Transformation of energy

Basic Concepts of Thermodynamics

Basic concepts of thermodynamics

Thermodynamics is the branch of physics that studies the effects of changes in temperature, pressure and volume of a physical system (a material, a liquid, a set of bodies, etc.), at a macroscopic level. The root "thermos" means heat and dynamics refers to movement, so thermodynamics studies the movement of heat in a body. Matter is made up of different particles that move in a disorderly way. Thermodynamics studies this disordered motion.

The practical importance lies fundamentally in the diversity of physical phenomena that it describes. Consequently, knowledge of this diversity has led to enormous technological productivity.

What Does Thermodynamics Study?

In thermodynamics the interactions between various thermodynamic systems are studied and classified.

A thermodynamic system is characterized by its properties, related to each other through equations of state. These can be combined to express internal energy and thermodynamic potentials, useful for determining equilibrium conditions between systems, spontaneous processes and the exchange of  energy with their environment.

The main elements we have for your study are:

  • The laws of thermodynamics. These laws define how energy can be exchanged between physical systems in the form of heat or work.
  • Entropy. Entropy is a quantity that can be defined for any system. Specifically, entropy defines the disorder in which the internal particles that make up matter move, that is, the kinetic energy that the particles that make up a body have .
  • Enthalpy. Enthalpy is a state function of the considered physical system. Actually, the first law of thermodynamics, depending on the enthalpy, takes the form dQ = dH - Vdp, that is, the amount of heat supplied to a system is used to increase the enthalpy and do external work - Vdp.

With these tools, thermodynamics describes how systems respond to changes in their environment. Often this study applies to gases. In this case, the ideal gases are often discussed.

What Is an Ideal Gas?

An ideal gas is a theoretical gas composed of a set of point particles with random displacement, which do not interact with each other, that is, the particles that compose it do not  exchange energy.

The ideal gas concept is useful because it behaves according to the ideal gas law, a simplified equation of state, which can be analyzed using statistical mechanics.

Under normal conditions such as normal pressure and temperature conditions, most real gases behave qualitatively as an ideal gas.

What Are the Laws of Thermodynamics?

The principles of thermodynamics were enunciated during the 19th century, which regulate thermodynamic transformations, their progress, their limits. Actually, they are real axioms based on the experience on which the whole theory is based.

Specifically, three basic principles can be distinguished, plus a "zero" principle that defines temperature and that is implicit in the other three.

Zero Principle of Thermodynamics

The zero 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, they are in equilibrium with each other and the shared property is temperature.

First Thermodynamic Principle

The first law of thermodynamics states that when one body comes into contact with another relatively colder body, a transformation occurs that leads to a state of equilibrium in which the temperatures of the two bodies are equal.

The first principle is, therefore, a principle of conservation of energy. In each thermal machine, a certain amount of energy is transformed into work: there cannot be a machine that produces work without consuming energy.

In short, the first thermodynamic principle is traditionally stated as: The variation of the internal energy of a closed thermodynamic system is equal to the difference between the heat supplied to the system and the work done by the system in the environment.

Second Thermodynamic Principle

There are several statements of the second law of thermodynamics, all equivalent, and each of the formulations emphasizes a particular aspect. First, he states that "it is impossible to make a cyclical machine that has the only result of transferring heat from a cold body to a warm body" (Clausius statement). Therefore,  heat transfer always goes from the hot body to the cold body.

On the other hand, it can also be stated, equivalently, that "it is impossible to carry out a transformation whose result is only that of converting the heat taken from a single source into mechanical work" ( Kelvin's statement).

Third Principle of Thermodynamics

The third principle of the laws of thermodynamics is closely related to the latter, and in some cases is considered as a consequence of the latter. In this sense, it can be stated by saying that "it is impossible to reach absolute zero with a finite number of transformations" and provides a precise definition of the quantity called entropy.

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

In the study of thermodynamics different concepts appear that should be known:

What Is a Thermodynamic System?

A thermodynamic system refers to a limited area used for thermodynamic research, and is the subject of the research. The outer space of the thermodynamic system is called the environment of this system.

The boundaries of a system separate the system from its exterior. This limit can be real or imaginary, but the system must be limited to a limited space. The system and its environment can transfer matter, work, heat or other forms of energy at the limit.

What Is a Thermodynamic Cycle?

A thermodynamic cycle is a circuit of thermodynamic transformations carried out in one or more devices intended to obtain work from two sources of heat at different temperatures, or conversely, to produce, through the contribution of work, the passage of heat from the lower temperature source at higher temperature.

The objective of a thermodynamic cycle is to obtain work from two thermal sources at different temperatures, for example, in a solar thermal energy installation. The work obtained is generally used to produce movement or to generate electricity.

Performance is the main parameter that characterizes a thermodynamic cycle. The thermal performance of a thermodynamic cycle is defined as the work obtained divided by the heat expended in the process.

What Are the Thermodynamic Properties?

Thermodynamic properties are the properties that define and intervene in the thermodynamic state of a system. Thermodynamics is characterized by having a state of equilibrium in which pressure, volume, temperature and composition are present.

These properties can be classified as extensive or intensive. Among these properties we find internal energy, entropy, enthalpy, heat, temperature, pressure, volume, etc.

What Does Thermal Performance Mean?

The thermal performance or efficiency of a thermal machine is a dimensionless coefficient or ratio calculated as the quotient of the energy produced (in one operating cycle) and the energy supplied to the machine (so that it can complete the thermodynamic cycle). It is designated by the Greek letter η

Depending on the type of thermal machine, the transfer of these energies will be in the form of heat, Q, or work, W.

In 1824, French physicist Sadi Carnot derived thermal efficiency for an ideal thermal machine as a function of the temperature of its hot and cold reservoirs:



Th is the temperature of the hot reservoir;
Tc is the temperature of the cold reservoir.

In conclusion, the thermal performance equation states that higher levels of efficiency are obtained with a higher temperature gradient between hot and cold fluids. In practice, the hotter the fluid, the greater the efficiency of the engine.

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.

Here are some examples of some of their applications:

  • Feeding. The hot kitchen is a constant example of chemical transformations through thermodynamic processes.
  • Material science. In this case, thermal processes are used to obtain new types of materials that have well-defined chemical and physical properties.
  • Industrial applications. In the industrial world there are many processes that transform raw materials into finished products using machinery and energy. An example is the ceramic industry where long tunnel kilns fire bricks at temperatures above 800 degrees Celsius.
  • Architecture. In the field of construction, it is very important to take into account the thermal transfers between the exterior and interior of the home. In bioclimatic architecture, thermodynamics plays a fundamental role, which is why all aspects of passive solar energy are studied in detail.
  • Generation of electricity. In all thermal power plants (fossil fuels, nuclear energy or solar plants) these concepts are used to drive steam turbines and electric generators.

The thermodynamic study is of great importance in the case of thermal solar energy because this type of solar installation is based on heat exchange.

Ultimately, thermodynamic results 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 Is Thermodynamic Solar Energy?

This application of solar energy is a technological system that takes advantage of the difference between the temperature of the liquid in the solar panels (in this case thermodynamic panels) and the ambient temperature.

In thermodynamic solar energy, solar panels carry a coolant at a very low temperature. The coolant, in contact with room temperature, undergoes a thermodynamic process of heat exchange as long as the outside temperature is not lower than that of the coolant.

The advantage of this system is that thermal energy can also be generated at night, in adverse weather conditions, rain, wind, etc.


Published: June 15, 2016
Last review: April 18, 2020