Conceptual Physics Fundamentals

Transcription

1 Conceptual Physics Fundamentals Chapter 9: HEAT TRANSFER AND CHANGE OF PHASE

2 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 Heat Transfer and Change of Phase You won t fully appreciate the frontiers of physics until you re familiar with its foothills. Iain MacInnes

4 Heat Transfer and Change of Phase Objects in thermal contact at different temperatures tend to reach a common temperature in three ways: conduction convection radiation

5 Conduction Conduction transfer of internal energy by electron and molecular collisions within a substance, especially a solid

6 Conduction Conductors good conductors conduct heat quickly substances with loosely held electrons transfer energy quickly to other electrons throughout the solid example: silver, copper, and other solid metals

7 Conduction Conductors (continued) poor conductors are insulators molecules with tightly held electrons in a substance vibrate in place and transfer energy slowly these are good insulators (and poor conductors) example: glass, wool, wood, paper, cork, plastic foam, air substances that trap air are good insulators example: wool, fur, feathers, and snow

8 Conduction CHECK YOUR NEIGHBOR 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

9 Conduction CHECK YOUR ANSWER 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.

10 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

11 Conduction Insulation (continued) dramatic example: walking barefoot without burning feet on red-hot coals is due to poor conduction between coals and feet

12 Convection Convection transfer of heat involving only bulk motion of fluids example: visible shimmer of air above a hot stove or above asphalt on a hot day visible shimmers in water due to temperature difference

13 Convection Reason warm air rises warm air expands, becomes less dense, and is buoyed upward it rises until its density equals that of the surrounding air example: smoke from a fire rises and blends with the surrounding cool air

14 Cooling by expansion Convection 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!

15 Convection CHECK YOUR NEIGHBOR 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 A and B D. none of the above

16 Convection CHECK YOUR ANSWER 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 A and B D. none of the above Explanation: Earth s atmosphere acts as a blanket, which keeps the valleys from freezing at nighttime.

17 Convection Winds result of uneven heating of the air near the ground absorption of Sun s energy occurs more readily on different parts of Earth s surface sea breeze The ground warms more than water in the daytime. Warm air close to the ground rises and is replaced by cooler air from above the water.

18 Radiation Radiation transfer of energy from the Sun through empty space

19 Radiation CHECK YOUR NEIGHBOR The surface of Earth loses energy to outer space due mostly to A. conduction. B. convection. C. radiation. D. radioactivity.

20 Radiation CHECK YOUR ANSWER The surface of Earth loses energy to outer space due mostly to A. conduction. B. convection. C. radiation. D. radioactivity. Explanation: Radiation is the only choice, given the vacuum of outer space.

21 Radiation CHECK YOUR NEIGHBOR Which body glows with electromagnetic waves? A. Sun B. Earth C. both A and B D. neither A nor B

22 Radiation CHECK YOUR ANSWER Which body glows with electromagnetic waves? A. Sun B. Earth C. both A and B D. neither A nor B Explanation: Earth glows in long-wavelength radiation, while the Sun glows in shorter waves.

23 Radiation Radiant energy transferred energy exists as electromagnetic waves ranging from long (radio waves) to short wavelengths (X-rays) in visible region, ranges from long waves (red) to short waves (violet)

24 Radiation Wavelength of radiation related to frequency of vibration (rate of vibration of a wave source) low frequency vibration produces longwavelength waves high frequency vibration produces shortwavelength waves

25 Radiation Emission of radiant energy every object above absolute zero radiates from the Sun s surface comes light, called electromagnetic radiation, or solar radiation from the Earth s surface is terrestrial radiation in the form of infrared waves below our threshold of sight

26 Radiation Emission of radiant energy (continued) frequency of radiation is proportional to the absolute temperature of the source ( ) f ~ T

27 Radiation Range of temperatures of radiating objects room temperature emission is in the infrared temperature above 500 C, red light emitted, longest waves visible about 600 C, yellow light emitted at 1500 C, object emits white light (whole range of visible light)

28 Radiation Absorption of radiant energy occurs along with emission of radiant energy effects of surface of material on radiant energy any material that absorbs more than it emits is a net absorber any material that emits more than it absorbs is a net emitter net absorption or emission is relative to temperature of surroundings

29 Radiation Absorption of radiant energy (continued) occurs along with emission of radiant energy good absorbers are good emitters poor absorbers are poor emitters example: radio dish antenna that is a good emitter is also a good receiver (by design, a poor transmitter is a poor absorber)

30 Radiation CHECK YOUR NEIGHBOR 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

31 Radiation CHECK YOUR ANSWER 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!

32 Radiation CHECK YOUR NEIGHBOR A hot pizza placed in the snow is a net A. absorber. B. emitter. C. both A and B D. none of the above

33 Radiation CHECK YOUR ANSWER A hot pizza placed in the snow is a net A. absorber. B. emitter. C. both A and B D. none of the above Explanation: The relation f ~ T tells us that high temperature sources emit high frequency waves. High frequency waves have short wavelength.

