1Phy107 Fall 20061ExamHour Exam 2: Wednesday, October 25th• In-class, covering waves, electromagnetism, andrelativity• Twenty multiple-choice questions• Will cover: Chapters 8, 9 10 and 11Lecture material• You should bring– 1 page notes, written single sided– #2 Pencil and a Calculator– Review Monday October 23rd– Review test will be available online on Monday(see exams page)Phy107 Fall 20062Topics covered in Exam 2• Waves, interference, resonance, andelectromagnetism• EM radiation, light, color• Special Relativity time &space• Special Rel. mass & energy• General RelativityPhy107 Fall 20063Wave properties• Amplitude is the maximum displacement ofstring above the equilibrium position• Wavelength, , is the distance between twosuccessive points that behave identically• Period: time required to complete one cycle• Frequency = 1/Period = rate at which cyclesare completed• Velocity = Wavelength/Period,v = / T, or v = fPhy107 Fall 20064Types of wave motion• Longitudinal wave– Vibrations are in the direction of motion• Transverse wave– Vibrations are perpendicular to the direction ofmotion.Phy107 Fall 20065Waves QuestionsWhen both transverse and longitudinal waves arepresent (such as s and p seismic waves in the Earth),the longitudinal wave usually travels faster.If the longitudinal wave travels about twice as fast asthe transverse wave of the same frequency, how aretheir wavelengths related?A. Longitudinal wavelength twice as longB. Longitudinal wavelength half as longC. Both same.Phy107 Fall 20066Interference of sound waves• Interference arises when waves change their‘phase relationship’.• Can vary phase relationship of two waves bychanging physical location of speaker.ConstructiveDestructive‘in-phase’‘1/2 phase diff’2Phy107 Fall 20067Interference of 2 speakerscresttroughconstructiveinterference,loud tonedestructiveinterferencequit tonePhy107 Fall 20068Interference questionYou are standing at a point where the signals from tworadio antennas cancel exactly.The towers broadcast at 1000 kHz.You walk around with a constant distance from tower 1.How much closer to tower 2 do you need to go to getfull constructive interference (strong radio signal)?Speed of sound: 340m/sA. 300 metersB. 150 metersC. 75 metersPhy107 Fall 20069Interference summary• Important quantity is distance differencein number of wavelengths.• A distance difference of a half wavelengthleads to destructive interference.• Whole wavelength differenceslead to constructive interference.But destructive interference also for3 half wavelengths, 5 half-wavelengths, etc.Constructive interference also occurs at differences of2 whole wavelengths, 3 whole wavelengthsPhy107 Fall 200610Doppler Effect• As the source movestoward the observer (A),the wavelength appearsshorter and the frequencyincreases• As the source moves awayfrom the observer (B),the wavelength appearslonger and the frequencyappears to be lowerPhy107 Fall 200611Most objects resonate,Closed tubePhy107 Fall 200612Resonance on string• First three natural vibrational modes ofa string fixed at both ends (e.g. aguitar string).• A normal pluck excites primarily thefirst vibrational mode.3Phy107 Fall 200613Force between chargesOpposite charges attractLike charges repel.• Other than the polarity, they interact much likemasses interact gravitationally.• Force is along the line joining the particles.Gravitational force: FG=GM1M2/ r2G=6.7x10-11 Nm2/kg2Electrostatic force: FE = k Q1 Q2 /r2k = 9x109 Nm2/C2+—Force on positive particle dueto negative particleCharge on 1 electron or proton= 1.6 x 10-19 CoulombPhy107 Fall 200614Coulomb force questionIf the distance between two positive charges isincreased by a factor of two, the forcebetween themA. Increases by a factor of 2B. Decreases by factor of 2C. Decreases by a factor of 4Phy107 Fall 200615Magnetic forces•North Pole and South Pole• This is the elementarymagnetic particle• Called magnetic dipole(North poleand south pole)• There are no magnetic‘monoples’ (magnetic charges)NSNSNSSNUnlikesattractLikesrepelPhy107 Fall 200616Source of magnetic field• Current in wire produces magnetic field.• That magnetic field aligns compass needleCurrentMagneticfieldPhy107 Fall 200617The electric force and field++Force on this charge……due to this charge+++Q1Q2F =kq1q2r2E =kQr2•Charge q1 can exert a forceon any number of charges.Would like to understand justthe part from q1.• Similar relationship betweenwork and VoltageF = EqPhy107 Fall 200618Magnetic Force• What does the magnetic force act on?– Electric field is from a charge and exerts a forceon other charges– Magnetic field is from a moving charge and exertsa force on other moving charges!• Magnetic field B• Magnetic force F = qvB– F perpendicular to both v and B4Phy107 Fall 200619Faraday’s law of inductionand Lenz’s Law• A changing(moving) magnetic field causes acurrent in a metal. However, electric fields arewhat causes electrons to move in a metal• Changing magnetic fields produce electric fields• The current produces a magnetic field,which repels the bar magnetPhy107 Fall 200620Amperes Law and Light• Finally: Changing electric fields cause magnetic fields!• Electric fields are from charges• Magnetic fields are from moving charges• Changing Magnetic fields cause Electric fields• Changing Electric fields cause Magnetic fields• All this was expressed in Maxwell’s equations• Maxwell and others realized that a changingmagnetic/electric field could cause a changing magnetic/electric field. The condition for one to cause the otherand vice-versa was for the two to change in a sin wavepattern and move at the velocity of light!Phy107 Fall 200621Properties of EM Waves• Light is a set of electric and magnetic fields where thechanging electric field creates the magnetic field and thechanging magnetic field creates the electric field• Only works when the fields change from up to down andback again at the speed of light• The speed of light is a special value - we’ll see this again inEinstein's relativity.• Has all properties of a wave:wavelength, frequency, interference…• Can be generated by spark jumping a gap.v =fc =Phy107 Fall 200622Types of EMwavesWe arefamiliarwith manydifferentwavelengthsof EM wavesAll are thesamephenomenaPhy107 Fall 200623EM wave questionAs an EM wave travels through empty space, itsspeed can be increased byA.
View Full Document
Unlocking...