Chapter 22 Heat Transfer

1. Chapter 22 Heat Transfer Conceptual Physics Chapter 22

2. Heat Transfer • Spontaneous heat flow is always from the warmer object to the cooler object. • Heat transfer continues until thermal equilibrium is reached. • Heat flow can occur in one of three ways: • Conduction • Convection • Radiation Conceptual Physics Chapter 22

3. Question Why does a tile floor feel colder on your bare feet than a carpeted floor? Conceptual Physics Chapter 22

4. Conduction • Conduction results from atoms transferring thermal energy from one atom to the next during collisions. • Examples: • Food coloring in hot water • Stirring a pot of soup • Tile floor Conceptual Physics Chapter 22

5. Thermal Conductors • Good conductors are made of atoms with valence electrons that are only loosely bound to the nucleus. • When a conductor is heated, the loosely bound electrons are caused to vibrate more rapidly and through larger amplitudes. • This makes collisions between neighboring atoms more likely. Conceptual Physics Chapter 22

6. Thermal Insulators • Insulators have valence electrons which are tightly bound to a specific nucleus. • The valence electrons in insulators do not have the same freedom of movement as those in conductors. • Collisions between neighboring atoms occur far less frequently and heat transfer is localized. Conceptual Physics Chapter 22

7. Conductors & Insulators • There are no perfect conductors nor are there perfect insulators. • Metals, as a group, are thought of as being good conductors. • Wood, plastic, styrofoam, paper and air are thought of as good insulators. • A good conductor is a poor insulator. Conceptual Physics Chapter 22

8. Insulating Properties of Air • Materials that are porous or have the ability to trap air pockets make good insulators • Examples: • Snow/Ice • Goose Down Jacket • Double Panel Glass Windows Conceptual Physics Chapter 22

9. Convection Convection occurs in fluids (liquids and gases) as a result of a net movement of a mass of the fluid in response to density differences. When a fluid is heated, it expands, becomes less dense, and rises above the more dense fluid surrounding it. Conceptual Physics Chapter 22

10. Convection Water at the top of the test tube comes to a boil and remains at the surface above the more dense water below it. Any heat that reaches the ice stuck at the bottom of the test tube must do so by conduction. Since water is a poor conductor, the ice remains unmelted! Conceptual Physics Chapter 22

11. Convection When the water at the bottom of the pan is heated, it expands, rises to the top and creates a convection current that causes the heating to occur more efficiently. The radiator forces hot air out near the floor where it rises toward the ceiling and sets up a natural circulation to heat the room more efficiently. Conceptual Physics Chapter 22

12. Convection During the day, the land heats up more readily than the water. The hot air above the shore rises and is replaced by cooler air from above the ocean resulting in an onshore breeze. During the night, the land cools more readily than the water. The warmer air above the water rises and is replaced by cooler air from above the shore resulting in an offshore breeze. Conceptual Physics Chapter 22

13. Convection • Rising warm air will expand because at higher altitudes less atmospheric pressure will squeeze on it. • As the rising air expands, it cools—this is why many mountain tops will stay cold enough to remain covered with snow. Conceptual Physics Chapter 22

14. Convection • Molecules in a region of expanding air collide more often with receding molecules than with approaching ones. • When a molecule collides with a molecule that is receding, its rebound speed after the collision is less than before the collision. Reduced kinetic energy means reduced temperature! Conceptual Physics Chapter 22

15. Convection • Convection occurs readily in gases and liquids, but does not occur at all in solid substances. Conceptual Physics Chapter 22

16. Radiation • Heat from the sun warms the earth after passing through millions of miles of empty space. • Since conduction and convection can not occur in a vacuum, this heating must take place by a different mechanism – radiation. Conceptual Physics Chapter 22

17. Radiation • Radiant energy is transmitted via electromagnetic waves. • These waves are capable of traveling through a vacuum – they do not rely in any way on a material medium in order to propagate. • Radio waves, infrared waves, light waves, microwaves and X-rays are all forms of radiation. Conceptual Physics Chapter 22

