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Ch. 25: EM Radiation

Ch. 25: EM Radiation

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Ch. 25: EM Radiation

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  1. Ch. 25:EM Radiation

  2. James Clerk Maxwell • E-field lines originate on positive charges and terminate on negative charges (Gauss’ Law) • B-field lines always form closed loops. • Changing B-field induces an emf, an E-field (Faraday’s Law). • B-fields are created by moving charges, currents (Ampere’s Law).

  3. Maxwell Predicted…(1865) • Electric and Magnetic fields are interrelated. • Varying E-field with time yields B-field. • Solution of Maxwell’s Equations in vacuum yield propagating, fluctuating, electric and magnetic fields • Each varying field induces the other.

  4. Heinrich Hertz (1887) • Experimentally validated Maxwell’s EM theories. • First to generate and detect EM waves in the laboratory.

  5. Radio Wave Transmission • E-field wave: • High-frequency reversal of the polarity of an electric dipole creates a varying electric field (time and space). E (x,t)

  6. Radio Wave Transmission • B-field Wave: • Changing polarity of a dipole creates a current. • Changing current induces a varying magnetic field. B (x,t)

  7. E-M Radiation

  8. Reception of Radio Waves • E-wave Reception: Antenna • E-wave supplies changing E-field at wire  Induces AC current • B-wave Reception: Loop Antenna • B-wave sets a changing magnetic field through the wire loop  varying magnetic flux induces a varying EMF  Induces AC current

  9. The Electromagnetic Spectrum c = λf

  10. Blackbody Radiation • All bodies emit thermal emission due to movement* of charges in matter. • The emission spectrum is temperature dependent. • Stars, fire, incandescent light bulbs, lava, hot coals, etc. peak in the visible part of the spectrum. • Humans’ thermal emission peaks in the infrared, ~12 μm.

  11. Polarization of EM Waves • EM waves from radio antenna are plane (linearly) polarized. • Created by charges oscillating in a line. • Thermal emission sources are non-polarized • Created by random thermal motion.

  12. Polarization of EM Waves • Non-polarized light can be polarized by use of Polaroid sheets. • Consist of plastic with needle-like crystals oriented lengthwise • Allow only one orientation of wave to pass • Sheet parallel to polarization passes light unimpeded. • Sheet perpendicular to polarization reduces flux to 0. • Two perpendicular Polaroid sheets reduces EM flux to 0. • Polarized sunglasses • 3D-movies!

  13. H-ITT • You observe two otherwise identical, thin tungsten filaments. Filament 1 glows red. Filament 2 glows yellow/white. What can you say about the respective currents through the filaments? A.) I1 = I2 B.) I1 < I2 C.) I1 > I2 D.) There is no current. E.) Not a thing.

  14. Fin.