Chapter 15 The Laws of Thermodynamics

1. Chapter 15 The Laws of Thermodynamics

2. Heat and work Thermodynamic cycle

3. Heat and work • Work is done by the system: • Work is done on the system :

4. The first law of thermodynamics • Work and heat are path-dependent quantities • Quantity Q + W = ΔEint(change of internal energy) is path-independent • 1st law of thermodynamics: the internal energy of a system increases if heat is added to the system or work is done on the system

5. The first law of thermodynamics • Adiabatic process: no heat transfer between the system and the environment • Isochoric (constant volume) process • Free expansion: • Cyclical process:

6. Work done by an ideal gas at constant temperature • Isothermal process – a process at a constant temperature • Work (isothermal expansion)

7. Work done by an ideal gas at constant volume and constant pressure • Isochoric process – a process at a constant volume • Isobaric process – a process at a constant pressure

8. Molar specific heat at constant volume • Heat related to temperature change: • Internal energy change:

9. Molar specific heat at constant pressure • Heat related to temperature change: • Internal energy change:

10. Free expansion of an ideal gas

11. Time direction • Irreversible processes – processes that cannot be reversed by means of small changes in their environment

12. Entropy • Entropy, loosely defined, is a measure of disorder in the system • Entropy is related to another fundamental concept – information. Alternative definition of irreversible processes – processes involving erasure of information • Entropy cannot noticeably decrease in isolated systems • Entropy has a tendencyto increase in open systems

13. Entropy in open systems • In open systems entropy can decrease: • Chemical reactions

14. Entropy in open systems • In open systems entropy can decrease: • Chemical reactions • Molecular self-assembly

15. Entropy in open systems • In open systems entropy can decrease: • Chemical reactions • Molecular self-assembly • Creation of information

16. Entropy in thermodynamics • In thermodynamics, entropy for open systems is • For isothermal process, the change in entropy: • For adiabatic process, the change in entropy:

17. The second law of thermodynamics • In closed systems, the entropy increases for irreversible processes and remains constant for reversible processes • In real (not idealized) closed systems the processes are always irreversible to some extent because of friction, turbulence, etc. • Most real systems are open since it is difficult to create a perfect insulation

18. Nicolas Léonard Sadi Carnot (1796–1832) Engines • In an ideal engine, all processes are reversible and no wasteful energy transfers occur due to friction, turbulence, etc. • Carnot engine:

19. Carnot engine (continued) • Carnot engine on the p-V diagram: • Carnot engine on the T-S diagram:

20. Engine efficiency • Efficiency of an engine (ε): • For Carnot engine:

21. Perfect engine • Perfect engine: • For a perfect Carnot engine: • No perfect engine is possible in which a heat from a thermal reservoir will be completely converted to work

22. Gasoline engine • Another example of an efficient engine is a gasoline engine:

23. Heat pumps (refrigerators) • In an ideal refrigerator, all processes are reversible and no wasteful energy transfers occur due to friction, turbulence, etc. • Performance of a refrigerator (K): • For Carnot refrigerator :

24. Perfect refrigerator • Perfect refrigerator: • For a perfect Carnot refrigerator: • No perfect refrigerator is possible in which a heat from a thermal reservoir with a lower temperature will be completely transferred to a thermal reservoir with a higher temperature

25. Questions?