18. Radiation • All objects emit radiant energy in a variety of wavelengths. • The sun and other extremely hot objects emit short wavelength radiation, in the range of visible light. • Objects at lower temperatures emit radiation of longer wavelength typically in the infrared range. Conceptual Physics Chapter 22

19. Radiation Short-wavelength infrared waves are absorbed by our skin, producing the sensation of heat. Conceptual Physics Chapter 22 19

20. Radiation An infrared thermometer measures the infrared radiant energy that is continuously emitted by all bodies in our environment and converts it to a temperature. The average frequency f of radiant energy is directly proportional to the Kelvin temperature T of the emitter. Conceptual Physics Chapter 22 20

21. Radiation • The slower moving molecules associated with a cooler body will emit lower frequency, longer wavelength radiation. Conceptual Physics Chapter 22

22. Absorption and Emission If everything in the world around us is constantly radiating energy, why don’t all objects eventually run out of energy? Conceptual Physics Chapter 22

23. Absorption and Emission • A good absorber of radiation is also a good emitter of radiation. • If a good absorber of radiation were not a good emitter, it would experience a net gain in thermal energy and never reach thermal equilibrium with its surroundings. Conceptual Physics Chapter 22

24. Absorption and Reflection • Absorption and reflection are opposite processes. • A good absorber of radiation is a poor reflector. • A good absorber therefore appears dark. • A perfect absorber reflects no radiant energy and appears perfectly black. Conceptual Physics Chapter 22

25. Absorption and Reflection • Openings (such as doorways of distant houses) appear black because the radiant energy that enters is reflected from the inside walls many times. • It is partly absorbed at each reflection until very little or none remains to come back out. Conceptual Physics Chapter 22

26. Absorption and Reflection • Even though the interior of the box has been painted white, the hole looks black. Conceptual Physics Chapter 22

27. Absorption and Reflection • Light-colored, highly polished surfaces make good reflectors of radiation. • Dark-colored, dull or flat surfaces make good absorbers of radiation. • A white or light-colored t-shirt will keep you much cooler during a hot summer day than a dark-colored shirt. Conceptual Physics Chapter 22

28. Absorption and Reflection • The silvered lining in a thermos reflects most of the radiant energy that reaches it. Conceptual Physics Chapter 22

29. Absorption and Reflection Which of the two pitchers will keep iced tea colder longer? Which of the two pitchers will keep hot coffee hotter longer? Conceptual Physics Chapter 22

30. Newton’s Law of Cooling • The rate of cooling of an object (whether by conduction, convection or radiation) depends on how much hotter the object is in comparison to its surroundings. rate of cooling ~ ΔT • Newton’s law of cooling also applies to objects which are being heated. Conceptual Physics Chapter 22

31. Question Since a hot cup of tea loses heat more rapidly than a lukewarm cup of tea, would it be correct to say that a hot cup of tea will cool to room temperature before a lukewarm cup of tea will? Conceptual Physics Chapter 22

32. The Greenhouse Effect • The greenhouse effect is a natural warming of the earth that takes place when solar radiation is trapped inside the earth’s atmosphere. • Greenhouse gases – carbon dioxide, methane and water vapor – are transparent to the short wavelength visible radiation that reaches the earth from the sun, allowing this radiant energy to penetrate the atmosphere and be absorbed by the earth. Conceptual Physics Chapter 22

33. The Greenhouse Effect • The earth emits terrestrial radiation of a longer wavelength (because the earth is cooler than the sun). This radiation is primarily in the infrared region of the electromagnetic spectrum. Conceptual Physics Chapter 22

34. The Greenhouse Effect • Some of this terrestrial radiation passes through the atmosphere and escapes back into space. The remainder is blocked by the atmosphere and warms the earth. • If the atmosphere were not opaque to this terrestrial radiation, the average global temperature would be approximately -18°C! Conceptual Physics Chapter 22

35. The Greenhouse Effect • This same effect occurs on a much smaller scale in an automobile left parked in the sun and in a florist’s greenhouse (although a florist’s greenhouse also restricts any heat transfer by convection). Conceptual Physics Chapter 22