34 Radiation CHECK YOUR NEIGHBOR Which melts faster in sunshine dirty snow or clean snow? A. dirty snow B. clean snow C. both A and B D. none of the above

35 Radiation CHECK YOUR ANSWER Which melts faster in sunshine dirty snow or clean snow? A. dirty snow B. clean snow C. both A and B D. none of the above Explanation: Dirty snow absorbs more sunlight, whereas clean snow reflects more.

36 Radiation Reflection of radiant energy opposite to absorption of radiant energy any surface that reflects very little or no radiant energy looks dark examples of dark objects: eye pupils open ends of pipes in a stack open doorways or windows of distant houses in the daytime

37 Radiation Reflection of radiant energy (continued) darkness often due to reflection of light back and forth many times partially absorbing with each reflection good reflectors are poor absorbers

38 Radiation CHECK YOUR NEIGHBOR 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.

39 Radiation CHECK YOUR ANSWER 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.

40 Eureka video of Conduction, Convection & Radiation Conduction, Convection and Radiation song

41 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 will warm faster in a warm room than in a cold room

42 Newton s Law of Cooling CHECK YOUR NEIGHBOR 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.

43 Newton s Law of Cooling CHECK YOUR ANSWER 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.

44 Global Warming and the Greenhouse Effect Greenhouse effect named for a similar temperature-raising effect in florists greenhouses

45 Global Warming and the Greenhouse Effect understanding 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

46 Global Warming and the Greenhouse Effect understanding greenhouse effect requires two concepts (continued) example: Excessive warming of a car s interior when windows are closed on a hot sunny day. Sun s rays are very short and pass through the car s windows. Absorption of Sun s energy warms the car interior. Car interior radiates its own waves, which are longer and don t transmit through the windows. Car s radiated energy remains inside, making the car s interior very warm.

47 Global Warming and the Greenhouse Effect Global warming energy absorbed from the Sun part reradiated by Earth as longer-wavelength terrestrial radiation

48 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

49 Global Warming and the Greenhouse Effect CHECK YOUR NEIGHBOR The greenhouse gases that contribute to global warming absorb A. more visible radiation than infrared. B. more infrared radiation than visible. C. visible and infrared radiation about equally. D. very little radiation of any kind.

50 Global Warming and the Greenhouse Effect CHECK YOUR ANSWER The greenhouse gases that contribute to global warming absorb A. more visible radiation than infrared. B. more infrared radiation than visible. C. visible and infrared radiation about equally. D. very little radiation of any kind. Explanation: Choice A has the facts backward. Choices C and D are without merit.

51 Heat Transfer and Change of Phase Matter exists in four common phases that involve transfer of internal energy: solid phase (ice) liquid phase (ice melts to water) gaseous phase (water burns to vapor) addition of more energy vaporizes water to vapor plasma phase (vapor disintegrates to ions and electrons)

52 Heat Transfer and Change of Phase Evaporation change of phase from liquid to gas

53 Heat Transfer and Change of Evaporation process Phase molecules in liquid move randomly at various speeds, continually colliding into 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

54 Heat Transfer and Change of Phase Important in cooling our bodies when we overheat sweat glands produce perspiration water on our skin absorbs body heat as evaporation cools the body helps to maintain a stable body temperature

55 Heat Transfer and Change of Phase Sublimation form of phase change directly from solid to gas example: dry ice (solid carbon dioxide molecules) mothballs frozen water

56 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

57 Heat Transfer and Change of Phase Condensation process (continued) Kinetic energy is absorbed by the liquid, resulting in increased temperature example: steam releases much energy when it condenses to a liquid and moistens the skin hence, it produces a more damaging burn than from same-temperature 100 C boiling water you feel warmer in a moist shower stall because the rate of condensation exceeds the rate of evaporation

58 Heat Transfer and Change of Phase Condensation process (continued) example: in dry cities, the rate of evaporation from your skin is greater than the rate of condensation, so you feel colder in humid cities, the rate of evaporation from your skin is less than the rate of condensation, so you feel warmer a cold soda pop can is wet in warm air because slowmoving molecules make contact with the cold surface and condense

59 Heat Transfer and Change of Phase CHECK YOUR NEIGHBOR If bits of coals do not stick to your feet when firewalking, it s best if your feet are A. wet. B. dry. C. sort of wet and sort of dry. D. none of these

60 Heat Transfer and Change of Phase CHECK YOUR ANSWER If bits of coals do not stick to your feet when firewalking, it s best if your feet are A. wet. B. dry. C. sort of wet and sort of dry. D. none of these Explanation: The energy that vaporizes water is energy that doesn t burn your feet.

