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# lecture15 - Physics 102 Lecture 15 Electromagnetic Waves...

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Electromagnetic Waves and Polarization Physics 102: Lecture 15 HE2: Monday March 15 Material covered: lectures through 12 NOT INCLUDING AC CIRCUITS! discussions through 6 homework through 6 labs through 5 Review: Sunday March 14, 3-4:30, room 141 Will go over Fall 2009 Exam

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Today: Electromagnetic Waves Energy Intensity Polarization
x z y E B Only the loop in the xy plane will have a magnetic flux through it as the wave passes. The flux will oscillate with time and induce an emf. (Faraday’s Law!!!) loop in xy plane loop in xz plane loop in yz plane 1 2 3 7 Preflight 15.1, 15.2 “In o rde r to  find the  lo o p that de te c ts  the  e le c tro m ag ne tic  wave ,  we  s ho uld find the  lo o p that has  the  g re ate s t flux thro ug h the   lo o p.” 32% 26% 42%

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Propagation of EM Waves Changing B field creates E field Changing E field creates B field E = c B x z y 10 If you decrease E, you also decrease B! This is important !
Preflight 15.4 Suppose that the electric field of an electromagnetic wave decreases in magnitude. The magnetic field: 1 increases 2 decreases 3 remains the same E=cB 17% 74% 8%

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Energy in E field 13 Electric Fields Recall Capacitor Energy: U = ½ C V 2 Energy Density (U/Volume): u E = ½ ε 0 E 2 Ad E d E d A CV U 2 0 2 1 2 2 0 2 1 2 2 1 ε ε = = = d A E d A C o ε = Ed V =
Energy in B field 13 Magnetic Fields Recall Inductor Energy: U = ½ L I 2 Energy Density (U/Volume): u B = ½ B 2 / μ 0 A l

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