Summary
4.1 Reversible and Irreversible Processes
- A reversible process is one in which both the system and its environment can return to exactly the states they were in by following the reverse path.
- An irreversible process is one in which the system and its environment cannot return together to exactly the states that they were in.
- The irreversibility of any natural process results from the second law of thermodynamics.
4.2 Heat Engines
- The work done by a heat engine is the difference between the heat absorbed from the hot reservoir and the heat discharged to the cold reservoir, that is,
- The ratio of the work done by the engine and the heat absorbed from the hot reservoir provides the efficiency of the engine, that is,
4.3 Refrigerators and Heat Pumps
- A refrigerator or a heat pump is a heat engine run in reverse.
- The focus of a refrigerator is on removing heat from the cold reservoir with a coefficient of performance
- The focus of a heat pump is on dumping heat to the hot reservoir with a coefficient of performance
4.4 Statements of the Second Law of Thermodynamics
- The Kelvin statement of the second law of thermodynamics: It is impossible to convert the heat from a single source into work without any other effect.
- The Kelvin statement and Clausius statement of the second law of thermodynamics are equivalent.
4.5 The Carnot Cycle
- The Carnot cycle is the most efficient engine for a reversible cycle designed between two reservoirs.
- The Carnot principle is another way of stating the second law of thermodynamics.
4.6 Entropy
- The change in entropy for a reversible process at constant temperature is equal to the heat divided by the temperature. The entropy change of a system under a reversible process is given by .
- A system’s change in entropy between two states is independent of the reversible thermodynamic path taken by the system when it makes a transition between the states.
4.7 Entropy on a Microscopic Scale
- Entropy can be related to how disordered a system is—the more it is disordered, the higher is its entropy. In any irreversible process, the universe becomes more disordered.
- According to the third law of thermodynamics, absolute zero temperature is unreachable.