1. HNRT 227 Fall 2015 Chapter 6 Wave Motion, Sound and Electromagnetism 17 September 2015 presented by Prof. Geller

2. Recall from Chapters 1-5 zUnits of length, mass and time, and metric Prefixes zDensity and its units zThe Scientific Method zSpeed, velocity, acceleration zForces zFalling objects, Newton’s Laws of Motion and Gravity zWork, Potential Energy and Kinetic Energy zConservation of Energy, Types/Sources of Energy zKinetic Molecular Theory, Temperature and Heat zPhases of matter and Thermodynamics zElectricity zMagnetism

3. Forces and Vibrations zVibration yback and forth motion zAmplitude yextent of displacement from the equilibrium position zCycle yone complete vibration zPeriod ytime required to complete one cycle zFrequency ynumber of cycles per second zRelationship between period and frequency yT = 1 / ff = 1 / T

4. Waves zLongitudinal ydisturbance that causes particles to move closer together or farther apart IN THE SAME DIRECTION the wave is moving zTransverse ydisturbance that causes motion PERPENDICULAR to the direction that the wave is moving zIn general, liquids carry longitudinal waves but not transverse waves yTransverse waves, such as water waves, dissipate all their energy at the phase interface zWe use same terms as vibrations, for waves  v =  * f

5. Sound Waves zSound does not travel in a vacuum zSound moves through solids faster than any gas zVelocity of sound is effected by composition and temperature of gas

6. Reflection, Refraction and Interference zReflection ywaves bouncing back off of a boundary zRefraction ychange in direction of wave crossing a boundary zInterference yinteraction of waves xdestructive interference xconstructive interference

7. Resonance zNatural frequency yfrequency of vibration determined by the object’s composition and shape zResonance ywhen frequency of external force matches natural frequency

8. Doppler Effect and Sonic Boom zDoppler Effect yApparent change in frequency of a wave caused by the relative motion of the source or observer xpitch of train approaching, departing zSonic boom yshock wave caused by object moving at speed of sound or faster

9. Question for Thought zWhat is a wave? zA wave is a disturbance that moves through a medium such as a solid or the air.

10. Question for Thought zA piano tuner hears three beats per second when a tuning fork and a note are sounded together and six beats per second after the string is tightened. What should the tuner do next? yATighten the string. yBLoosen the string? zLoosen. Since the beat frequency depends upon the difference between the two frequencies, you wish to go in the direction of fewer beats per second.

11. Question for Thought zDo astronauts on the Moon have to communicate by radio even when close to one another? yAYes yBNo zYes, because there is no medium such as air to transmit sound on the moon.

12. Question for Thought zWhat is resonance? zThe condition where the frequency of an external force matches the frequency of an object is resonance.

13. Question for Thought zDoes sound travel faster in warm air than in cool air? yATrue yBFalse zTrue. Gas molecules have a greater kinetic energy and move faster in warm air than in cold air. These molecules are able to transfer an impulse from one molecule to the next faster.

14. Question for Thought zDo all frequencies of sound travel with the same velocity? yAYes yBNo zLonger wavelengths have lower frequencies. Since the velocity of sound is equal to the product of the frequency times the wavelength, the velocity is a constant.

15. Question for Thought zWhat eventually happens to a sound wave traveling through the air? zThe energy of the sound wave is eventually dissipated into heat.

16. Question for Thought zWhat gives a musical note its characteristic quality? zThe presence and strength of various overtones determine the characteristic sound of a musical note.

17. Question for Thought zDoes a supersonic aircraft make a sonic boom only when it cracks the sound barrier? yATrue yBFalse zThe sonic boom is from the building up of a pressure wave in front of the moving aircraft. Since this pressure wave is present as long as the plane is moving faster than the speed of sound, the aircraft continually makes a sonic boom.

18. Question for Thought zWhat is an echo? zAn echo is the return of a sound wave to its source after the wave has been reflected.

19. Question for Thought zWhy are fundamental frequencies and overtones also called resonant frequencies? zThey all produce standing waves or resonance in whatever is oscillating.

20. Question zThe distance between the center of a condensation and the center of an adjacent rarefaction is 65.23 cm. If the frequency is 256.0 Hz, how fast are these waves moving? yA171 cm/s yB171 m/s yC334 cm/s yD334 m/s yE686 cm/s

21. Answer 334

22. Question zA warning buoy is observed to rise every 5.0 seconds as crests of waves pass by it. What is the period of these waves? xA10 seconds xB20 seconds xC5 seconds xD2.5 seconds xE5 Hertz zWhat is the frequency? xA2 Hertz xB0.2 Hertz xC5 Hertz xD0.5 Hertz xE10 Hertz

23. Answer

24. Question zSound from the siren of an emergency vehicle has a frequency of 750 Hz and moves with a velocity of 343 meters per second. What is the distance from one condensation to the next? yA0.23 m yB0.46 m yC0.69 m yD0.92 m yE1.15 m

