8B - 12_EMRad_11.ppt

8B - 12_EMRad_11.ppt - Electromagnetic radiation Maxwell...

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Electromagnetic radiation Maxwell predicted EM waves and H. Hertz first demonstrated them experimentally. We now discuss them further.
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Describing Traveling Sine Wave W / W 0 = sin2 π (x/ λ - t/T) = sin2 π (x/ λ - f t) = sin(kx - ω t) = sink[x - ( ω /k)t] = sink[x - vt] When x increases by λ , angle increases by 2 π and sin has same value. When t increases by T, angle increases by 2 π , and sin has same value. Velocity of wave, v = λ / T = f λ = ω / k W 0 x λ T = 1/f
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General Properties 0f Waves Energy Momentum Pressure Polarization Reflection Total Internal Reflection - Frustrated (Tunnel) Refraction Diffraction Interference Standing waves (modes of vibration)
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Properties of EM Waves 8 0 0 1 3 10 m v c s μ ! = = = " 0 0 E E v B B = = Travel (through vacuum) with speed of light, c, in all inertial frames, for all f and λ , with c = f λ = but in materials v varies with f and λ . At every point in the wave and any instant of time, E and B are in phase with one another, with E and B fields are perpendicular to one another, and to the direction of propagation (they are transverse ): Direction of propagation = Direction of ! E B r r
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Electromagnetic Waves f c ! = Hz
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Generating Electric Dipole Electromagnetic Waves Animations of EM wave generation: http://www.upscale. utoronto . ca/PVB/Harrison/Flash/E M/EMWave/EMWave .html More detailed: http://www. falstad .com/emwave1/
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Quarter-Wavelength Antenna
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Accelerated charges are the source of EM waves Most commonly electric dipole radiation. Example: quarter-wavelength antenna t = 0 t = T/4 t = T/2 t = T 4 ! 4 Animation:http://www-antenna.pe.titech.ac.jp/~hira/hobby/edu/em/smalldipole/smalldipole.html
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Radiation from a L = λ / 4 Antenna
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Why are Radio Towers Tall? AM Radio stations have frequencies 535 – 1605 kHz. (WLW 700 Cincinnati is at 700 kHz.) 8 3 3 10 m/s 429m 700 10 Hz / 4 107m 350ft c f !
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This note was uploaded on 04/10/2011 for the course PHYS 8B taught by Professor Catherinebordel during the Spring '10 term at Berkeley.

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8B - 12_EMRad_11.ppt - Electromagnetic radiation Maxwell...

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