61 Boiling Boiling process rapid evaporation from beneath the surface of a liquid

62 Boiling Boiling process (continued) rapid form of 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 (vapor pressure is insufficient)

63 Boiling Boiling process (continued) boiling water at 100 C is in thermal equilibrium boiling water is being cooled as fast as it is being warmed in this sense, boiling is a cooling process

64 Boiling Boiling point depends on pressure example: buildup of vapor pressure inside a pressure cooker prevents boiling, thus resulting in a higher temperature that cooks the food Boiling point is lower with lower atmospheric pressure example: water boils at 95 C in Denver, CO (high altitude) instead of at 100 C (sea level)

65 Boiling demonstration of cooling effect of evaporation and boiling

66 Boiling CHECK YOUR NEIGHBOR The process of boiling A. cools the water being boiled. B. depends on atmospheric pressure. C. is a change of phase below the water surface D. all of the above

67 Boiling CHECK YOUR ANSWER The process of boiling A. cools the water being boiled. B. depends on atmospheric pressure. C. is a change of phase below the water surface. D. all of the above

68 Boiling and Freezing Freezing by evaporation dish of room temperature water is placed in a vacuum jar as pressure in the jar is slowly reduced by a vacuum pump, water begins to boil molecules with highest kinetic energy escape and the remaining water is cooled pressure is reduced further, more faster-moving molecules boil away until the remaining water reaches 0 C

69 Boiling and Freezing Freezing by evaporation (continued) as cooling continues by boiling, ice forms over the surface of the bubbling water, resulting in frozen bubbles of boiling water

70 Melting and Freezing Melting occurs when a substance changes phase from a solid to a liquid opposite of freezing When heat is supplied to a solid, added vibration breaks molecules loose from the structure and melting occurs.

71 Melting and Freezing Freezing occurs when a liquid changes to a solid opposite of melting When energy is continually removed from a liquid, molecular motion decreases until the forces of attraction bind them together and formation of ice occurs.

72 Energy and Change of Phase Energy and Change of Phase From solid to liquid to gas phase add energy From gas to liquid to solid phase remove energy

73 Energy and Change of Phase example of both vaporization and condensation processes Cooling cycle of refrigerator pumps a special fluid that vaporizes and draws heat from stored food. The gas that forms along with its energy is directed to the condensation coils outside the fridge where heat is released and the fluid condenses back to liquid. air conditioner pumps heat energy from one part of the unit to another

74 Energy and Change of Phase Heat of fusion amount of energy needed to change any substance from solid to liquid and vice versa example: heat of fusion for water is 334 joules/g farmers in cold climates replaced frozen tubs of water with unfrozen ones in their cellars to prevent jars of food from freezing

75 Energy and Change of Phase Heat of vaporization amount of energy needed to change any substance from liquid to gas and vice versa example: heat of vaporization for water is 2256 joules/g In briefly touching a hot skillet, energy that normally would flow into your finger instead vaporizes water. Hence, you re not burned.

76 Change of Phases for Water

77 Backup

78 Energy and Change of Phase CHECK YOUR NEIGHBOR When snow forms in clouds, the surrounding air is A. cooled. B. warmed. C. insulated. D. thermally conducting.

79 Energy and Change of Phase CHECK YOUR ANSWER When snow forms in clouds, the surrounding air is A. cooled. B. warmed. C. insulated. D. thermally conducting. Explanation: The change of phase is from gas to solid, which releases energy.

80 Energy and Change of Phase CHECK YOUR NEIGHBOR Which involves the greatest number of calories? A. condensing 1 gram of 100 C steam to 100 water B. cooling 1 gram of 100 C water to 1 gram of 0 C ice C. cooling 1 gram of 0 C ice to near absolute zero D. all about the same

81 Energy and Change of Phase CHECK YOUR ANSWER Which involves the greatest number of calories? A. condensing 1 gram of 100 C steam to 100 C water B. cooling 1 gram of 100 C water to 1 gram of 0 C ice C. cooling 1 gram of 0 C ice to near absolute zero D. all about the same Explanation: 540 calories is more than the 100 calories for B, and half of 273 calories to cool ice (the specific heat of ice is about half that for liquid water).

82 Energy and Change of Phase CHECK YOUR NEIGHBOR Ice is put in a picnic cooler. To speed up the cooling of cans of beverage, it is important that the ice A. melts. B. is prevented from melting. C. be in large chunks. D. none of these

83 Energy and Change of Phase CHECK YOUR ANSWER Ice is put in a picnic cooler. To speed up the cooling of cans of beverage, it is important that the ice A. melts. B. is prevented from melting. C. be in large chunks. D. none of these Explanation: For each gram of ice that melts, 540 calories is taken from the beverage.