25. Answer

26. Electromagnetism zMagnetic Fields ygenerated by electric current zEnergy conversion yelectric motors yelectric generators yspeakers zMaxwell’s Equations ysummary of electromagnetic laws and interactions…

27. iClicker Question zThe presence of a uniform magnetic field may be detected by using a yAstationary charge yBsmall mass yCbeam of neutrons yDmagnetic compass

28. Question for Thought zExplain why a balloon that has been rubbed sticks to a wall for a while. zThe balloon has a net charge as a result of being rubbed. When the balloon is brought near a wall, the net charge on the balloon moves electrons around in the wall. As a result, a small region near the balloon has a net charge of opposite sign than the balloon. The overall wall is still electrically neutral; there are now small regions that have net charges. The force from the opposite signed charges in the balloon and the wall causes the balloon to stick to the wall. There it will stay until enough charge has leaked away to cancel the charge on the balloon.

29. Question for Thought zExplain what is happening when you walk across a carpet and receive a shock when you touch a metal object. zExcess charge is building up on your body from the carpet as you walk across it. When a metal object is touched, the charge flows out of your body, through the lower resistance of the metal. It finds a path into the ground, which supplied the charge to make up for what you removed from the carpet.

30. Question for Thought zWhy does a positively or negatively charged object have multiples of the fundamental charge? zAn electron carries a negative charge and can be moved to and from objects relatively easy. Since electrons cannot be divided into parts that can move separately, the smallest charge it is possible to have or to move is the charge of one electron. The charge of one electron is sometimes referred to as the fundamental charge.

31. Question for Thought zExplain how you know that it is an electric field, not electrons, that moves rapidly through a circuit. zThe electrons move rapidly inside a wire bouncing against each other like molecules in a gas. Since so many collisions occur, an individual electron cannot move from one end of a wire to another rapidly. The electric field inside the wire, which exerts a force on the electrons, can move rapidly though the wire because it does not require something to carry it. The force from the electric field gives the electrons a drift velocity.

32. Question for Thought zA kWhr is a unit of yApower yBenergy yCwork yDboth B and C above zA kWhr is work multiplied by time. Since a watt is energy per time, a kWhr is a unit of energy or work.

33. Question for Thought zWhat is the difference between AC and DC? zIn direct current (DC), the current always flows in a single direction. In alternating current (AC), the flow of current changes direction with a regular frequency.

34. Question for Thought zWhat is a magnetic pole? How are magnetic poles named? zA magnetic pole is a region where the force of magnetic attraction seems to be concentrated. The pole that seeks, or points to a generally north direction, is called a north pole, and the other pole is called a south pole.

35. Question for Thought zHow is an unmagnetized piece of iron different from the same piece of iron when it is magnetized? zIn an unmagnetized piece of iron, the magnetic domains are pointing in random directions such that the net field is zero. In a magnetized piece of iron, most of the domains are aligned so that their fields add to make a larger field.

36. Question for Thought zExplain why the electric utility company increases the voltage of electricity for long- distance transmission. zIf the voltage is small, the current is large for a particular amount of power. Increasing the voltage decreases the current. Large currents promote many collisions of electrons inside the wire with other electrons and positive ions. Each collision takes energy from the electric field, diverting it into kinetic energy of the positive ions and heating the wire, so there are fewer power losses with lower currents. Thus a higher voltage means less power loss since the current is lower.

37. Question for Thought zDescribe how an electric generator is able to generate an electric current. zThe electromagnetic generator uses induction to generate a current in loops of wire moving in a magnetic field. Electrons in the loops of wire are forced toward one end by the magnetic field, which sets up a potential difference.

38. Question for Thought zWhy does the north pole of a magnet point to the geographic North Pole if like poles repel? zThe earth's north magnetic pole is actually a magnetic south pole located near the geographic North Pole.

39. Question for Thought zExplain what causes an electron to move toward one end of a wire when the wire is moved across a magnetic field. zThe electron is moving, creating its own magnetic field. The interaction between the magnetic field of the electron and the external magnetic field creates a force on the electron, causing it to move.

40. Question zWhat is the force between two balloons with a negative charge of 1.6 x 10 -10 C if the balloons are 5.0 cm apart?

41. Answer

42. Question  What is the voltage across a 60.0  resistor with a current of 3.33 amps?

43. Answer

44. Question  A 10.0  lightbulb is connected to a 12.0 Volt battery. What is the current flowing through the bulb? What is the power of the bulb?

