1. College Physics, 7th Edition
Lecture Outline
Chapter 11
College Physics, 7th Edition
Wilson / Buffa / Lou
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2. Chapter 11 Heat
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3. Units of Chapter 11 Definition and Units of Heat
Specific Heat and Calorimetry
Phase Changes and Latent Heat
Heat Transfer
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4. 11.1 Definition and Units of Heat
Heat is a form of energy, and therefore is measured in joules. There are other units of heat, though; the most common one is the kilocalorie:
One kilocalorie (kcal) is defined as the amount of heat needed to raise the temperature of 1 kg of water by 1 C° (from 14.5°C to 15.5°C).
One calorie is .001 kilocalorie.
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5. 11.1 Definition and Units of Heat
Confusingly, the calories listed on nutrition labels in the U.S. are really kilocalories (sometimes called Calories). Some other labels are more accurate (left, Australia; right, Germany).
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6. 11.1 Definition and Units of Heat
This figure illustrates the three most common units of heat.
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7. 11.1 Definition and Units of Heat
So, if heat is energy, how do kilocalories convert to joules? Careful experimentation using the apparatus below gives the answer, as the work done by the falling weights raises the temperature of the water.
This relationship is called the mechanical equivalent of heat.
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8. 11.2 Specific Heat and Calorimetry
The amount of heat needed to increase the temperature of a solid or liquid depends on the amount of the substance, the temperature change, and the properties of the substance itself.
The constant c is called the specific heat of the substance.
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9. 11.2 Specific Heat and Calorimetry
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10. 11.2 Specific Heat and Calorimetry
Calorimetry is the quantitative measurement of heat exchange; it is done using a calorimeter.
A calorimeter is insulated from the environment, minimizing heat exchange. Therefore, heat lost by one object in the calorimeter must be gained by another. This is one way of measuring specific heats.
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11. 11.2 Specific Heat and Calorimetry
Specific heat can be defined for gases as well, but gases do not have constant volume or pressure.
We therefore define two specific heats for gases—one at constant volume (cV), and one at constant pressure (cP).
For a particular gas, cP is always greater than cV.
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12. 11.3 Phase Changes and Latent Heat
Three phases of matter: solid, liquid, and gas.
Solid has a definite shape and the strongest intermolecular bonds.
Liquid flows but is relatively incompressible, so it has a definite volume.
Gas is compressible, and will expand to fill a container.
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13. 11.3 Phase Changes and Latent Heat
Solid → liquid: melting
Liquid → gas: evaporating, boiling
Gas → liquid: condensing
Liquid → solid: freezing
Solid → gas: sublimating
Latent heat is the amount of heat absorbed or released when a substance undergoes a phase transition.
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14. 11.3 Phase Changes and Latent Heat
During a phase transition, the heat energy goes to changing the intermolecular bonds, and the temperature does not change.
The heat needed for a phase change is:
Here, L is the latent heat; Lf is the latent heat of fusion (solid ↔ liquid) and Lv the latent heat of vaporization.
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15. 11.3 Phase Changes and Latent Heat
Latent heat is a property of a particular substance.
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16. 11.3 Phase Changes and Latent Heat
Heat added as ice becomes steam:
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17. Example (similar to 11.4, p.392)
Students in a physics lab are to determine the specific heat of copper experimentally. They heat 0.150kg of cooper shot to 100 C and then carefully pour the hot shot into a calorimeter cup containing 0.200kg of water at 20.0 C. The final temperature of the mixture in the cup is measured to be 25 C. If the aluminum cup has a mass of kg, what is the specific heat of cooper? (Assume that there is no heat exchange with the surroundings.)
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18. Problem
A 0.30 kg piece of ice at 0 C is placed in a liter of water at room temperature (20 C) in an insulated container. Assuming that no heat is lost to the container, what is the final temperature of the water?
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19. 11.4 Heat Transfer Heat transfer takes place via three mechanisms:
Conduction
Convection
Radiation
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20. 11.4 Heat Transfer
Conduction is the transfer of heat through a substance, such as the disposable cup that holds your hot coffee. If the cup is a good conductor of heat, you will need a sleeve to keep from burning your hand.
Typically, metals are good conductors of heat—they have electrons that are free to move throughout the material—and nonmetals are not.
Nonconductors of heat are also called insulators.
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21. 11.4 Heat Transfer
The heat flow rate through a slab of material is proportional to its surface area and to the temperature difference, and inversely proportional to its thickness.
The constant k is called the thermal conductivity.
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22. 11.4 Heat Transfer
This diagram illustrates the geometry of heat transfer by conduction.
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23. 11.4 Heat Transfer
The thermal conductivities of substances vary widely.
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24. 11.4 Heat Transfer
When insulating a house, we want materials whose thermal conductivity is as low as possible.
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25. 11.4 Heat Transfer
Heat transfer in fluids is mostly by convection, which is the result of mass transfer; that is, heat is transferred as warmer fluid moves to replace cooler fluid. Convection may be spontaneous (as below) or forced.
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26. 11.4 Heat Transfer
Many homes are heated using forced hot air; this is an example of forced convection.
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27. 11.4 Heat Transfer
Radiation is the only type of heat transfer that can take place through a vacuum. You can feel the radiation of heat when you stand near a fireplace. This radiation is in the form of electromagnetic waves, in the infrared part of the spectrum.
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28. 11.4 Heat Transfer
The rate of energy radiation is given by Stefan’s law:
A is the object’s surface area, T is its temperature, and e is a number between 0 and 1 called the emissivity. σ is the Stefan–Boltzmann constant:
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29. 11.4 Heat Transfer
A good emitter of radiation is also a good absorber:
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30. 11.4 Heat Transfer Here we see the three types of heat transfer:
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31. 11.4 Heat Transfer
A Thermos bottle minimizes all three types of heat transfer.
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32. 11.4 Heat Transfer
Passive solar heating uses the changing angle of the Sun to warm buildings in the winter but not in the summer.
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33. Summary of Chapter 11
Heat is the energy exchanged between objects due to a temperature difference.
Specific heat tells the energy needed to raise 1 kg of a substance by 1 C°.
Calorimetry uses the heat transfer between objects to measure specific heats.
Latent heat is the heat needed per kilogram to change the phase of a substance.
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34. Summary of Chapter 11
Conduction is the transfer of heat between objects that are in direct contact.
Convection is the transfer of heat via mass movement of the molecules of a fluid.
Radiation is the transfer of heat via electromagnetic radiation.
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35. Chapter 11, Problem
A volume of 0.50 L of water at 16 C is put into an aluminum ice cube tray of mass kg at the same temperature. How much energy must be removed from this system by the refrigerator to turn the water into ice at -8.0 C?
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36. (Assume that there is no heat loss except to the water itself.)
Chapter 11, Problem
An electric immersion heater has a power rating of 1500 W. If the heater is placed in a liter of water at 20 C, how many minutes will it take to bring the water to a boil?
(Assume that there is no heat loss except to the water itself.)
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37. Chapter 11, Problem
A kg piece of ice at -20 C is converted to steam at 115 C. How much heat must be supplied to this?
To convert the steam back to ice at -5 C, how much heat would have to be removed?
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38. Which bar is longer, and how many times longer?
Chapter 11, Problem 49
An aluminum bar and a copper bar of identical cross-sectional area have the same temperature difference between their ends and conduct heat at the same rate.
Which bar is longer, and how many times longer?
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39. © 2010 Pearson Education, Inc.