Physics 208, Fall 2008Your Professor and TAsLecturesDiscussions and LabsPowerPoint PresentationExamsGradingPhysics Learning CenterLast semester: mechanical wavesCommon wave propertiesWave propertiesPeriodic waveMoving wave crestsSame period, slower wave velocityFreq., wavelength, velocity are relatedWave relationsInterferenceDestructive Interference in a StringDestructive interference in a continuous waveSetting up destructive interferenceQuestionInterference of 2 speakersA little more detailOther angles?Interference engineeringLine array works by interferenceTuesday, Sep. 2 Phy208 Lecture 11Physics 208, Fall 2008Waves: Interference and Diffraction (Chap. 20-22)Colors in soap films, CDs, and butterfly wingsRefraction of Light, and Optics (Chap. 23-24)Telescopes, microscopes, and eyesElectricity (Chap. 26-32)Capacitors & cell membranes, battery-less flashlightsMagnetism (Chap. 33-34)Electromagnetic Waves (Chap. 35)Light and polarizationModern Physics (Chap. 37-42)Radiation and matter, radioactivity, nuclear fission http://www.physics.wisc.edu/undergrads/courses/fall08/208Tuesday, Sep. 2 Phy208 Lecture 12Your Professor and TAsQuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.TA consultation hours are in 2331 Chamberlin. Hours will be posted.Tuesday, Sep. 2 Phy208 Lecture 13LecturesTuesday, Thursday 12:05-12:55 2103 ChamberlinIncludesDemonstrationsExamplesQuick questionsHonors sectionFriday 12:05-12:55, 2103 ChamberlinFirst meeting Sep. 12 (next week)Required for those signed up for honors(B or better course grade also required for honors)Tuesday, Sep. 2 Phy208 Lecture 14Discussions and LabsDiscussions Monday and Wednesday (begin tomorrow, Sep. 3) Weekly Q&A, group problem, quiz.Labs - start next week (3254 Chamberlin). - No lab notebook: question sheets distributed. - 10 of 11 labs must be completed to pass the course. - Complete only 10 -> lab grade reduced by 30%.•If you must miss a lab:•Immediately contact your TA•Try to arrange for a different lab section that week•If not possible, lab missed (for a good reason) can be made up only during the immediately following exam week.Tuesday, Sep. 2 Phy208 Lecture 15 Homework: (Mastering Physics)Should be able to use your login from Physics 207–Follow email instructions for registering •Assigned each Thursday, due 8 days later Friday at midnight. •First assignment: HW1 assigned Thurs, Sep. 4 due Friday, Sep. 12 at midnightText: (Same as Physics 207 last semester)Tuesday, Sep. 2 Phy208 Lecture 16ExamsExam 1: Mon. Sep. 29 5:30-7 pm Exam 2: Tue. Oct. 28 5:30-7 pm Exam 3: Tue. Nov. 25 5:30-7 pm Cumulative Final Exam: Mon. Dec 15, 10:05 am -12:05 pmExams make up most of your grade (70%)Three midterm exams, 1 hr 30 min eachTypically 5-6 problems (old exams at web site)Multiple choiceShort answerLonger calculationTuesday, Sep. 2 Phy208 Lecture 17GradingWeight CommentsLabs10% 10 of the 11 labs required to pass the courseLab grade reduced 30% if only 10 labs completedDiscussion10%Mon & WedHomework10%Mastering PhysicsExams:MTE1MTE2MTE3Final15 %15%15%25%1:30 hrs1:30 hrs1:30 hrs2:00 hrsTuesday, Sep. 2 Phy208 Lecture 18Physics Learning Center•Structured small group sessions•Review sessions•Practice problems•Focus on concepts•Opportunity to meet study partners•Room 2338 Chamberlin•Contact Larry WatsonRoom [email protected], Sep. 2 Phy208 Lecture 19Last semester: mechanical wavesExamples Sound wavesWater wavesEarthquake wavesMedium requiredWave describes displacement of mediumThis semester: electromagnetic wavesExamples Light wavesRadio wavesMicrowavesX-raysNo medium requiredCan propagate in vacuumWave describes local electric and magnetic fieldsTuesday, Sep. 2 Phy208 Lecture 110Common wave propertiesReflectionSuperpositionTuesday, Sep. 2 Phy208 Lecture 111Wave properties•Wave has both space and time dependenceShows constant time intervalsTuesday, Sep. 2 Phy208 Lecture 112crestPeriodic wave•Repeats continuously, moves in space•Wavelength : distance to complete one cycle•Period T: time to complete one cycle•Frequency f: 1/Period trough€ Asin2πλx −2πTt ⎛ ⎝ ⎜ ⎞ ⎠ ⎟Mathematically:Tuesday, Sep. 2 Phy208 Lecture 113Moving wave crests•Period = 1 second•Propagation speed = vTuesday, Sep. 2 Phy208 Lecture 114Same period, slower wave velocity•Period = 1 second•Propagation speed = v/2Tuesday, Sep. 2 Phy208 Lecture 115Freq., wavelength, velocity are related•Source emits a crest once every period T.•This crest propagates at velocity v.•By the time the source emits another crest T seconds later, the first crest has moved vT.•So the distance between crests is vT.–This is the wavelength of the wave•Wavelength = velocity periodTuesday, Sep. 2 Phy208 Lecture 116Wave relations•Velocity = Wavelength / Period•v = / T, or v = f•f = Frequency = 1 / Period = 1/T€ Asin2πλx −2πTt ⎛ ⎝ ⎜ ⎞ ⎠ ⎟Sinusoidal traveling wave:€ Asin kx −ωt( )or€ k = 2π /λ ω = 2πf = 2π /TTuesday, Sep. 2 Phy208 Lecture 117Interference•Water drop is a source of circular waves (two-dimensions here)•When the waves overlap, they superimpose.•In some areas they cancel, in others they reinforce.•This is called interferenceTuesday, Sep. 2 Phy208 Lecture 118Destructive Interference in a String•Two pulses are traveling in opposite directions•Net displacement = sum of individual displacements•Pulses unchanged after interferenceTuesday, Sep. 2 Phy208 Lecture 119Destructive interference in a continuous wave•Two waves, a and b, have the same amplitude and frequency•They are 1/2 wavelength out of phase•When they combine, the waveforms cancel+=Tuesday, Sep. 2 Phy208 Lecture 120Setting up destructive interference•Sound takes two paths to arrive at ear•One path 1/2 wavelength longer than other•Results in destructive interferenceDestructiveDestructive interference for frequencies such that path length difference is 1/2 wavelength.Tuesday, Sep. 2 Phy208 Lecture 121QuestionTwo speakers each emit a 340 Hz sound wave. At what speaker separation would the sound from two speakers cancel? (sound velocity = 340 m/s)A. 0.1 mB. 0.25 mC. 0.5 mD. 1.0 mE. 2.0 mTuesday, Sep. 2 Phy208 Lecture 122Interference of 2 speakerscresttroughconstructive interference,loud tonedestructive interferencequit
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