Engine engineering

What is thermodynamics?

What is thermodynamics?

Thermodynamics is the part of physics that is responsible for the relationship between heat and work. Study the effects of the variation of pressure, temperature and volume of a physical system (we understand by physical system a liquid, a material, a set of bodies, etc.), at a macroscopic level, that is, it can be observed.

The thermodynamic name comes from the thermo root, which means heat and dynamics that refers to movement. The movement of heat in a body.

Matter is composed of different particles that move in a disorderly manner (what is called entropy). Thermodynamics studies this messy movement.

Thermodynamics is of great importance that describes a large number of physical phenomena, including the operation of thermal motors.

What is thermodynamics for?

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

The study of thermodynamics is of great importance in thermal engines. The diesel cycle, the Otto cycle or the steam engine or the Stirling engine have a close relationship with thermodynamics and its laws because there is a heat exchange in all of them.

The results of thermodynamics are essential for other fields of physics and chemistry, chemical engineering, aerospace engineering, mechanical engineering, cell biology, biomedical engineering, and materials science to name a few.

What are the laws of thermodynamics?

The three laws of thermodynamics are:

  1. First law of thermodynamics: when energy passes, as work, as heat or with matter, inside or outside a system, the internal energy of the system changes according to the law of conservation of energy.
  2. Second law of thermodynamics: In a natural thermodynamic process, the sum of the entropies of the interacting thermodynamic systems increases.
  3. Third law of thermodynamics: the entropy of a system approaches a constant value as the temperature approaches absolute zero.

In addition, a "zero law" is conventionally added, which defines the thermal equilibrium:

Zero law of thermodynamics: if two systems are in thermal equilibrium with a third system, they are in thermal equilibrium with each other. This law helps define the concept of temperature.

What law says that energy is not created or destroyed, only transformed?

The law of conservation of energy. This law is a consequence of the first law of thermodynamics. The law of conservation of energy establishes that the total amount of energy in an isolated system remains constant at all times. A direct consequence of this is that energy cannot be created or destroyed, but it can only be converted from one form to another.

An example of the application of the law of conservation of energy is the conversion of energy from chemical energy to kinetic energy. Another consequence of this law is that a perpetual mobile device can only continue to move on its own if it does not provide power to its environment, in practice that is not feasible.

According to Albert Einstein's special theory of relativity, mass is a form of energy, and energy and mass can be fused. Therefore, there is little distinction between particles of matter and particles of energy, since they can change each other by, among other things, annihilation. However, this does not affect the total amount of energy in a closed system. Therefore, the Energy Conservation Law seeks to preserve the total amount of energy in an isolated system, including resting mass energy and all other forms of energy (kinetic, chemical, thermal, etc.).

Study of thermodynamics

The main elements we have to study thermodynamics are:

  • The laws of thermodynamics. These laws of thermodynamics define the way in which energy can be exchanged between physical systems in the form of heat or work. There are 4 laws of thermodynamics that are listed starting with the zero law of thermodynamics.
  • Entropy Entropy defines the disorder in which the internal particles that make up matter move. Entropy is a magnitude that can be defined for any system. 

In thermodynamics the interactions between different systems are studied and classified. This study leads to define concepts such as thermodynamic system and its contour. A thermodynamic system is characterized by its properties, related to each other through state equations. These can be combined to express internal energy and thermodynamic potentials, useful for determining the equilibrium conditions between systems and spontaneous processes.

With these tools, thermodynamics describes how systems respond to changes in their environment.

Historical evolution of thermodynamics

In the beginning, technological developments, such as so-called steam machines or thermometers, were carried out empirically. But it wasn't until the 19th century when scientists like Carnot and Joule formalized their results and determined the theoretical causes of their operation.

Initially, thermodynamics began to be studied in order to increase the efficiency of the first steam engines.

Publication Date: August 30, 2017
Last Revision: February 28, 2020