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# 4: The Second Law of Thermodynamics

• 4.0: Prelude to The Second Law of Thermodynamics
The second law of thermodynamics limits the use of energy within a source. Energy cannot arbitrarily pass from one object to another, just as we cannot transfer heat from a cold object to a hot one without doing any work. We cannot unmix cream from coffee without a chemical process that changes the physical characteristics of the system or its environment. We cannot use internal energy stored in the air to propel a car without disturbing something around that object.
• 4.1: Reversible and Irreversible Processes
A reversible process is a process in which the system and environment can be restored to exactly the same initial states that they were in before the process occurred, if we go backward along the path of the process. An irreversible process is what we encounter in reality almost all the time. The system and its environment cannot be restored to their original states at the same time.
• 4.2: Heat Engines
A heat engine is a device used to extract heat from a source and then convert it into mechanical work that is used for all sorts of applications. For example, a steam engine on an old-style train can produce the work needed for driving the train. Several questions emerge from the construction and application of heat engines. For example, what is the maximum percentage of the heat extracted that can be used to do work?
• 4.3: Refrigerators and Heat Pumps
The cycles we used to describe the engine in the preceding section are all reversible, so each sequence of steps can just as easily be performed in the opposite direction. In this case, the engine is known as a refrigerator or a heat pump, depending on what is the focus: the heat removed from the cold reservoir or the heat dumped to the hot reservoir. Either a refrigerator or a heat pump is an engine running in reverse.
• 4.4: Statements of the Second Law of Thermodynamics
The second law of thermodynamics can be stated in several different ways, and all of them can be shown to imply the others. In terms of heat engines, the second law of thermodynamics may be stated as follows: It is impossible to convert the heat from a single source into work without any other effect.
• 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 second law of thermodynamics is best expressed in terms of a change in the thermodynamic variable known as entropy, which is represented by the symbol S. Entropy, like internal energy, is a state function. This means that when a system makes a transition from one state into another, the change in entropy ΔS is independent of path and depends only on the thermodynamic variables of the two states.
• 4.7: Entropy on a Microscopic Scale
Entropy can be related to how disordered or randomized a system is—the more it is disordered, the higher is its entropy.
• 4.A: The Second Law of Thermodynamics (Answer)
• 4.E: The Second Law of Thermodynamics (Exercise)
• 4.S: The Second Law of Thermodynamics (Summary)

Thumbnail: The work done on the gas in one cycle of the Carnot refrigerator.