3: The Second Law of Thermodynamics
- Page ID
- 32017
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- 3.1: Carnot Cycle
- A thermodynamic engine operates by taking in heat from a hot reservoir and performing certain work and then giving up a certain amount of heat into a colder reservoir. If it can be operated in reverse, it can function as a refrigerator. The Carnot cycle is a reversible cyclic process. However, this is an idealized engine, no real engine can be perfectly reversible. The utility of the Carnot engine is to give the framework and logic of the arguments related to the second law of thermodynamics.
- 3.2: The Second Law
- The second law of thermodynamics is a statement of what we know by direct experience. It is not something that is derived from more fundamental principles, even though a better understanding of this law has emerged over time. There are several ways to state the second law, the most common ones being the Kelvin statement and the Clausius statement.
- 3.3: Consequences of the Second Law
- Once we take the second law as an axiom of thermodynamics, there are some important and immediate consequences. The first result is about the efficiency of the Carnot cycle, captured as the following theorem.
- 3.4: Absolute Temperature and Entropy
- Another consequence of the second law is the existence of an absolute temperature. Although we have used the notion of absolute temperature, it was not proven. Now we can show this just from the laws of thermodynamics.
- 3.5: Some Other Thermodynamic Engines
- The automobile engine operates in four steps, with the injection of the fuel-air mixture into the cylinder. It undergoes compression which can be idealized as being adiabatic. The ignition then raises the pressure to a high value with almost no change of volume. The high pressure mixture rapidly expands, which is again almost adiabatic. This is the power stroke driving the piston down. The final step is the exhaust when the spent fuel is removed from the cylinder.