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Exam 2 cutoffs A 80 B 70 C 60 D 50 42 +/- 4 72.3 +/- 9 What can I do to help my grade? Bonus points Special WEBQUIZ like last time ·one time only ·30 minute limit ·noon-midnight, this Thursday ·ONLY for those scoring below 70 points on Exam 2 Weighted to close the gap between the dual distribution: Exam Score Possible gain in points 40 30 50 20 60 10 70 0 This week’s regular WEBQUIZ cancelled! The electric field in an EM wave traveling northward oscillates up and down. In what plane does the magnetic field oscillate? 1) In the north - south plane. 2) In the up - down plane. 3) In the northeast - southwest plane. 4) In the northwest - southeast plane. 5) In the east - west plane. Electromagnetic Waves EM Waves are possible! What produces a magnetic field? current What produces a changing magnetic field? changing current Accelerating or oscillating charge changing current oscillating charge oscillating E field oscillating B field Direction of wave motion Recall: speed of mechanical waves along an elastic medium ~ T m Tension linear density E directly pushes charges (connecting distance charges), conveying the disturbance on one charge to others B induces back emfs to resist the changing E-fields v= E B while the mutually induced fields guaranteed EB Maxwell concluded disturbances in E, B field lines ripple out at constant speed 1 v= m 0e 0 In 1849 (Maxwell’s time) v= 1 m 0e 0 = 311,000,000 m/sec and Fizeau’s best measurement: c=315,000,000 m/sec Maxwell: “The velocity of the transverse undulations calculated from the electromagnetic measurements of MMs. Kohlrausch and Weber, agrees so exactly with the velocity of light calculated from the optical experiments of M. Fizeau, that we can scarcely avoid the inference that light consists in the transverse undulations of electric & magnetic phenomena.” The wavelength of microwaves generated by a microwave oven is about 3 cm. At what frequency do these waves cause the water molecules in your breakfast burrito to vibrate ? 1) 1 GHz 2) 10 GHz 1 GHz = 109 Hz 3) 100 GHz Infrared Photography Infrared photography is used to check heat insulation of houses in winter. If you look at an infrared picture of a house which areas would need better insulation? 1) the bright areas 2) the dark areas 3) need more information Some answers to previous slides: 5) In the east - west plane. In an electromagnetic wave, the E and B fields are perpendicular to each other and to the direction the wave propagates. 2) 10 GHz f = v / = 3 108 m/s / 0.03 m = 1010 Hz = 10 GHz 1) the bright areas