1. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley. Chapter 7: HEAT TRANSFER AND CHANGE OF PHASE

2. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley This lecture will help you understand: Conduction Convection Radiation Newton’s Law of Cooling Global Warming and the Greenhouse Effect Heat Transfer and Change of Phase Boiling Melting and Freezing Energy and Change of Phase

3. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Heat Transfer Processes of thermal energy transfer: Conduction Convection Radiation

4. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Conduction Transfer of internal energy by electron and molecular collisions within a substance

5. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Heat Transfer: Conduction Conduction occurs predominately in solids where the molecules remain in relatively restricted locations. When you stick a nail into ice, does cold flow from the ice to your hand, or heat from your hand to the ice?

6. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley If you hold one end of a metal bar against a piece of ice, the end in your hand will soon become cold. Does cold flow from the ice to your hand? A.Yes. B.In some cases, yes. C.No. D.In some cases, no. Heat Transfer: Conduction CHECK YOUR NEIGHBOR

7. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley If you hold one end of a metal bar against a piece of ice, the end in your hand will soon become cold. Does cold flow from the ice to your hand? A.Yes. B.In some cases, yes. C.No. D.In some cases, no. Explanation: Cold does not flow from the ice to your hand. Heat flows from your hand to the ice. The metal is cold to your touch, because you are transferring heat to the metal. Heat Transfer: Conduction CHECK YOUR ANSWER

8. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Conduction Insulation Doesn’t prevent the flow of internal energy Slows the rate at which internal energy flows Example: Rock wool or fiberglass between walls slows the transfer of internal energy from a warm house to a cool exterior in winter, and the reverse in summer

9. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Conduction Application Snow patterns on the roof of a house show areas of conduction and insulation. Bare parts show where heat from inside has conducted through the roof and melted the snow.

10. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley When thermal insulation, such as spun glass or rock wool, is placed beneath the roof of a house, then in cold weather the insulation will A.create heat to warm the house. B.keep the cold from coming through the roof. C.slow the flow of heat from inside the house to the outside. D.stop the flow of heat from inside the house to the outside. Energy Transfer CHECK YOUR NEIGHBOR

11. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Energy Transfer CHECK YOUR ANSWER When thermal insulation, such as spun glass or rock wool, is placed beneath the roof of a house, then in cold weather the insulation will A.create heat to warm the house. B.keep the cold from coming through the roof. C.slow the flow of heat from inside the house to the outside. D.stop the flow of heat from inside the house to the outside. Explanation: No insulation can stop heat flow. Insulation only slows it. (A fortune awaits the inventor who can come up with the “perfect” insulator!)

12. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Heat Transfer: Conduction Good conductors: Composed of atoms with “loose” outer electrons Known as poor insulators Examples—all metals to varying degrees Poor conductors: Delay the transfer of heat Known as good insulators Examples—wood, wool, straw, paper, Styrofoam, cork, liquid, gases, air, or materials with trapped air

13. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Conduction Dramatic example: Author John Suchocki walks barefoot without burning his feet on red-hot coals,due to poor conduction between the coals and his feet

14. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Convection Transfer of heat involving only bulk motion of fluids Examples: Visible shimmer of air above a hot stove or above asphalt on a hot day Visible shimmers in water due to temperature difference

15. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Convection Cooling by expansion Opposite to the warming that occurs when air is compressed Example: The “cloudy” region above hot steam issuing from the nozzle of a pressure cooker is cool to the touch (a combination of air expansion and mixing with cooler surrounding air). Careful, the part at the nozzle that you can’t see is steam—ouch!

16. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Convection Currents Convection currents produced by unequal heating of land and water. During the day, warm air above the land rises, and cooler air over the water moves in to replace it. At night, the direction of air flow is reversed.

17. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Convection Reason warm air rises Warm air expands, becomes less dense, and is buoyed upward Air rises until its density equals that of the surrounding air Example: Smoke from a campfire rises and blends with the surrounding cool air.

18. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Although warm air rises, why are mountaintops cold and snow covered, while the valleys below are relatively warm and green? A.Warm air cools when rising. B.There is a thick insulating blanket of air above valleys. C.Both of the above. D.None of the above. Heat Transfer: Convection CHECK YOUR NEIGHBOR

19. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Although warm air rises, why are mountaintops cold and snow covered, while the valleys below are relatively warm and green? A.Warm air cools when rising. B.There is a thick insulating blanket of air above valleys. C.Both of the above. D.None of the above. Explanation: Earth’s atmosphere acts as a blanket, which for one important thing, keeps Earth from freezing at nighttime. Heat Transfer: Convection CHECK YOUR ANSWER

20. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Radiation Transfer of energy via electromagnetic waves that can travel through empty space

21. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Heat Transfer: Radiation Wavelength of radiation is related to the frequency of vibration. Low - frequency vibrations  long waves High - frequency vibrations  short waves

22. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Radiation Emission of radiant energy Every object above absolute zero radiates From the Sun’s surface comes light, or solar radiation From the Earth’s surface is terrestrial radiation in the form of infrared waves below our threshold of sight

23. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Wave Frequency - Temperature (a) A low-temperature (cool) source emits primarily low-frequency, long wavelength waves. (b) A medium-temperature source emits primarily medium-frequency. (c) A high-temperature source emits primarily high-frequency, short wavelength waves.

24. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Radiation Emission of radiant energy Peak frequency of radiation is proportional to the absolute temperature of the source ( )

25. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Emission and Absorption The surface of any material both absorbs and emits radiant energy. When a surface absorbs more energy than it emits, it is a net absorber, and temperature tends to rise. When a surface emits more energy than it absorbs, it is a net emitter, and temperature tends to fall.

26. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Emission and Absorption Absorption of Radiant Energy: The ability of a material to absorb and radiate thermal energy is indicated by its color. Good absorbers and good emitters are dark in color. Poor absorbers and poor emitters are reflective or light in color.

27. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Emission and Absorption Whether a surface is a net absorber or net emitter depends on whether its temperature is above or below that of its surroundings. A surface hotter than its surroundings will be a net emitter and tends to cool. A surface colder than its surroundings will be a net absorber and tends to warm.

28. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley If a good absorber of radiant energy were a poor emitter, its temperature compared with its surroundings would be A.lower. B.higher. C.unaffected. D.None of the above. Emission and Absorption CHECK YOUR NEIGHBOR

29. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley If a good absorber of radiant energy were a poor emitter, its temperature compared with its surroundings would be A.lower. B.higher. C.unaffected. D.None of the above. Explanation: If a good absorber were not also a good emitter, there would be a net absorption of radiant energy, and the temperature of a good absorber would remain higher than the temperature of the surroundings. Nature is not so! Emission and Absorption CHECK YOUR ANSWER

30. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Radiation Reflection of radiant energy Darkness is often due to reflection of light back and forth many times partially absorbing with each reflection Good reflectors are poor absorbers

31. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Which is the better statement? A.A black object absorbs energy well. B.An object that absorbs energy well is black. C.Both say the same thing, so both are equivalent. D.Both are untrue. Absorption of Radiant Energy CHECK YOUR NEIGHBOR

32. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Which is the better statement? A.A black object absorbs energy well. B.An object that absorbs energy well is black. C.Both say the same thing, so both are equivalent. D.Both are untrue. Explanation: This is a cause-and-effect question. The color black doesn’t draw in and absorb energy. It’s the other way around—any object that does draw in and absorb energy, will, by consequence, be black in color. Absorption of Radiant Energy CHECK YOUR ANSWER

33. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Which of the following does NOT emit radiation? A.A lit fluorescent lamp. B.A lit incandescent lamp. C.A burned out incandescent lamp. D.None of the above. Emission of Radiant Energy CHECK YOUR NEIGHBOR

34. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Which of the following does NOT emit radiation? A.A lit fluorescent lamp. B.A lit incandescent lamp. C.A burned out incandescent lamp. D.None of the above. Explanation: Everything continually emits radiation—and everything continually absorbs radiation. When emission is greater than absorption, temperature of the emitter drops. When absorption is greater than emission, temperature increases. Everything is emitting and absorbing radiation continually. That’s right—everything! Emission of Radiant Energy CHECK YOUR ANSWER

35. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Newton’s Law of Cooling Approximately proportional to the temperature difference  T between the object and its surroundings In short: Rate of cooling ~  T Examples: Hot apple pie cools more quickly in a freezer than if left on the kitchen table Warmer house more quickly leaks thermal energy to the outside than a cooler house

36. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Newton’s Law of Cooling Newton’s Law of Cooling (continued) Applies to rate of warming –Object cooler than its surroundings warms up at a rate proportional to  T Example: Frozen food warm quicker in a warm room than in a cold room

37. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley It is commonly thought that a can of beverage will cool faster in the coldest part of a refrigerator. Knowledge of Newton’s law of cooling A.supports this knowledge. B.shows this knowledge is false. C.may or may not support this knowledge. D.may or may not contradict this knowledge. Newton’s Law of Cooling CHECK YOUR NEIGHBOR

38. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley It is commonly thought that a can of beverage will cool faster in the coldest part of a refrigerator. Knowledge of Newton’s law of cooling A.supports this knowledge. B.shows this knowledge is false. C.may or may not support this knowledge. D.may or may not contradict this knowledge. Newton’s Law of Cooling CHECK YOUR ANSWER

39. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Global Warming and the Greenhouse Effect Greenhouse Effect Named for a similar temperature-raising effect in florists’ greenhouses

40. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Global Warming and the Greenhouse Effect Understanding the greenhouse effect requires two concepts: All things radiate at a frequency (and therefore wavelength) that depends on the temperature of the emitting object transparency of things depends on the wavelength of radiation

41. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Global Warming and the Greenhouse Effect Global Warming Energy absorbed from the Sun Part reradiated by Earth as longer-wavelength terrestrial radiation

42. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Global Warming and the Greenhouse Effect Global warming (continued) Terrestrial radiation absorbed by atmospheric gases and re-emitted as long-wavelength terrestrial radiation back to Earth Reradiated energy unable to escape, so warming of Earth occurs Long-term effects on climate are of present concern

43. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley If Earth radiated more energy than it absorbs from the Sun, Earth’s average temperature would A.decrease. B.increase. C.likely not change. D.None of these. Global Warming and the Greenhouse Effect CHECK YOUR NEIGHBOR

44. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Global Warming and the Greenhouse Effect CHECK YOUR ANSWER If Earth radiated more energy than it absorbs from the Sun, Earth’s average temperature would A.decrease. B.increase. C.likely not change. D.None of these. Explanation: The Sun emits short-wavelength radiation and the Earth absorbs it. Earth emits long-wavelength radiation, and we have an energy balance that determines Earth’s temperature. If Earth radiates more than it absorbs, then like any system, its temperature would decrease. That’s the physics!

45. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Phases of Matter Matter exists in the three common phases: solid, liquid, and gas (a fourth phase of matter is plasma). When matter changes from one phase to another, energy is transferred.

46. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Heat Transfer and Change of Phase Evaporation Change of phase from liquid to gas

47. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Heat Transfer and Change of Phase Evaporation process Molecules in liquid move randomly at various speeds, continually colliding with one another Some molecules gain kinetic energy while others lose kinetic energy during collision Some energetic molecules escape from the liquid and become gas Average kinetic energy of the remaining molecules in the liquid decreases, resulting in cooler water

48. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Evaporation Application Pigs have no sweat glands and therefore cannot cool by the evaporation of perspiration. Instead, they wallow in mud to cool themselves.

49. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Heat Transfer and Change of Phase Sublimation Form of phase change directly from solid to gas Examples: dry ice (solid carbon dioxide molecules) mothballs frozen water

50. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Heat Transfer and Change of Phase Condensation process Opposite of evaporation Warming process from a gas to a liquid Gas molecules near a liquid surface are attracted to the liquid They strike the surface with increased kinetic energy, becoming part of the liquid

51. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Condensation Application If you’re chilly outside the shower stall, step back inside and be warmed by the condensation of the excess water vapor in the shower Evaporation cools you. Condensation warms you!

52. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Evaporation-Condensation Toy The toy drinking bird operates by the evaporation of ether inside its body and by the evaporation of water from the outer surface of its head. The lower body contains liquid ether, which evaporates at room temperature. (a) vaporizes, (b) creates pressure, ether goes up the tube, (c) tips over, condensed ether runs back to body. (d) each pivot wets the head and the cycle repeats.

53. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley When a liquid changes phase to a gas, it A.absorbs energy. B.emits energy. C.neither absorbs nor emits energy. D.becomes more conducting. Energy Transfer CHECK YOUR NEIGHBOR

54. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Energy Transfer CHECK YOUR ANSWER When a liquid changes phase to a gas, it A.absorbs energy. B.emits energy. C.neither absorbs nor emits energy. D.becomes more conducting.

55. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Boiling Boiling process Rapid evaporation occurs beneath the surface of a liquid

56. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Boiling Boiling process (continued) evaporation beneath the surface forms vapor bubbles bubbles rise to the surface if vapor pressure in the bubble is less than the surrounding pressure, then the bubbles collapse hence, bubbles don’t form at temperatures below boiling point when vapor pressure is insufficient

57. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Boiling Heating warms the water from below. Boiling cools the water from above.

58. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Pressure Cooker The tight lid of a pressure cooker holds pressurized vapor above the water surface, which inhibits boiling. In this way, the boiling point of water is greater than 100°C.

59. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley When a liquid is brought to a boil, the boiling process tends to A.resist a further change of phase. B.heat the liquid. C.cool the liquid. D.radiate energy from the system. Boiling CHECK YOUR NEIGHBOR

60. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Boiling CHECK YOUR ANSWER When a liquid is brought to a boil, the boiling process tends to A.resist a further change of phase. B.heat the liquid. C.cool the liquid. D.radiate energy from the system.

61. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Equilibrium (a) In a mixture of ice and water at 0°C, ice crystals gain and lose water molecules at the same time. The ice and water are in thermal equilibrium. (b) This gaining-and-losing process is inhibited when salt is added to the water. Then with fewer water molecules at the interface, fewer enter the ice.

62. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley A spectacular classroom (or home) demo is placing a soda- pop can with a bit of water in it on a hot stove. Soon steam comes from the opening. Invert the can into a bath of water and atmospheric pressure crushes the can. The key to this dramatic whopping of the can is mainly A.temperature reduction of the steam in the can. B.rapid condensation of steam in the can. C.reduced pressure due to cooling of steam in the can. D.increase in atmospheric pressure on the can. Can Crunch! CHECK YOUR NEIGHBOR

63. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison Wesley Can Crunch! CHECK YOUR ANSWER A spectacular classroom (or home) demo is placing a soda- pop can with a bit of water in it on a hot stove. Soon steam comes from the opening. Invert the can into a bath of water and atmospheric pressure crushes the can. The key to this dramatic whopping of the can is mainly A.temperature reduction of the steam in the can. B.rapid condensation of steam in the can. C.reduced pressure due to cooling of steam in the can. D.increase in atmospheric pressure on the can. Comment: Quick reduction in pressure occurs as steam molecules rapidly condense upon the water in the bath (they don’t condense on the hot inner metal walls). With lowered pressure inside, the outside atmospheric pressure produces the whop! Similarly, the condensation cycle of steam turbines reduces pressure on the back side of turbine blades. Greater steam pressure on the front side of the blades turns the turbine.