HNRT 227 Fall 2015 Chapter 6

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

2. Recall from Chapters 1-5 • Units of length, mass and time, and metric Prefixes • Density and its units • The Scientific Method • Speed, velocity, acceleration • Forces • Falling objects, Newton’s Laws of Motion and Gravity • Work, Potential Energy and Kinetic Energy • Conservation of Energy, Types/Sources of Energy • Kinetic Molecular Theory, Temperature and Heat • Phases of matter and Thermodynamics • Electricity • Magnetism

3. Forces and Vibrations • Vibration • back and forth motion • Amplitude • extent of displacement from the equilibrium position • Cycle • one complete vibration • Period • time required to complete one cycle • Frequency • number of cycles per second • Relationship between period and frequency • T = 1 / f f = 1 / T

4. Waves • Longitudinal • disturbance that causes particles to move closer together or farther apart IN THE SAME DIRECTION the wave is moving • Transverse • disturbance that causes motion PERPENDICULAR to the direction that the wave is moving • In general, liquids carry longitudinal waves but not transverse waves • Transverse waves, such as water waves, dissipate all their energy at the phase interface • We use same terms as vibrations, for waves • v = l * f

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

6. Reflection, Refraction and Interference • Reflection • waves bouncing back off of a boundary • Refraction • change in direction of wave crossing a boundary • Interference • interaction of waves • destructive interference • constructive interference

7. Resonance • Natural frequency • frequency of vibration determined by the object’s composition and shape • Resonance • when frequency of external force matches natural frequency

8. Doppler Effect and Sonic Boom • Doppler Effect • Apparent change in frequency of a wave caused by the relative motion of the source or observer • pitch of train approaching, departing • Sonic boom • shock wave caused by object moving at speed of sound or faster

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

10. Question for Thought • A 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? • A Tighten the string. • B Loosen the string? • Loosen. 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 • Do astronauts on the Moon have to communicate by radio even when close to one another? • A Yes • B No • Yes, because there is no medium such as air to transmit sound on the moon.

12. Question for Thought • What is resonance? • The condition where the frequency of an external force matches the frequency of an object is resonance.

13. Question for Thought • Does sound travel faster in warm air than in cool air? • A True • B False • True. 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 • Do all frequencies of sound travel with the same velocity? • A Yes • B No • Longer 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 • What eventually happens to a sound wave traveling through the air? • The energy of the sound wave is eventually dissipated into heat.

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

17. Question for Thought • Does a supersonic aircraft make a sonic boom only when it cracks the sound barrier? • A True • B False • The 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 • What is an echo? • An echo is the return of a sound wave to its source after the wave has been reflected.

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

20. Question • The 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? • A 171 cm/s • B 171 m/s • C 334 cm/s • D 334 m/s • E 686 cm/s

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22. Question • A warning buoy is observed to rise every 5.0 seconds as crests of waves pass by it. What is the period of these waves? • A 10 seconds • B 20 seconds • C 5 seconds • D 2.5 seconds • E 5 Hertz • What is the frequency? • A 2 Hertz • B 0.2 Hertz • C 5 Hertz • D 0.5 Hertz • E 10 Hertz

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24. Question • Sound 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? • A 0.23 m • B 0.46 m • C 0.69 m • D 0.92 m • E 1.15 m

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26. Electromagnetism • Magnetic Fields • generated by electric current • Energy conversion • electric motors • electric generators • speakers • Maxwell’s Equations • summary of electromagnetic laws and interactions…

27. iClicker Question • The presence of a uniform magnetic field may be detected by using a • A stationary charge • B small mass • C beam of neutrons • D magnetic compass

28. Question for Thought • Explain why a balloon that has been rubbed sticks to a wall for a while. • The 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 • Explain what is happening when you walk across a carpet and receive a shock when you touch a metal object. • Excess 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 • Why does a positively or negatively charged object have multiples of the fundamental charge? • An 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 • Explain how you know that it is an electric field, not electrons, that moves rapidly through a circuit. • The 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 • A kWhr is a unit of • A power • B energy • C work • D both B and C above • A 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 • What is the difference between AC and DC? • In 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 • What is a magnetic pole? How are magnetic poles named? • A 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 • How is an unmagnetized piece of iron different from the same piece of iron when it is magnetized? • In 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 • Explain why the electric utility company increases the voltage of electricity for long-distance transmission. • If 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 • Describe how an electric generator is able to generate an electric current. • The 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 • Why does the north pole of a magnet point to the geographic North Pole if like poles repel? • The earth's north magnetic pole is actually a magnetic south pole located near the geographic North Pole.

39. Question for Thought • Explain what causes an electron to move toward one end of a wire when the wire is moved across a magnetic field. • The 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 • What 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?

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

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

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46. Waves • Types of waves • longitudinal • e.g. sound • transverse • e.g. electromagnetic waves • Velocity, frequency and wavelength • wave velocity = wavelength times frequency • watch your units

47. Electromagnetism • Electricity according to Gauss • relates electricity to electric charge • Faraday’s Law • relates electric fields to magnetic fields • Magnetism according to Gauss • relates magnetism to electricity

48. Maxwell’s Equations • Ampere-Maxwell Law • relates magnetic field to electricity • Maxwell • unifies electricity and magnetism into electromagnetism

49. Electromagnetic Interactions • Transmission vs. opacity • Absorption vs. emission • Scattering • refraction • reflection • diffraction • interference

50. Electromagnetic Spectrum • Visible • Red (~7000 A or 700 nm) • Orange, Yellow, Green, Blue, Indigo • Violet (~4000 A or 400 nm) • More than meets the eye • radio, microwave, infrared, ROYGBIV (visible), ultraviolet, X-rays, gamma rays • from lowest energy to highest energy • from longest to shortest wavelength • from lowest to highest frequency