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7.2: Objective 6.b.

Last updated

Apr 10, 2024
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- 7.1.8: Temperature and Heat (Summary)
- 7.2.1: Prelude to The First Law of Thermodynamics

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Energy is conserved in all processes, including those associated with thermodynamic systems. The roles of heat transfer and internal energy change vary from process to process and affect how work is done by the system in that process. We will see that the first law of thermodynamics explains that a change in the internal energy of a system comes from changes in heat or work. Understanding the laws that govern thermodynamic processes and the relationship between the system and its surroundings is therefore paramount in gaining scientific knowledge of energy and energy consumption.

7.2.1: Prelude to The First Law of Thermodynamics
In a car engine, heat is produced when the burned fuel is chemically transformed into mostly CO₂ and H₂O, which are gases at the combustion temperature. These gases exert a force on a piston through a displacement, doing work and converting the piston’s kinetic energy into a variety of other forms—into the car’s kinetic energy; into electrical energy to run the spark plugs, radio, and lights; and back into stored energy in the car’s battery.
7.2.2: Thermodynamic Systems
A thermodynamic system includes anything whose thermodynamic properties are of interest. It is embedded in its surroundings or environment; it can exchange heat with, and do work on, its environment through a boundary, which is the imagined wall that separates the system and the environment. In reality, the immediate surroundings of the system are interacting with it directly and therefore have a much stronger influence on its behavior and properties.
7.2.3: Work, Heat, and Internal Energy
Positive (negative) work is done by a thermodynamic system when it expands (contracts) under an external pressure. Heat is the energy transferred between two objects (or two parts of a system) because of a temperature difference. Internal energy of a thermodynamic system is its total mechanical energy.
7.2.4: First Law of Thermodynamics
Now that we have seen how to calculate internal energy, heat, and work done for a thermodynamic system undergoing change during some process, we can see how these quantities interact to affect the amount of change that can occur. This interaction is given by the first law of thermodynamics, which argues you cannot get more energy out of a system than you put into it. We will see in this chapter how internal energy, heat, and work all play a role in the first law of thermodynamics.
7.2.5: Thermodynamic Processes
The thermal behavior of a system is described in terms of thermodynamic variables. For an ideal gas, these variables are pressure, volume, temperature, and number of molecules or moles of the gas. For systems in thermodynamic equilibrium, the thermodynamic variables are related by an equation of state. A heat reservoir is so large that when it exchanges heat with other systems, its temperature does not change.
7.2.6: Heat Capacities of an Ideal Gas
We learned about specific heat and molar heat capacity previously; however, we have not considered a process in which heat is added. We do that in this section. First, we examine a process where the system has a constant volume, then contrast it with a system at constant pressure and show how their specific heats are related.
7.2.7: Adiabatic Processes for an Ideal Gas
When an ideal gas is compressed adiabatically, work is done on it and its temperature increases; in an adiabatic expansion, the gas does work and its temperature drops. Adiabatic compressions actually occur in the cylinders of a car, where the compressions of the gas-air mixture take place so quickly that there is no time for the mixture to exchange heat with its environment.
7.2.8: The First Law of Thermodynamics (Summary)
7.2.9: 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.
7.2.10: 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.
7.2.11: 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.
7.2.12: 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?
7.2.13: 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.
7.2.14: 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.
7.2.15: 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.
7.2.16: 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.
7.2.17: The Second Law of Thermodynamics (Summary)

Thumbnail: Different thermodynamic paths taken by a system in going from state A to state B. For all transitions, the change in the internal energy of the system $ΔE_{int}=Q−W$ is the same.

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