnetic Waves

Physics 1161: Lecture 18. Electromagnetic Waves. Textbook Sections 25-1 – 25-5. Direction wave travels. Electromagnetic Waves. Generator creates E field up and down. - +. Direction wave travels. Electromagnetic Waves. Generator creates E field up and down. - - + +.

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Electromagnetic Waves

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  1. Physics 1161:Lecture 18 Electromagnetic Waves • Textbook Sections 25-1 – 25-5

  2. Direction wave travels Electromagnetic Waves Generator creates E field up and down. - +

  3. Direction wave travels Electromagnetic Waves Generator creates E field up and down. - - + +

  4. Direction wave travels Electromagnetic Waves Generator creates E field up and down. - +

  5. Direction wave travels Electromagnetic Waves Generator creates E field up and down.

  6. Direction wave travels Electromagnetic Waves Generator creates E field up and down. + -

  7. Direction wave travels Electromagnetic Waves Generator creates E field up and down. + + - -

  8. Direction wave travels Electromagnetic Waves Generator creates E field up and down. + -

  9. + - y x z Electromagnetic Waves

  10. Direction wave travels + - Which direction should I orient my antenna to receive a signal from a vertical transmission tower? • Vertical • Horizontal • 45o angle Alternating E field moves charges up and down thru antenna!

  11. Preflight 18-1 • An electromagnetic wave is travelling along the x-axis, with its electric field oscillating along the y-axis. • In what direction does the magnetic field oscillate?      along the x-axis      along the z-axis      along the y-axis

  12. + - y x z Electromagnetic Waves Generator also creates B field into and out of the page!

  13. Preflight 18-8 Which of the loops will detect the electromagnetic wave?    (1) x-y plane  (2) x-z plane  (3) y-z plane

  14. y x z Electromagnetic Waves • Transverse (vs. sound waves – longitudinal) • E perpendicular to B and always in phaseE & B increase and decrease at same times • Can travel in empty space (sound waves can’t!) • “Speed of light”: v = c = 1/ (e0m0) = 3 x 108 m/s (186,000 miles/second!) • Frequency:f = v/l = c/l

  15. Preflight 18.2 – 18.7 Which of the following are transverse waves? • sound • light • radio • X-ray • microwave • “The Wave” (i.e. at football games)

  16. Preflight 14.6 – 14.12 Which of the following are transverse waves?  • sound • light • radio • X-ray • microwave • “The Wave” (i.e. at football games)   All but sound!   

  17. E x Example EM Waves Practice Shown below is the E field of an EM wave broadcast at 96.1 MHz and traveling to the right. (1) What is the direction of the magnetic field? (2) Label the two tic marks on the x axis (in meters).

  18. E x Example EM Waves Practice Shown below is the E field of an EM wave broadcast at 96.1 MHz and traveling to the right. (1) What is the direction of the magnetic field? (2) Label the two tic marks on the x axis (in meters). Perpendicular to E, v: Into/out of the page 3.1 6.2

  19. Light -- an Electromagnetic Wave

  20. Electromagnetic Wave • Transverse • Traveling oscillating electric field and magnetic field • Can travel through space • Generated by accelerated charges • Emitted by excited atoms when they return to ground state

  21. Electromagnetic Spectrum

  22. Visible Part of Spectrum 4 7

  23. Spring Model • Electrons can be modeled as particles connected to the atomic nucleus by springs • Electrons of atoms in glass have certain natural frequencies of vibration

  24. Light Transmission

  25. Glass Transparency Ultraviolet matches natural frequency of electrons in glass Infrared vibrates entire atom or molecule Both result in warming of the glass

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