From last time…Interference of sound wavesSuperimposing sound wavesInterference of 2 speakersInterference engineeringDoppler EffectDoppler Effect for a moving sourceShock Waves and Sonic BoomsSonic BoomBreaking the sound barrierBreaking the ‘sound’ barrier in a canoe!ResonanceMost objects resonateClosed tube resonanceResonance on stringWine glass resonancesDriving at resonanceTacoma Narrows BridgeElectricity and MagnetismElectrical ChargeElectrical chargePositive and negative seperatedSeparating chargeCharge by conduction (touching)Interactions between chargesForce between chargesElectrostatic force is strongMagnetism: Permanent magnetsField lines of a magnetThe Earth is a Magnet!Phy107 Fall 20061Wavelength, frequency, and velocity are all related.Waves can add up, either giving a wave of larger amplitude, or one of smaller amplitude.HW#4: Chapter 8: Conceptual: # 19, Problems: # 2, 7 Chapter 9: Conceptual: # 4, 10 Problems: # 2, 4, 8From last time…Phy107 Fall 20062Interference of sound waves•Interference arises when waves change their ‘phase relationship’. •Can vary phase relationship of two waves by changing physical location of speaker.ConstructiveDestructive‘in-phase’‘1/2 phase diff’Phy107 Fall 20063Superimposing sound waves•Depending on your relative distance from two identical sound sources, the sound intensity can vary.•Important aspect is relative distance from each source in wavelengths!Destructive interference for 1 half wavelength, also for3 half wavelengths, 5 half-wavelengths, etc.Constructive interference also occurs at differences of 2 whole wavelengths, 3 whole wavelengthsPhy107 Fall 20064Interference of 2 speakerscresttroughconstructive interference,loud tonedestructive interferencequit tonePhy107 Fall 20065Interference engineeringPhy107 Fall 20066Doppler Effect•A Doppler effect is experienced whenever there is relative motion between a source of waves and an observer.•For instance, a fire engine or train passing you.–When the source and the observer are moving toward each other, the observer hears a higher frequency–When the source and the observer are moving away from each other, the observer hears a lower frequency•Although the Doppler Effect is commonly experienced with sound waves, it is a phenomena common to all wavesPhy107 Fall 20067Doppler Effect for a moving source•As the source moves toward the observer (A), the wavelength appears shorter and the frequency increases•As the source moves away from the observer (B), the wavelength appears longer and the frequency appears to be lowerPhy107 Fall 20068Shock Waves and Sonic Booms•A shock wave results when the source velocity exceeds the speed of the wave itself•The circles represent the wave fronts emitted by the sourcePhy107 Fall 20069Sonic Boom•Source of sound approaching the listener is equal to or faster than the speed of sound•Each successive wave is superimposed on the previous one•Shock wave results as air compression in crest gets very largePhy107 Fall 200610Breaking the sound barrier•No sound received till after the source passes the listener - then a sonic boom - followed by normal sound from the source•Conical bow wake from condensed water vapor at high pressure shock wave front.QuickTime™ and aYUV420 codec decompressorare needed to see this picture.Phy107 Fall 200611Breaking the ‘sound’ barrier in a canoe!If the canoe moves faster than the water wave velocity, shock wave also builds up where all the crests line up.For water wave velocity ~1 m/s, so Mach 2 is 2 m/s= 4.5 mph !!Phy107 Fall 200612Resonance•So far have been talking about waves traveling in media that extend in all directions.•In a finite object, the boundaries cause reflections.•The reflected wave interferes with rest of wave,causing destructive or constructive interference.•For destructive interference,the wave tends to die away. •But for constructive interference, the wave builds up.•Which one happens depends on wavelength.Phy107 Fall 200613Most objects resonate•But even complicated objects havesome natural frequency of oscillation•Pendulum•Wine glass•Musical instruments•Natural frequency has to do with size and materials properties of object.Phy107 Fall 200614Closed tube resonancePhy107 Fall 200615Resonance on string•First three natural vibrational modes of a string fixed at both ends (e.g. a guitar string).•A normal pluck excites primarily the first vibrational mode.Phy107 Fall 200616Wine glass resonancesHolographic interferometry showing contour map of vibration for different modes. Points of maximum motion appear as bull’s eyes.Phy107 Fall 200617Driving at resonance•Can tune a speaker to the fundamental resonant frequency of the wine glass (here 1210 Hz).•More and more energy poured into glass - the glass vibrates with larger and larger amplitude.•The glass shatters as the vibration amplitude becomes too large.Stroboscopic movie of fundamental vibration mode of a wineglass.QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.Phy107 Fall 200618Tacoma Narrows Bridge•Even a non-resonant drive can transfer energy.•Driven by 40 mph wind•Causes vibration of bridge at its natural (resonant) frequency.Movie of bridge torsional vibrationsQuickTime™ and aYUV420 codec decompressorare needed to see this picture.Phy107 Fall 200619Electricity and Magnetism•Electric charge and electric forces•Magnetic forces•Unification of electric and magnetic forces–Understanding how they combine together–Electromagnetic wavesPhy107 Fall 200620Electrical Charge•Charge: intrinsic property of matter•Two types: –Positive Charge: Protons–Negative Charge: electrons–Opposites Attract! (likes repel)•Atoms are neutral–Positively charged central nucleus r~10-15 m–Negatively charged electrons orbit r~10-10 m•Charge is quantized(one electron or proton)Phy107 Fall 200621Electrical charge•Electrons carry electrical charge, and can be moved from one material to another.•The electrons have a negative charge.•The unit of electric charge is the Coulomb•One electron carries only a tiny amount of chargeCharge on 1 electron = 1.6 x 10-19 CoulombTransferring 1 Coulomb of charge means that 6,250,000,000,000,000,000 electrons have moved!Phy107 Fall 200622Positive and negative seperated•Triboelectric – friction •Conduction – contact•Induction–Proximity/ground•PolarizationPhy107 Fall
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