Physics 22: Electromagnetic Waves

# Physics 22: Electromagnetic Waves

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## Physics 22: Electromagnetic Waves

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1. Physics 22: Electromagnetic Waves Christopher Chui Physics 22: Electromagnetic Waves - Christopher Chui

2. 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

3. 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

4. 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

5. 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