Summary of Thermodynamics: Thermal Machines

Relevance of the Theme

Thermodynamics: Thermal Machines is a central pillar of Physics. This theme encapsulates the processes that drive our modern civilization, from generating electrical energy in coal, gas, and nuclear power plants to propelling motor vehicles. It is the science of transformations that occur between heat and mechanical work. Mastering this subject is essential to understand the complexity and applicability of physical principles in the real world.

Contextualization

Thermal Machines are the backbone of Thermodynamics. They give a tangible expression to the laws of Thermodynamics, setting in motion the heat that flows from a hot system to a cold system. The study of Thermal Machines permeates 2nd-year High School Physics and extends across the academic spectrum, finding applications even in Quantum Physics and the Theory of Relativity.

Theoretical Development

Components:

• Thermal Machines: Devices designed to convert heat into mechanical work, which powers operations such as generating electricity, propelling vehicles, and even household appliances. Their operation is based on two or more thermal reservoirs (a hot source and a cold one) and is governed by thermodynamic principles.

• Thermodynamic Cycles: Sequences of processes that a thermal machine completes to convert heat into work. The most common cycles are Carnot, Stirling, Otto, and Diesel, each with its own peculiarities and applications.

• Efficiency: It is the ratio between the work done by the machine (that is, the supplied energy minus the wasted energy) and the amount of heat absorbed by it. Due to the Law of Conservation of Energy, the efficiency of thermal machines is never 100%.

• Laws of Thermodynamics: The First Law (Law of Conservation of Energy) establishes that the change in internal energy of a system is equal to the heat supplied minus the work done. The Second Law (Entropy Law) imposes limits on the efficiency of thermal machines, suggesting that, in an isolated system, entropy (i.e., disorder) always increases.

Key Terms:

• Heat: Form of energy transferred from one body to another due to a temperature difference.
• Entropy: Measure of the disorder or randomness of a system.
• Thermal Reservoir: Bodies that have a large thermal capacity and, therefore, do not undergo significant temperature changes when transferring or receiving heat.

Examples and Cases:

• Thermal Power Plant: Electric energy is generated through a thermal machine in thermal power plants. In this case, heat is generated by burning fossil fuel (coal, natural gas, oil) and then used to generate steam. The high-pressure steam moves a turbine, which in turn drives a power generator.
• Automobile Engine: The engine of a car, whether gasoline (Otto cycle) or diesel (Diesel cycle), is a thermal machine that converts the chemical energy of the fuel into mechanical work. The heat generated by the combustion of the fuel inside each cylinder is what performs the work.
• Refrigerator: Although contrary to the principle of converting heat into work, the operation of a refrigerator or air conditioner is controlled by a thermal machine that extracts heat from the air or food and transfers it to the external environment, using electrical work.

Detailed Summary

Relevant Points:

• Importance of Thermal Machines: Thermal Machines are vital to our society because they enable the transformation of heat into mechanical work. They are present in various aspects of our daily lives, from energy generation to vehicle propulsion.

• Thermodynamic Cycles: The entire process of a thermal machine, from heat absorption to mechanical work, occurs through thermodynamic cycles. Each cycle has its own characteristics, aimed at specific applications.

• Efficiency of Thermal Machines: The efficiency of a thermal machine is a crucial aspect. It is determined as the ratio between the work done and the heat absorbed. Due to the Law of Conservation of Energy, efficiency can never be 100%, and is limited by the Second Law of Thermodynamics.

• Laws of Thermodynamics: The Laws of Thermodynamics are the basis for the study of thermal machines. The First Law relates the change in internal energy of the system to the heat supplied and the work done. The Second Law, in turn, establishes the impossibility of a heat process occurring from the cold source to the hot source without the application of work.

Conclusions:

• Transformative Role of Thermal Machines: Thermal machines are essential for the transformation of heat into mechanical work, and this is one of the main applications of thermodynamic laws in our world.

• Thermodynamic Limitations: The Second Law of Thermodynamics defines a fundamental limitation for thermal machines: the impossibility of completely converting heat into mechanical work. This translates into the limited efficiency of these machines.

• Diversity of Applications: Thermal machines can be conceived and operate in different ways, each with its own characteristics and practical applications. This allows them to be used in numerous situations, becoming a fundamental pillar of modern technology.

Exercises:

1. Describe the operation of a thermal power plant from the perspective of thermal machines and thermodynamic cycles.

2. Explain the operating principle of a gasoline car engine, identifying the adopted thermodynamic cycle and discussing the engine's efficiency.

3. Differentiate the First and Second Laws of Thermodynamics in terms of Thermal Machines, and explain how these laws influence the operation of such machines.