45. Answer

46. Waves zTypes of waves ylongitudinal xe.g. sound ytransverse xe.g. electromagnetic waves zVelocity, frequency and wavelength ywave velocity = wavelength times frequency xwatch your units

47. Electromagnetism zElectricity according to Gauss yrelates electricity to electric charge zFaraday’s Law yrelates electric fields to magnetic fields zMagnetism according to Gauss yrelates magnetism to electricity

48. Maxwell’s Equations zAmpere-Maxwell Law yrelates magnetic field to electricity zMaxwell yunifies electricity and magnetism into electromagnetism

49. Electromagnetic Interactions zTransmission vs. opacity zAbsorption vs. emission zScattering yrefraction yreflection ydiffraction yinterference

50. Electromagnetic Spectrum zVisible yRed (~7000 A or 700 nm) yOrange, Yellow, Green, Blue, Indigo yViolet (~4000 A or 400 nm) zMore than meets the eye yradio, microwave, infrared, ROYGBIV (visible), ultraviolet, X-rays, gamma rays xfrom lowest energy to highest energy xfrom longest to shortest wavelength xfrom lowest to highest frequency

51. iClicker Question zWhich of the following groups have electromagnetic wavelengths, all of which are shorter than visible light: zAultraviolet, microwave, radio zBultraviolet, x-ray, gamma ray zCinfrared, microwave, radio zDall of the above have wavelengths shorter than visible light zEnone of the above have wavelengths with all shorter than visible light

52. Reflection and Refraction (not all in text) zReflection ythe angle of incidence is equal to the angle of reflection   i =  r zVirtual image ylight rays appear to originate from zReal image ylight rays really do meet here zRefraction ychange of direction of light yn = c / v [defines index of refraction]

53. Diffraction, Interference and Polarization (not all in text) zDiffraction ylight rays appear to bend around the edge of an object zInterference ylight rays interacting with other light rays causing reinforcement or canceling or some combination of the two zPolarization yvibrates/oscillates in a single plane

54. Doppler Shift zA change in measured frequency caused by the motion of the observer or the source yclassical example of pitch of train coming towards you and moving away ywrt light it is either red-shifted (away) or blue- shifted (towards)

55. Question for Thought zWhat determines if an electromagnetic wave emitted from an object is a visible light wave or a wave of infrared radiation? zThe frequency of the wave. Alternately, the wavelength of the wave.

56. Question for Thought zWhat carries more energy, red light or blue light? yA Red LightB Blue light zShould this mean anything about the preferred color of warning and stop lights? Explain. zBlue light carries more energy. No it shouldn’t have anything to do with preferred colors of warning lights because the energy difference between these two colors is very, very small. The number of photons determines the intensity of the light.

57. Question for Thought zWhat happens to light that is absorbed by matter? zThe energy in the light is transferred to the absorbing material.

58. Question for Thought zOne star is reddish and the other is bluish. Do you know anything about the relative temperatures of the two stars? Explain. zThe bluish star is at a higher temperature because higher temperature objects emit more photons of shorter wavelengths (and higher energies) than objects with lower temperatures.

59. Question for Thought zWhy does a highway sometimes appear wet on a hot summer day when it is not wet? zThe hot air above the surface of the highway has a lower index of refraction than the air above it, so light striking the warmer air is refracted upward. This light is interpreted by your brain to be reflected light.

60. Question for Thought zHow can you tell if a pair of sunglasses is polarizing or not? zLook at the clear sky at an angle of about 90° from the sun. The scattered light from this direction is partially polarized, so if the sky appears to darken as the glasses are turned, the glasses are polarized. If you have a pair of polarizing sunglasses, turn a lens of the unknown pair over a stationary lens of the known, polarizing pair. If the unknown pair is polarizing, light coming through the lens will appear to darken then brighten.

61. Question for Thought zWhat conditions are necessary for two light waves to form an interference pattern of bright lines and dark areas? zTwo light beams from a single source striking a card with two small parallel slits in phase with each other are necessary. Light is diffracted through these slits, landing on a screen. Regions where the diffracted light from both of the slits is in phase have a bright line. Regions where the diffracted light from one slit is out of phase with the light from the other slit have a dark line.

62. Question for Thought zExplain why the intensity of reflected light appears to change if you tilt your head from side to side while wearing polarizing sunglasses. zReflected light is slightly polarized. When the polarization of the reflected light is parallel with the polarizing sunglasses, it appears brighter. When the polarization of the light is perpendicular with the sunglasses it appears darker.

63. Question for Thought zWhy do astronauts in orbit around Earth see a black sky with stars that do not twinkle but see a blue Earth? zThere is no atmosphere to suspend particles above the astronauts to scatter the light from the sun, so they see a black sky. The stars do not twinkle because there is no atmospheric turbulence above them to refract the light in various ways. The earth appears blue because light of that frequency is being scattered the most by the atmosphere below them.

64. Question zHow much time is required for reflected sunlight to travel from the Moon to Earth if the distance between Earth and the Moon is 3.85 x 10 5 kilometers? yA7.8 x 10 2 s yB1.28 x 10 3 s yC1.28 x 10 -3 s yD7.8 x 10 -4 s yE7.8 x 10 3 s

65. Answer t = d / v

66. Question zThe wavelength of light from a monochromatic source is measured to be 6.80 x 10 -7 meters. What is the frequency of the light? What is the color that you would observe?

67. Answer