Содержание
- 2. Irreversibility of processes There exist many processes that are irreversible: the net transfer of energy by
- 3. Heat Engines A heat engine is a device that takes in energy by heat and, operating
- 4. Thermal Efficiency of a Heat Engine
- 5. Heat Pumps or Refrigerators In a heat engine a fraction of heat from the hot reservoir
- 6. Refrigerator W – work done on the heat pump Qh – heat, put into the hot
- 7. Coefficient of performance of a refrigerator The effectiveness of a refrigerator is described in terms of
- 8. The Second Law of Thermodynamics The Kelvin form: It is impossible to construct a cyclic engine
- 9. The Second Law of Thermodynamics The Clausius form: It is impossible to construct a cyclic engine
- 10. Carnot cycle 1. A-B isothermal expansion B-C adiabatic expansion 3. C-D isothermal compression 4. D-A adiabatic
- 11. Carnot Efficiency Using the equation of state and the first law of thermodynamics we can easily
- 12. So, the work done on a gas during an isothermal process A → B is: (1)
- 13. For adiabatic processes: So, statement (3) gives us:
- 14. So, using the last expression and the expression for efficiency: Thus we have proved that the
- 15. Carnot theorem The Carnot engine is the most efficient engine possible that operates between any two
- 16. Carnot Theorem Proof Let’s prove it from the contrary: let’s have Carnot engine A to be
- 17. Entropy Measures the amount of disorder in thermal system. It is a function of state, and
- 18. Entropy change calculations Entropy is a state variable, the change in entropy during a process depends
- 19. So for infinitesimal changes: The subscript r on the quantity dQr means that the transferred energy
- 20. Change of Entropy in a Carnot Cycle Carnot engine operates between the temperatures Tc and Th.
- 21. Reversibility of Carno Cycle Using equality, proved for the Carnot Cycle (slide N13): We eventually find
- 22. Reversible Cycle Now consider a system taken through an arbitrary (non-Carnot) reversible cycle. Because entropy is
- 23. Ideal Gas Reversible Process Suppose that an ideal gas undergoes a quasi-static, reversible process from an
- 24. - This expression demonstrates that ΔS depends only on the initial and final states and is
- 25. The Second Law of Thermodynamics The total entropy of an isolated system that undergoes a change
- 26. Microscopic States Every macrostate can be realized by a number of microstates. Each molecule occupies some
- 27. Entropy on a Microscopic Scale Let’s have an ideal gas expanding from Vi to Vf. Then
- 28. After further transformations: n – number of moles, R=kbNa. Then we use the equation for isothermal
- 29. Entropy is a measure of Disorder The more microstates there are that correspond to a given
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