2. Heat Transfer Conduction, Convection, and Radiation

3. Review: Temperature Tells us how hot or cold something is compared with a standard scale Tells us how hot or cold something is compared with a standard scale A measure of the average kinetic energy of the molecules of a substance A measure of the average kinetic energy of the molecules of a substance

4. Hot or Cold?

5. Review: Thermal Energy The sum of the energies of all of the molecules).

6. Do warmer objects always have more thermal energy (internal energy) than cooler objects? Do warmer objects always have more thermal energy (internal energy) than cooler objects?

7. Which has more thermal energy?

8. Review: Heat The flow of thermal energy Higher temperature (HOT) Lower temperature (COLD) Heat flows until thermal equilibrium is reached

13. Heat always flows from HOT --> COLD

14. Which direction does heat transfer?

16. Conduction

17. Ice Blocks -- which will melt the ice faster?

18. Conduction: How does this method work? Conduction requires contact. Objects must be touching each other…

19. Conduction Conduction is explained by collisions between atoms or molecules. Energy moves from one place to another but molecules do not.

20. Diamonds are awesome conductors

21. Conduction transfer of energy from molecule to molecule OUTCH!!!

27. Heat Conductors Materials that conduct heat well… Metals are the best heat conductors. If you touch a piece of metal that is at room temperature it often "feels cold." This is because metal is a good conductor of heat. It quickly conducts heat away from your body.

28. Don’t ever let anyone convince you to try this…

29. Heat Insulators Liquids and gases are in general good insulators. Porous materials with lots of small spaces are good insulators. (styrofoam…or a fuzzy coat…) Poor conductors of heat….

30. Polar Bear Fur

31. Double Pane Windows

32. Home Insulation

33. Heat Insulators Insulation delays heat transfer, it can not prevent it! A blanket on your bed does not provide your body with heat. It just slows the conduction of your body heat to the colder air.

37. Heat Transfer Conduction, Convection, and Radiation

38. Convection: How does this method work? Convection: How does this method work? When a fluid is heated, it expands and becomes less dense. In the presence of gravity, less dense fluids rise...

41. Hot water rises, cools, and falls Convection

42. Convection

43. Convection Heated air rises, cools, then falls. Air near the heater is replaced by cooler air and then falls

44. Convection Convection Fluids can be liquids such as molten rock inside the Earth...

45. Convection Fluids can also be gases…such as the air above the warm ground.

46. Convection: Example Land warms and cools more quickly than water. Thus convection currents are often evident at the shore in sea breezes and land breezes.

47. Lava Lamp: Convection

49. Heat Transfer Conduction, Convection, and Radiation

50. Radiation: How does this method work? Radiation is heat transfer through electromagnetic waves.

51. Radiation Any object that has temperature radiates heat. Radiation is the only form of heat transfer that can occur in a vacuum (empty space).

52. Radiation “Light” energy visible “Heat” energy infrared

53. We receive lots of warmth from the sun as radiation. Go outside on a sunny day and the warmth you feel of the sun on your face is radiation.

54. Much of the heat we feel near a fire is radiation

57. Radiation: Wavelengths Emitted Shorter wavelength infrared waves absorbed by our skin produce the sensation of heat. If an object is hot enough, some of the radiant energy it emits is in the form of visible light. At a temperature of about 500˚C an object begins to emit the longest wavelengths we can see, red light. At about 1200˚C all the different waves to which the eye is sensitive are emitted and we see an object as white hot.

58. Radiation: Good Absorbers are Good Emitters Good absorbers of radiant energy are also good emitters (and poor absorbers are poor emitters.). A black mug will allow warm liquids to cool really quickly but it will also allow cool liquids to warm really quickly.

59. Would you like your coffee with cream?

60. Igloo… How can ice “keep us warm?” Why does the doorway appear dark?

61. Mpemba effect A boy from Tanzania discovered that his hot ice cream mix froze before his classmates’ cooler mixes?

62. Mpemba effect Convection accelerates heat transfer Convection accelerates heat transfer Frost on sides and bottom has insulating effects Frost on sides and bottom has insulating effects

63. How does a Thermos work?

64. Is it better to wear light or dark colors in the desert to stay cool?

65. Why do Emperor Penguins huddle together to stay warm? Gilbert et al. 2006