1. 1 Heat Engines and Refregerators Readings: Chapter 19

2. 2 Heat Engine Heat engine is the device which transform heat into work. Heat transfer from hot to cold. How can we produce the work during this heat transfer? Hot reservoir.Cold reservoir.

3. 3 Example: Work during heat transfer. This is not enough. We want to do it many times. So the gas should be return the initial state.

4. 4 Example: Work during heat transfer. Heat engine: any CLOSED-CYCLE device that extract heat from hot reservoir, does useful work, and exhausts heat to a cold reservoir. To return (gas) system to the initial state we need to transfer some heat to the cold reservoir.

5. 5 Heat engine: Work during heat transfer. Work done by the heat engine (gas) is minus the work done by external forces.

6. 6 Heat engine: Ideal gas. Heat engine is determined by the pV (closed) cycle. Different cycles – different heat engines.

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9. 9 Heat engine: Thermal efficiency The efficiency of heat to work transfer is characterized by thermal efficiency, defined as

10. 10 Heat engine: Thermal efficiency

11. 11 Heat engine: Ideal gas. Thermal efficiency depends on the type of the cycle.

12. 12 Heat engine: Ideal gas: Thermal efficiency Thermal efficiency depends on the type of the cycle. To find thermal efficiency you need to Method A 1.find the work done by the system – It is equal to the area enclosed by the cycle on a pV diagram; 2.find the heat transferred from the hot reservoir (positive heat during elementary subprocess). Method B find the heat transferred during all elementary subbprocesses. Then 1.sum of all positive heat is the net heat transferred from the hot reservoir. 2.sum of all negative heat is the net heat transferred to cold reservoir. Elementary subprocess

13. 13 Example: Thermal efficiency

14. 14 Example: Thermal efficiency

15. 15 Thermal efficiency What type of process has the maximum thermal efficiency? Carnot Cycle: It consists of two adiabatic processes and two isothermal processes.

16. 16 Thermal efficiency For any type of heat engine Where is the temperature of cold reservoir (the lowest temperature of the gas during the cycle); is the temperature of the hot reservoir (the highest temperature of the gas during the cycle). The equality is only for Carnot cycle.

17. 17 Example: What is the maximum thermal efficiency if Where the temperature is in Kelvin: then

18. 18 Refrigerators: Heat transfer from cold reservoir to hot reservoir. Refrigerator is an any closed-cycle device that uses external work to remove heat from a cold reservoir and exhaust heat to a hot reservoir. is a positive heat transfer to the gas during the cycle, and is the (minus) negative heat transfer, is the work done by external forces (this work is positive). Coefficient of performance:

19. 19 Refrigerators: Coefficient of performance. For any refrigerator The equality is for Carnot cycle: Example: What is the maximum coefficient of performance if