Name: ______________________________ 1 Lab #3 HNRS 228 Laboratory with Ms. Jahangeer 14 February 2011 Heat Energy, Thermodynamics and the Carnot Cycle Open the software for this laboratory experiment by running the program called Exploration of Physics from your desktop. You will find this experiment under the "heat" tab of the program's menu. Please remember to answer all questions in complete sentences with necessary graphics or calculations as requested. The 1st Law of Thermodynamics and Energy Conversion This experiment allows you to explore the relationship between mechanical energy and heat. The falling weight drives a paddle which stirs the water in a thermally insulated bucket. The stirring raises the temperature of the water by a measurable amount, converting gravitational potential energy into heat. You will be performing the experiment several times, varying the parameters each time. Be sure to record the water mass, initial water temperature, hanging mass height and gravity each time. Energy Varying Height and Mass Perform the experiment first starting with 2 kilograms for the hanging mass. Then change only the hanging mass, 2 kilograms greater each time, until you get to 10 kilograms. Fill in your data table below: Water Mass Initial Temp. Hanging Mass Height Gravity Final Temp.Name: ______________________________ 2 1. As the hanging block mass increases how is the final temperature of the water changing? Explain what is happening. 2. As you increase block mass, what is happening to the potential energy of the block? Explain. Now explore the changes that occur due to changes in the height. Changing only the height, fill in the data table for this experiment. Water Mass Initial Temp. Hanging Mass Height Gravity Final Temp. 3. As the block height increases how is the final temperature of the water changing? Explain what is happening.Name: ______________________________ 3 4. As you increase block height, what is happening to the potential energy of the block? Explain. Now explore the changes that occur due to changes in the water mass. Changing only the water mass, fill in the data table for this experiment. Water Mass Initial Temp. Hanging Mass Height Gravity Final Temp. 5. As you increase water mass, does it take more or less energy to raise the water to a given temperature? Explain what is happening.Name: ______________________________ 4 Now explore the changes that occur due to changes in the initial water temperature. Changing only the initial water temperature, fill in the data table for this experiment. Water Mass Initial Temp. Hanging Mass Height Gravity Final Temp. 6. As you increase the initial water temperature, what happens to the energy in the system? Explain. 7. How does the increasing initial water temperature affect the final temperature? Explain.Name: ______________________________ 5 8. What conclusions can you draw from the experiments conducted in this laboratory exercise, regarding heat, conversion of energy and the conditions of the experiment? Experiment #2 – Carnot Cycle You will find this experiment under the "heat" tab of the program's menu. After clicking the Carnot Cycle link within the Heat tab menu you will get to the experimental setup. This experiment shows a Carnot cycle in action. You can choose the values of pressure and volume between which the cycle will operate, and a mono-atomic ideal gas in cylinder fitted with a piston is then taken clockwise around the cycle. A table shows the heat absorbed by the gas, the change in internal energy of the gas, and the work done by the gas for each branch, as well as the efficiency of the cycle. The Theory This experiment shows a Carnot cycle in action. The engine which performs this cycle (the Carnot Engine) represents an idealized heat engine in which heat is converted into work by means of a cyclic process. Taking P to be pressure and V to be volume of an ideal gas, the relationship between P and V for an isothermal process is PV = constant. The change in internal energy Eint of an ideal gas undergoing an isothermal process is zero. The change in internal energy of a mono-atomic ideal gas undergoing an adiabatic process from initial values PiVi to final values PfVf is Eint = (3/2)PfVf – (3/2)PiVi.Name: ______________________________ 6 A Carnot engine cycle is a clockwise thermodynamic cycle made up of two isothermal and two adiabatic processes. Equipped with the above information as well as the First Law of Thermodynamics you have Q = Eint + W Q is the heat exchange from/to the gas and W is the work done by the gas. You are then able to calculate Q, Eint, and W for each branch of the cycle, enabling you to calculate the efficiency of the cycle, which is equal to the total work done by the cycle divided by the total heat put into the gas. For the Carnot cycle, the efficiency of the cycle can more easily be found by c =1 – Tc/Th, where Tc and Th are the coldest and hottest temperatures of the gas during the cycle. The Experiment and Questions The green curves represent adiabatic processes and the red curves represent the isothermal processes. The loop on the PV diagram is controlled by moving the upper left and lower right corners of the cycle. The position of the upper left is independent of the position of the lower right. However, if one of these corners is fixed, there are limitations on where the other corner can be. 9. Why, if one corner of the PV diagram is fixed, are there limitations on where the other corner can be? Note that on the PV diagram, the area enclosed by the complete cycle is equal to the net amount of useful work extracted from the process.Name: ______________________________ 7 The efficiency of the cycle is calculated by taking the total work done by the cycle and dividing it by the heat added to the cycle, that is, all the positive values of Q added together. You can find out what Tc and Th are for the particular cycle by clicking on the upper left and lower right corners of the cycle. 10. Calculate the Carnot efficiency using the formula 1 – Tc/Th and compare it to the efficiency given in the program using W/Qh. How does your calculation compare to the efficiency provided in the program? Why do you think there may be some difference? 11. Change the upper left and lower right bounds of