Physics 22: Electromagnetic Waves

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Physics 22 Electromagnetic Waves

• Christopher Chui

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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

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Displacement Current and Amperes Law

• The generalized Amperes law
• SB//Dl moIC moeo (DFE/Dt) Maxwells 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

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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

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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 signals amplitude
• In frequency modulation (FM), the frequency of
  the carrier signal is made to change in
  proportion to the audio signals amplitude