# Thermodynamics in physics

Thermodynamics is the branch of physics that describes the effects of changes in temperature, pressure, and volume of a physical system at a macroscopic level. This branch of physics focuses on the study of energy transformations, particularly with regard to heat and work.

Matter is composed of different particles that move in a disorderly manner. According to the definition of thermodynamics, this science does not study the behavior of each particle but rather studies the global behavior of all these particles.

This science allows us to understand many natural and technological phenomena, from the efficiency of a machine to the functioning of the climate and metabolism in living beings.

## What does thermodynamics study?

Physical thermodynamics studies how energy is transformed and transferred in the form of heat and work between different systems. It focuses on understanding how these transfers affect matter, especially in terms of temperature, pressure, and volume. The fundamental principles of thermodynamics are organized into 4 fundamental laws.

This branch of physics studies how energy behaves and how it is converted from one form to another, particularly in relation to heat and work. It does this by analyzing the properties of physical systems and how variations in temperature, pressure, and volume influence these systems. In addition to the fundamental laws, thermodynamics is applied to various processes and technologies, such as the creation of heat engines, refrigerators, and heating systems.

## Laws of thermodynamics

The principles of thermodynamics regulate the transformations related to heat, its progress and its limits. In reality, they are real axioms based on experience on which the entire theory is based.

Specifically, three basic principles can be distinguished, plus a "zero" principle.

### Zeroth law

The Zeroth law of thermodynamics states that when two interacting systems are in thermal equilibrium, they share some properties, which can be measured by giving them a precise numerical value. Consequently, when two systems are in thermal equilibrium with a third system, they are in equilibrium with each other, and the shared property is temperature.

### The first law

The first law states that when two bodies at different temperatures are in contact, heat transfer occurs until a state of equilibrium is reached. In this new state, the temperatures of the two bodies are equal.

The first principle is the principle of conservation of energy which states that energy is neither created nor destroyed; it only transforms.

### The second law

The second law of thermodynamics states that heat always flows from a hot object to a cold object and that it is not possible to create a 100% efficient heat engine. This is known as the principle of the impossibility of a perpetual motion machine of the second kind.

Furthermore, the second law introduces the concept of entropy, which measures the amount of disorder in a system. Entropy always tends to increase in an isolated system.

### The third law

The third law states that it is impossible to reach absolute zero with a finite number of transformations and provides a precise definition of the magnitude called entropy.

Additionally, the third law also states that the entropy for a perfectly crystalline solid, at a temperature of 0 kelvin, is equal to 0.

## Thermodynamic systems

A thermodynamic system is a specific part of the universe that we choose to study and analyze from a thermodynamics point of view. This system is separated from its surroundings by boundaries, which may be real (such as the walls of a container) or imaginary (such as a border defined only for the purpose of the study).

The environment is everything that is outside the system and can interact with it.

System boundaries are crucial because they determine how the system can exchange energy and matter with its surroundings. For example, these boundaries may allow the passage of heat, work, or even matter, depending on the type of system we are analyzing.

## Thermodynamic cycles

A thermodynamic cycle is a series of processes by which a thermodynamic system passes from an initial state, through several state changes, and finally returns to the initial state. These cycles are performed in devices such as engines and refrigerators, with the purpose of converting thermal energy into useful work, or vice versa.

The main parameter evaluated in these cycles is the thermal efficiency. The thermal efficiency (η) is calculated as the work obtained divided by the heat supplied.

## Thermodynamic properties

Thermodynamic properties are the characteristics that describe the state and behavior of a thermodynamic system at any point in a cycle. These properties allow us to study how a system interacts with its environment and how it changes as energy in the form of heat or work is applied to it. These properties fall into two main categories: intensive and extensive:

1. Intensive properties: These do not depend on the amount of matter in the system. Examples of intensive properties include temperature which measures how hot or cold a system is, pressure which represents the force exerted by the system per unit area, and density which is the mass per unit volume.

2. Extensive properties: These depend on the amount of matter in the system. Examples of extensive properties include mass (amount of matter in the system), volume (space occupied by the system), or internal energy (the sum of all the energies of the molecules within the system.

Some properties, such as entropy and enthalpy, can be more abstract:

• Entropy: Measures the disorder or randomness of particles in a system. The greater the entropy, the greater the disorder.

• Enthalpy: It is the sum of the internal energy and the product of the pressure and volume of the system. It represents the total useful energy content in a system to perform work at constant pressure.

## Use and applications

Thermodynamics can be applied to a wide variety of science and engineering topics, such as engines, phase transitions, chemical reactions, transport phenomena, and even black holes.

Below we list some examples of some of its applications:

• In cooking food.
• In the automotive industry, most engines are thermal engines.
• In materials science, we obtain new types of materials that possess well-defined chemical and physical properties.
• Industrial applications to transform raw materials into finished products using machinery and energy.
• In architectural design, thermal transfers between the exterior and interior of the home are taken into account, especially in bioclimatic architecture and passive solar energy.
• Generation of electricity in thermal power plants where thermodynamic processes allow heat to be converted into electricity.
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Data de publicació: June 15, 2016
Última revisió: August 5, 2024