THERMODYNAMICS

1. THERMODYNAMICS Understanding P vs V curves is crucial to understanding the processes of heat engines. The four types of processes are: Isobaric Isovolumetric Isothermal Adiabatic

2. Isobaric Change Constant pressure If volume increases, work is done by the gas (-W) If work is done on the gas (+W), volume decreases

3. PV CURVES If heat is added to a gas at constant pressure, volume must increase P = F/A F = PA Wout = Fd = PADy PDV = -W

4. PV CURVES PDV = - W The minus W indicates work done by the system. From PV = nRT� �if P is constant and volume increases, temperature must be increasing. For work to be done, heat must be added to the gas

5. Isovolumetric Change Constant volume If internal energy goes up, pressure goes up If internal energy goes down, pressure goes down

6. Cyclic Processes and Energy Conservation Combining the isobaric and isovolumetric processes results in a cycle illustration how heat can be converted into work.

7. Cyclic Processes and Energy Conservation Step 1 from A to B Heat the gas (+Q) at constant pressure Volume increases Gas does work (-WAB) Work is represented by the area under the curve: DWAB. PV = nRT

8. Cyclic Processes and Energy Conservation Step 2 from B to C Cool the gas (-Q) at constant volume Pressure decreases Gas does no work (WBC = 0) Notice, there is no area under the curve! PV = nRT

9. Cyclic Processes and Energy Conservation Step 3 from C to D Cool the gas (-Q) at constant pressure Volume decreases Work is done on the gas (positive work) The blue area under the curve WCD positive. PV = nRT

10. Cyclic Processes and Energy Conservation Step 4 from D to A Heat the gas (+Q) at constant Volume Pressure increases No Work is done, why? PV = nRT

11. Cyclic Processes and Energy Conservation THE TOTAL CYCLE Net work by the system = WAB-WCD Work = area enclosed by A to B to C to D to A The net heat added to the system: DQ = QAB - QBC - QDC + QDA

12. THE TOTAL CYCLE DQ = QAB - QBC - QCD + QDA DU = 0 because the gas returns to its original state, so� from DU = Q + W DQ = -W Qin � Qout = -W

13. Isothermal Change Constant temperature Internal energy remains the same If volume increases, work is done by the gas If work is done on the gas, volume decreases.

14. The Isothermal Process Isothermal expansion or compression ADD or TAKE AWAY HEAT at constant temperature. From PV = nRT, constant temp = constant PV This is a reversible process

15. The Isothermal Process U, the internal energy of the system depends directly on temperature. The higher the temperature, the higher the internal energy. How can the internal energy of a gas be changed?

16. The First Law of Thermodynamics The first law of thermodynamics states DU = DQ + W Internal energy can be changed by adding heat or doing work on the gas.

17. The Isothermal Process DU = DQ + W For Isothermal processes DU = 0 DQ = -W If heat goes in (+DQ), work must come out (-W) If work goes in (+W), heat must come out of the gas (-DQ)

18. Adiabatic Change No heat flows in or out Pressure changes Volume changes Temperature changes

19. The Adiabatic Process Adiabatic means without heat flow No heat is gained or lost by the system, DQ = 0 How are work and internal energy related?

20. The Adiabatic Process Work done on the system raises the gas temperature +W = DU Work done by the system lowers the gas temperature -W = DU

21. The Adiabatic Process Although it looks similar, this curve is different from the isothermal because temperature is not constant.

22. Heat Engines Click on the following link to visit the web site - Hyperphysics.

23. Heat Engine Efficiency

24. Heat Engines

25. Second Law of Thermodynamics The second law of thermodynamics is a profound principle of nature which affects the way energy can be used. There are several approaches to stating this principle qualitatively. Here are some approaches to giving the basic sense of the principle. 1. Heat will not flow spontaneously from a cold object to a hot object. 2. Any system which is free of external influences becomes more disordered with time. This disorder can be expressed in terms of the quantity called entropy. 3. You cannot create a heat engine which extracts heat and converts it all to useful work. 4. There is a thermal bottleneck which constrains devices which convert stored energy to heat and then use the heat to accomplish work. For a given mechanical efficiency of the devices, a machine which includes the conversion to heat as one of the steps will be inherently less efficient than one which is purely mechanical.