Physics 22: Electromagnetic Waves

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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

Christopher Chui

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
• 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
• 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
• 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