1. Conceptual Physical Science 5th Edition Chapter 7: HEAT TRANSFER AND
CHANGE OF PHASE
© 2012 Pearson Education, Inc.

2. This lecture will help you understand:
Conduction
Convection
Radiation
Newton’s Law of Cooling
Climate Change and the Greenhouse Effect
Heat Transfer and Change of Phase
Boiling
Melting and Freezing
Energy and Change of Phase

3. Heat Transfer Processes of thermal energy transfer: Conduction
Convection
Radiation

4. Conduction Conduction
Transfer of internal energy by electron and molecular collisions within a substance

5. 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. Heat Transfer: 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.
In some cases, yes.
No.
In some cases, no.
C. No.

7. Heat Transfer: 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.
In some cases, yes.
No.
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.
C. No.

8. 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. 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. Energy Transfer CHECK YOUR NEIGHBOR
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.
keep the cold from coming through the roof.
slow the flow of heat from inside the house to the outside.
stop the flow of heat from inside the house to the outside.
C. slow the flow of heat from inside the house to the outside.

11. 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.
keep the cold from coming through the roof.
slow the flow of heat from inside the house to the outside.
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!)
C. slow the flow of heat from inside the house to the outside.

12. 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. 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. Convection 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. 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. 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. 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. Heat Transfer: 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.
There is a thick insulating blanket of air above valleys.
Both of the above.
None of the above.
C. Both of the above.

19. Heat Transfer: 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.
There is a thick insulating blanket of air above valleys.
Both of the above.
None of the above.
Explanation:
Earth’s atmosphere acts as a blanket, which for one important thing, keeps Earth from freezing at nighttime.
C. Both of the above.

20. Radiation
Radiation
Transfer of energy via electromagnetic waves that can travel through empty space

21. Heat Transfer: Radiation
Wavelength of radiation is related to the frequency of vibration.
Low-frequency vibrations  long waves
High-frequency vibrations  short waves

22. 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. 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. Radiation Emission of radiant energy
Peak frequency of radiation is proportional to the absolute temperature of the source ( )

25. 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. 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. 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. Emission and Absorption
CHECK YOUR NEIGHBOR
If a good absorber of radiant energy were a poor emitter, its temperature compared with its surroundings would be
A. lower.
higher.
unaffected.
None of the above.
B. higher.

29. Emission and Absorption
CHECK YOUR ANSWER
If a good absorber of radiant energy were a poor emitter, its temperature compared with its surroundings would be
A. lower.
higher.
unaffected.
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!
B. higher.

30. 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. Which is the better statement?
Absorption of Radiant Energy
CHECK YOUR NEIGHBOR
Which is the better statement?
A. A black object absorbs energy well.
An object that absorbs energy well is black.
Both say the same thing, so both are equivalent.
Both are untrue.
B. An object that absorbs energy well is black.

32. Which is the better statement?
Absorption of Radiant Energy
CHECK YOUR ANSWER
Which is the better statement?
A. A black object absorbs energy well.
An object that absorbs energy well is black.
Both say the same thing, so both are equivalent.
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.
B. An object that absorbs energy well is black.

33. Which of the following does NOT emit radiation?
Emission of Radiant Energy
CHECK YOUR NEIGHBOR
Which of the following does NOT emit radiation?
A. A lit fluorescent lamp.
A lit incandescent lamp.
A burned out incandescent lamp.
None of the above.
D. None of the above.

34. Which of the following does NOT emit radiation?
Emission of Radiant Energy
CHECK YOUR ANSWER
Which of the following does NOT emit radiation?
A. A lit fluorescent lamp.
A lit incandescent lamp.
A burned out incandescent lamp.
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!
D. None of the above.

35. 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. 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. 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.
shows this knowledge is false.
may or may not support this knowledge.
may or may not contradict this knowledge.
A. supports this knowledge.

38. 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.
shows this knowledge is false.
may or may not support this knowledge.
may or may not contradict this knowledge.
A. supports this knowledge.

39. Climate Change and the Greenhouse Effect
Named for a similar temperature-raising effect in florists’ greenhouses

40. Climate Change 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. Climate Change and the Greenhouse Effect
Climage Change
Energy absorbed from the Sun
Part reradiated by Earth as longer-wavelength terrestrial radiation

42. Climate Change and the Greenhouse Effect
Climate Change (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. Climate Change and the Greenhouse Effect
CHECK YOUR NEIGHBOR
If Earth radiated more energy than it absorbs from the Sun, Earth’s average temperature would
A. decrease.
increase.
likely not change.
None of these.
A. decrease.

44. Climate Change 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.
increase.
likely not change.
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!
A. decrease.

45. 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. Heat Transfer and Change of Phase
Evaporation
Change of phase from liquid to gas

47. 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. 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. 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. 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. 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. 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. When a liquid changes phase to a gas, it
Energy Transfer
CHECK YOUR NEIGHBOR
When a liquid changes phase to a gas, it
A. absorbs energy.
emits energy.
neither absorbs nor emits energy.
becomes more conducting.
A. absorbs energy.

54. When a liquid changes phase to a gas, it
Energy Transfer
CHECK YOUR ANSWER
When a liquid changes phase to a gas, it
A. absorbs energy.
emits energy.
neither absorbs nor emits energy.
becomes more conducting.
A. absorbs energy.

55. Boiling Boiling process
Rapid evaporation occurs beneath the surface of a liquid

56. 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. Boiling Heating warms the water from below.
Boiling cools the water from above.

58. 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. When a liquid is brought to a boil, the boiling process tends to
CHECK YOUR NEIGHBOR
When a liquid is brought to a boil, the boiling process tends to
A. resist a further change of phase.
heat the liquid.
cool the liquid.
radiate energy from the system.
C. cool the liquid.

60. When a liquid is brought to a boil, the boiling process tends to
CHECK YOUR ANSWER
When a liquid is brought to a boil, the boiling process tends to
A. resist a further change of phase.
heat the liquid.
cool the liquid.
radiate energy from the system.
C. cool the liquid.

61. 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. Can Crunch! CHECK YOUR NEIGHBOR
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.
rapid condensation of steam in the can.
reduced pressure due to cooling of steam in the can.
increase in atmospheric pressure on the can.
B. rapid condensation of steam in the can.

63. 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.
rapid condensation of steam in the can.
reduced pressure due to cooling of steam in the can.
increase in atmospheric pressure on the can.
Explanation:
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.
B. rapid condensation of steam in the can.