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

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?