Lesson Plans

Physics 4th Edition ©2002

by Jerry Wilson and Anthony Buffa

Week 14

Chapter 12: Thermodynamics

College Board Performance Objectives:

• Define a thermodynamic system.
• Differentiate between state and phase.
• Give two examples in which the internal energy of a system can be changed.
• State the First Law of Thermodynamics, give two examples in which the law is demonstrated, and represent the first law mathematically.
• Define and give illustrated examples of each of the following thermodynamic processes: (a) adiabatic, (b) isochoric, (c) isothermal, and (d) isobaric.
• Explain the significance of a P-V diagram in describing (a) adiabatic, (b) isochoric, (c) isothermal, and (d) isobaric thermodynamic processes.
• State the Second Law of Thermodynamics.
• Define the entropy of a system.
• Explain the operation and the limitations of the efficiency of a heat engine.
• Determine the efficiency of a heat engine in terms of heat input and heat output.
• Determine the efficiency of a heat engine in terms of input temperature and output temperature.
• Differentiate between Carnot Efficiency and actual efficiency as applied to heat engines.

College Board Lab Objectives:

• Devise an experiment to measure the work done in an isothermal process.
• Interpret and make calculations from PV diagrams.

Suggested Labs:

• Phase Change

Resources:

• Student Edition — pp. 388–411
• Student Study Guide — pp. 150–161
• Instructor's Solution Manual — pp. 158–169
• Test Items File — pp. 208–225

Pacing Guide:

• Functions Necessary for the First Law—day 1
• The First Law of Thermodynamics—days 1 and 2
• P-V Diagrams and Adiabatic Processes—days 2 and 3
• Isochoric and Isothermic Processes—days 2 and 3
• Heat Engines and efficiency—day 3 and 4
• The Second Law of Thermodynamics and Entropy—days 3 and 4
• The Carnot Cycle—day 4
• Lab — day 5
• Block Scheduling
  The first law of thermodynamics, P-V diagrams, and adiabatic and isothermal processes require one and one-half blocks. Stress P-V diagrams. Heat engines, the Second Law of Thermodynamics, and the Carnot cycle require one and one-half blocks. Emphasize heat engines and associated P-V diagrams.

Key Words:

Critical Thinking Questions:

1. A special case of an adiabatic process is called the throttling process. An example of this is air trapped in a basketball escaping through a needle valve. Write the First Law of Thermodynamics for such a process. Can you find any another examples of this type of process? If so, do any involve a phase change?
2. A gas expands against a movable piston, lifting it 5.0 cm at constant speed. If the piston has a mass of 90.0 kg and a cross-section of 70 cm²:
   1. How much work does the expanding gas do?
   2. If the process is adiabatic, what is the change in internal energy?
   3. Is U an increase or decrease in the internal energy?
3. Exactly 5.00 g of water at 100°C is converted to steam at 1.0 atm of pressure.
   1. What thermal energy is added?
   2. What work is done in expansion?
   3. Find the change in the internal energy.
   4. What is the change in entropy?
4. One mole of an ideal gas at STP undergoes a series of steps in a cyclic process.
   Step 1: The gas is heated at constant pressure until its volume triples.
   Step 2: It is then heated at constant volume until the pressure doubles.
   Step 3: It is compressed to its original volume.
   Step 4: Finally, it is cooled until it reaches its original pressure.
   
   1. Graph P vs. V indicating the pressure, volume, and temperature at the end of each step.
   2. What is the net work done?
   3. What is the net heat into the system?
   4. What is the change in the internal energy?
   Calculate W and U for a 1.00 L cube of aluminum as it is heated from 0°C to 320°C. The specific heat of aluminum is 920 J/kg C°, its coefficient of volumetric expansion is 7.2x10^–5 1/C°, and its mass is 2.70 kg.

Troubleshooting Tips/Error Traps:

• Remind students that all temperatures must be converted to Kelvin.
• Students may be confused recognizing which functions are state functions.
• Many students will have difficulty identifying the processes on a P-V diagram. Provide many examples and graphs.
• Students may confuse heat and internal energy. Remind them that heat transfer depends on a temperature difference.
• Students who do not understand the difference between heat and temperature will not grasp the significance of an adiabatic process.
• Entropy causes difficulty for most students. Guide students through several clear examples.

End of Chapter Activity:

1. Diesel engines are more efficient than gasoline engines. Consider one entire cycle for each engine and make a statement concerning their compression ratios.
2. A car salesman tells you a new device is on the market that will increase the efficiency of the engine to 100%. Should you buy this device? If you had such a device installed on you car, what would be the temperature of the exhaust gas?
3. Eat an ice cream cone and describe the entropy changes in the process.
4. Take a deck of ordered cards and drop the deck on the floor. What happens to the order? Pick them up at random and drop them again. Do conditions for order improve? Why or why not?
5. Take the time to go over problems 19, 22, 23, 24, 25, 41, 42, and 84 on pp. 413, 414, and 416 in the textbook until you understand them. These are important P-V diagram problems related to similar problems on past AP* B Physics Exams.

Suggested Problem Assignments: