TOPS Physics - Thermodynamics

Transcription

1 TOPS Physics - Thermodynamics Pressure-Volume Work Determination - Conceptual In this activity, you will use a heat engine to lift a mass. Purpose: To explore the concept of converting thermal energy to physical work. Equipment: Pasco Heat Engine Ice and water containers Aluminum Air Chamber 50g and 100g masses Cautions: This equipment is delicate. Everything should go together with the lightest of touches. Do not force anything! You may find that some of the setup procedure has already been done for you. Check each step to make sure that it is done properly. The success of your work depends upon correct setup! Procedure to set up the heat engine 1. Prepare two containers of water, one at room temperature and one with ice. 2. Loosen the thumbscrew holding the heat engine piston and allow the piston to move to the lowest position 3. Place the aluminum air chamber into the ice water. 4. Connect the tube with the check valves from the aluminum air chamber to one port of the heat engine. Push and twist slightly to lock the tube in place. 5. Push down firmly on the white plastic clamp to close the unused port of the heat engine. Mass Platform Heat Engine Aluminum Air Chamber Close this port with its clamp TOPS_ThermoConceptP_V_Work_07_08.doc (Lyle, Adler) - DRAFT Page 1

2 Operating the heat engine: 1. Place a 50g mass on the platform of the heat engine. You will be moving the aluminum air chamber back and forth between the ice water and room temperature water. You need to count the number of times you switch containers. Make tick marks on a piece of paper so you don t lose count in the process. 2. Move the aluminum air chamber from the ice water to the room temperature water. 3. Observe what happens. Determine which way the air flows through the tubing. Hint: The check valves allow air to move in one direction only, from blue toward clear. 4. Wait for a count of 10. The platform should move upward a bit. 5. Immerse the aluminum air chamber in the ice water bath. The piston should stay where it is. 6. Observe what happens. Determine which way the air flows through the tubing 7. Wait for a count of Move the aluminum air chamber back and forth between the ice and room temperature water until the piston is at the top of the cylinder. 9. Record the number of switches here: 10. Place the aluminum air chamber in the ice water bath. 11. Disconnect the tube from the Heat Engine. Turn the connector slightly counterclockwise and pull it out. 12. Allow the piston and mass to go to the bottom. 13. Reconnect the tube to the Heat Engine. 14. Place a 100g mass on the platform of the Heat Engine. 15. Repeat steps 2-6 until the piston reaches the top of the cylinder. 16. Record the number of switches here: 17. Place the aluminum air chamber in the ice water bath. 18. Disconnect the tube from the Heat Engine. Turn the connector slightly counterclockwise and pull it out. 19. Allow the piston and mass to go to the bottom. 20. Reconnect the tube to the Heat Engine. 21. Pour out the room temperature water. Fill the container with hot water from a hot water faucet. 22. Leave the 100g mass on the platform of the heat engine. 23. Repeat steps 2-6 until the piston reaches the top of the cylinder. 24. Record the number of switches here: 25. Place the aluminum air chamber in the ice water bath. 26. Open the clamp on the heat engine. It is harmful to leave the clamp closed. General Questions: 1. Did the 100g mass rise as quickly as the 50g mass? 2. What was the effect of using hot water in place of cold water to raise the 100g mass? TOPS_ThermoConceptP_V_Work_07_08.doc (Lyle, Adler) - DRAFT Page 2

3 Detailed Questions: 1. When you moved the aluminum air chamber from the ice water to the warmer water, what happened to the temperature of the air inside the chamber? 2. Did heat flow into or out of the aluminum air chamber? 3. What happened to the volume of the air? 4. Did the aluminum air chamber get visibly larger when it was heated? 5. Where did the extra air volume go and what did it do? Hint: Look at the arrangement of the check valves in the tubing! 6. When you moved the aluminum air chamber from the warm water to the ice water what happened to the temperature of the air inside the aluminum air chamber? 7. Did heat flow into or out of the aluminum air chamber? 8. What happened to the volume of the air inside the aluminum air chamber? TOPS_ThermoConceptP_V_Work_07_08.doc (Lyle, Adler) - DRAFT Page 3

4 9. Did the aluminum air chamber get visibly smaller when it was cooled? 10. Where did the extra air come from? Hint: Look at the arrangement of the check valves in the tubing! 11. Considering an entire cycle, did all the heat energy absorbed from the warm water get turned into mechanical work? If not, where did the wasted energy go? 12. Could you make an engine that works without a cold reservoir to which to reject heat? Why or why not? 13. Did the engine lift the weight faster with hotter water? How does this relate to the Carnot efficiency equation? TOPS_ThermoConceptP_V_Work_07_08.doc (Lyle, Adler) - DRAFT Page 4

