Electricity and Magnetism- Generating E.M.waves

1 P32-Class 32: OutlineHour 1:Generating Electromagnetic WavesPlane EM WavesElectric Dipole EM WavesHour 2:Experiment 12: MicrowavesReview Exam 3 Results2 P32-Recall:Electromagnetic Radiation3 P32-Recall Electromagnetic Radiation: Plane Waveshttp://ocw.mit.edu/ans7870/8/8.02T/f04/visualizations/light/07-EBlight/07-EB_Light_320.html4 P32-Properties of EM Waves8001310mvcsµε===×00EEcBB==Travel (through vacuum) with speed of lightAt every point in the wave and any instant of time, E and B are in phase with one another, withE and B fields perpendicular to one another, and to the direction of propagation (they are transverse):Direction of propagation = Direction of ×EB􀁇􀁇5 P32-Generating Plane Electromagnetic Radiation6 P32-Shake A Sheet of Charge Up and Down 7 P32-Java Applet for Generation of Plane Waves http://ocw.mit.edu/ans7870/8/8.02T/f04/visualizations/light/09-planewaveapp/09-planewaveapp320.html8 P32-First Pull The Sheet of Charge Down At Speed v10tanEvTvEcTcθ===100ˆˆ2vvEccσε⎛⎞⎛⎞==⎜⎟⎜⎟⎝⎠⎝⎠Ejj􀁇When you pull down, there is a back force up!9 P32-Rate of Work Done?2100ˆˆ()22evvdAddqdAccσσσεε⎛⎞⎛⎞===⎜⎟⎜⎟⎝⎠⎝⎠FEjj􀁇􀁇2ext0ˆ2evdAddcσε⎛⎞=−=−⎜⎟⎝⎠FFj􀁇􀁇()2222extext00ˆˆ22dWddvvvdAdtdAdtccσσεε⎛⎞=⋅=−⋅−=⎜⎟⎝⎠Fsjj􀁇􀁇10 P32-What About B Field?010ˆ(2), 0 ˆ(2), 0vxvxµσµσ⎧+>⎪=⎨−<⎪⎩kBk􀁇100ˆˆ2vvEccσε⎛⎞⎛⎞==⎜⎟⎜⎟⎝⎠⎝⎠Ejj􀁇11 P32-Rate Energy Carried Away?22011000011ˆˆˆ224vvvccµσσσµµεε⎛⎞⎛⎞⎛⎞=×=×=⎜⎟⎜⎟⎜⎟⎝⎠⎝⎠⎝⎠SEBjki􀁇􀁇􀁇Energy radiated to left and right is exactly equal to the rate of work required to move sheet down12 P32-To generate plane wave, move sheet up and down sinusoidallyThe work you do in moving the sheet is carried away as electromagnetic radiation, with 100% efficiency.13 P32-Generating Plane Wave Applet14 P32-PRS Question:Generating A Plane Wave15 P32-Generating Electric Dipole Electromagnetic Waves16 P32-Generating Electric Dipole Radiation Applet17 P32-Quarter-Wavelength AntennaAccelerated charges are the source of EM waves. Most common example: Electric Dipole Radiation.4λ4λt = 0t = T/4t = T/2t = T18 P32-Why are Radio Towers Tall?AM Radio stations have frequencies 535 –1605 kHz. WLW 700 Cincinnati is at 700 kHz. 83310m/s429m70010Hz/4107m350ftcfλλ×===×≈≈The WLW 700 Cincinnati Tower is 747 ft tall19 P32-Quarter-Wavelength Antenna20 P32-Quarter-Wavelength Antennahttp://ocw.mit.edu/ans7870/8/8.02T/f04/visualizations/light/04-QuarterWaveAntenna/04-MicrowaveDLICS_320.html21 P32-Spark Gap Transmitter22 P32-Spark Gap Generator:An LC OscillatorFirst: Example of “lumped” LC Oscillator (Capacitor & Inductor together as one)23 P32-Group Problem: Lumped LC CircuitHeight (into page): hParallel Plate CapacitorCylindrical InductorQuestion: What is the resonance frequency?Recall:01LCω=0ACdε=LIΦ=[]solenoidcurrentperunitlengthoBµ=24 P32-Our spark gap antenna6124(4.510)(3310F)1.510sτRC−−==×Ω×=×rad14cfTl==10310cm/s12.4cm×=92.410Hz2.4GHz=×=1) Charge gap (RC)2) Breakdown! (LC)3) Repeat25 P32-Spark Gap Transmitter26 P32-PRS Question:Spark Gap Antenna27 P32-Spark Gap Antennahttp://ocw.mit.edu/ans7870/8/8.02T/f04/visualizations/light/03-AntennaPattern/03-MicrowaveAntenna_320.html28 P32-Spark Gap Antenna29 P32-PRS Questions:Angular Distribution & Polarization of Radiation30 P32-Demonstration:Antenna31 P32-Polarization32 P32-Polarization of TV EM WavesWhy oriented as shown?Why different lengths?33 P32-Demonstration:Microwave Polarization34 P32-Experiment 12:Measure Wavelength by Setting Up Standing Wave35 P32-Experiment 12:Microwaves36 P32-Exam 3 Results