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Physics 22: Electromagnetic Waves. Christopher Chui. Changing Electric Fields produce Magnetic Fields. James Clerk Maxwell (1831-1879) made predictions of electromagnetic waves and unified all phenomena of electricity and magnetism

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physics 22 electromagnetic waves

Physics 22: Electromagnetic Waves

Christopher Chui

Physics 22: Electromagnetic Waves - Christopher Chui

changing electric fields produce magnetic fields
Changing Electric Fields produce Magnetic Fields
  • James Clerk Maxwell (1831-1879) made predictions of electromagnetic waves and unified all phenomena of electricity and magnetism
  • Maxwell equations: 1) a form of Gauss’ law that relates electric field to its sources, electric charge; 2) magnetic field lines are always continuous; 3) an electric field is produced by a changing magnetic field; 4) a magnetic field is produced by an electric current, or by a changing electric field

Physics 22: Electromagnetic Waves - Christopher Chui

displacement current and ampere s law
Displacement Current and Ampere’s Law
  • The generalized Ampere’s law:
  • SB//Dl = moIC + moeo (DFE/Dt) – Maxwell’s 4th Law
  • A sinusoidal signal on a dipole antenna produces electric fields, which in turn, produce magnetic field lines perpendicular to the electric fields. These fields form closed loops.
  • EM waves are produced by accelerating electric charges
  • Speed of EM waves, v = 1 / sqrt (moeo) = c, speed of light
  • E = vB where v is the speed of the wave

Physics 22: Electromagnetic Waves - Christopher Chui

light as an em wave
Light as an EM Wave
  • Basic equation: fl = c
  • EM spectrum:
  • radio wavesmicrowavesinfraredvisible lightultravioletx-raysgamma rays
  • Speed of light ~ 3.00 x 108 m/s
  • Energy density in EM waves = u = ½ eoE2 + ½ B2/mo = B2/mo =eoE2 = sqrt (eo / mo)EB
  • Energy transported at any t = S = eocE2 =cB2/ mo = EB/ mo
  • Average S = ½ eocEo2 = ½ cBo2/ mo = EoBo/ 2mo =EmaxBmax/ mo

Physics 22: Electromagnetic Waves - Christopher Chui

radio and television
Radio and Television
  • Marconi (1874-1937) invented the wireless telegraph
  • Radio waves are produced by a radio transmitter
  • Sound waves are encoded into an audio signal, which is amplified. An RF oscillator produces an RF signal as a carrier, which is mixed into a modulated signal. The RF signal is amplified and radiated by the antenna.
  • In amplitude modulation (AM), the amplitude of the carrier signal is made to vary in proportion to the audio signal’s amplitude
  • In frequency modulation (FM), the frequency of the carrier signal is made to change in proportion to the audio signal’s amplitude

Physics 22: Electromagnetic Waves - Christopher Chui