5 Teacher Reference Pages Introduction: A heat engine works by absorbing heat from a warm source and rejecting heat to a colder sink. It requires both of these to operate continuously This experiment makes this clear to students because they have to work both with a warm source and a cold sink, without which the engine cannot be made to operate. Also demonstrated is Carnot efficiency as the difference between source and sink temperatures is varied. Experimental goals: After completing this experiment, students will be able to describe the need for both a heat source and a heat sink for a heat engine. They will be able to describe the operation of the simple heat engine. They will be able to describe the effect of varying temperature difference on efficiency. California Science Standards addressed in this laboratory activity: 3(a) Students know heat flow and work are two forms of energy transfer between systems. 3(b) Students know that the work done by a heat engine that is working in a cycle is the difference between the heat flow into the engine at high temperature and the heat flow out at a lower temperature (first law of thermodynamics) and that this is an example of the law of conservation of energy. 3(g) Students know how to solve problems involving heat flow, work, and efficiency in a heat engine and know that all real engines lose some heat to their surroundings. Investigation & Experimentation: 1(a) Select and use appropriate tools and technology (such as computer-linked probes, spreadsheets, and graphing calculators) to perform tests, collect data, analyze relationships, and display data. 1(c) Identify possible reasons for inconsistent results, such as sources of error or uncontrolled conditions. 1(d) Formulate explanations by using logic and evidence. 1(l) Analyze situations and solve problems that require combining and applying concepts from more than one area of science. TOPS_ThermoConceptP_V_Work_07_08.doc (Lyle, Adler) - DRAFT Page 5

6 Equipment: Pasco Heat Engine Ice and water containers Key words: work, pressure, volume, heat Procedure notes: Each lab group needs a minimum of 2 students Answers to questions: General Questions: 1. Did the 100g mass rise as quickly as the 50g mass? No, It required more swaps between hot and cold. Typically, 50g will need 17 swaps versus 23 for 100g. Remember, the 35g piston is also being raised. 2. What was the effect of using hot water in place of room temperature water to raise the 100g mass? The weight was raised much faster Detailed Questions: 1. When you moved the aluminum air chamber from the ice water to the warmer water, what happened to the temperature of the air inside the aluminum air chamber? The temperature increased 2. Did heat flow into or out of the aluminum air chamber? Heat flowed into the aluminum air chamber 3. What happened to the volume of the air? The volume of air increased 4. Did the aluminum air chamber get visibly larger when it was heated? No 5. Where did the extra air volume go and what did it do? Hint: Look at the arrangement of the check valves in the tubing! The extra air went out the tube, through the in-line check valve and into the bottom of the cylinder. It did not go out of the T because the check valve in the T prevented it. TOPS_ThermoConceptP_V_Work_07_08.doc (Lyle, Adler) - DRAFT Page 6

7 TOPS Physics 6. When you moved the aluminum air chamber from the warm water to the ice water what happened to the temperature of the air inside the aluminum air chamber? The temperature decreased 7. Did heat flow into or out of the aluminum air chamber? Out 8. What happened to the volume of the air inside the aluminum air chamber? It decreased 9. Did the aluminum air chamber get visibly smaller when it was cooled? No 10. Where did the extra air come from? Hint: Look at the arrangement of the check valves in the tubing! The extra air came in through the check valve in the T. Air could not come in via the in-line check valve because that valve only allows air to flow toward the cylinder. 11. Considering an entire cycle, did all the heat energy absorbed from the warm water get turned into mechanical work? If not, where did the wasted energy go? Not all the energy went to work. Some energy was lost as heat to the ice water. 12. Could you make an engine that works without a cold reservoir to which to reject heat? Why or why not? No. If you do not reject some heat, the working gas cannot be replenished and the engine will simply stop. 13. Did the engine lift the weight faster with hotter water? How does this relate to the Carnot efficiency equation? The carnot equation is (T hot -T cold )/T hot. The greater the difference between the hot and cold temperatures, the greater the efficiency and the faster the weight is raised. References Pasco equipment guide California Science Standards TOPS_ThermoConceptP_V_Work_07_08.doc (Lyle, Adler) - DRAFT Page 7