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Understanding Electromagnetic Radiation and Transmission in the EM Spectrum

Learn about EM radiation, Maxell's Equations, E-M Wave Reception, Blackbody Radiation, and Polarization of EM Waves. Discover the interrelation between Electric and Magnetic fields, EMF induction, and propagation behaviors of EM waves. Dive into topics such as radio wave transmission, reception methods, and the fascinating world of the Electromagnetic Spectrum. Find out how changing fields induce each other and how polarization affects light transmission. Explore the practical applications and theoretical foundations of electromagnetic phenomena.

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Understanding Electromagnetic Radiation and Transmission in the EM Spectrum

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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.

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