# What is Energy?

Energy is a scalar physical quantity. Energy is a unique measure of various forms of movement and interaction of matter, a measure of the transition of matter from one form to another. The introduction of the concept of energy is important because if the physical system is closed, then its energy is stored in this system during the time during which the system will be closed.

A closed system is a system that does not exchange energy with the outside. This statement is called the law of conservation of energy: energy is neither created nor destroyed, it can only be transformed. It is only possible to convert one type of energy into another type of energy. For example, nuclear energy in electrical energy.

Typically, energy is denoted by the symbol E. To indicate the amount of heat (the amount of energy transmitted by heat transfer), the symbol Q is generally used. To designate work as the amount of energy transferred, generally the symbol W is used. The symbol U is generally used to indicate the internal energy of the body (the origin of the symbol must be specified).

## Types of energy

Energy can be presented in various forms, which, by the principle of conservation of energy, can be transformed into each other. Therefore, we can talk about different types of energy more depending on their effects than their natural origin. In this sense, energy can be classified as mechanical, thermodynamic, electromagnetic and nuclear.

### Definition of mechanical energy

Mechanical energy is due to the geometric and dynamic variables of the system, from a macroscopic point of view, and is the one that responds to the simplest mathematical scheme.

The definition of mechanical energy in a material body of constant mass (m) is the sum of its kinetic energy and its potential energy. The kinetic energy is proportional to the speed of the body while the potential energy depends on the position of the body in the surrounding force field. Thus, in the case of the free fall, the position would be determined by the height and the force field by the force of gravity. The potential strain energy due to the elastic properties of a deformed body such as a compressed spring could also be considered.

### Definition of thermodynamic energy

The definition of thermodynamic energy makes sense from a molecular point of view. Taking into account the thermodynamic variables, we can define the internal energy U of a system as the sum of the kinetic energies of the molecules that compose it and the potential energy of the forces between them. Each of the atoms or molecules of a body or of a substance are in continuous movement, whether of rotation, translation or vibration, with respect to the equilibrium position, more or less intense depending on the temperature. From this point of view we can affirm that thermal energy is equivalent to the kinetic energy of molecules, as formulated by L. Boltzmann in his kinetic theory of gases.

Internal energy is not an absolute measureable quantity, but only the variations in energy between two states of the system (first principle of thermodynamics) are measured.

The name given to it depends on the manifestation of this energy or the nature of the phenomenon that generates it. We define

• bond energy or formation energy the difference between the energy of a molecule and that of the atoms that form it (bond),
• dissociation energy the energy released in the dissociation of a compound,
• activation energy the increase in energy required for a chemical reaction (activation energy) to occur and
• resonance energy the difference between the theoretical formation energy and the actual formation energy of a resonant compound (resonance).

Free energy relates the variation of internal energy (U) or enthalpy (H) with the variation of the entropy (S) of a system and serves to indicate in which direction the system will evolve spontaneously (affinity).

### Definition of electromagnetic energy

The definition of electromagnetic energy is energy derived from the electromagnetic nature of matter.

Electromagnetic energy manifests itself basically in two ways: transforming into kinetic energy of electrical charges that are in its zone of influence - which can be converted into heat (Joule effect) or mechanical energy (electric motors) - or propagating as radiant energy outside the medium where it has been generated in the form of electromagnetic waves - which can then be converted into light energy, etc. - or also, at atomic scale, emitting carrier particles of a certain amount of energy determined by the Planck equation (photoelectric effect).

### Definition of nuclear energy

The last definition of energy is nuclear energy. This type of energy keeps the components of the atomic nucleus very close together. The mass of an atomic nucleus is less than the sum of the masses of the elementary particles that form it (mass defect). This mass defect is due to the appearance of a cohesion energy absorbed by the nuclear structure and that can be calculated using Einstein's theory of relativity.

This energy can be released in the form of radiant energy and kinetic energy from the particles that are expelled from the nucleus.

## Renewable energy and non-renewable energy

There are also two definitions of energy that refer to the form of use. In this sense we can distinguish renewable energy and non-renewable energy. Usually we refer to these two definitions to talk about the generation of electrical energy but the use of these types of energy is not always to generate electricity.

Renewable energy is energy that comes from inexhaustible sources. Some examples of this type of energy are solar energy (photovoltaic and thermal), wind energy, hydraulic energy.

Non-renewable energy is energy that comes from limited sources. In non-renewable ones, the consumption of this type of energy is greater than its regeneration capacity. Some examples of this type of energy are fossil fuels (coal, oil and natural gas) and nuclear energy among others. In the case of nuclear energy, and more specifically with nuclear fission, although a large amount of energy can be obtained per unit of nuclear fuel, uranium is a limited natural resource that does not regenerate on its own.

## How is energy used? Uses and applications of energy

The use of energy as a source of work has always been an essential need for man and one of the fundamental factors of modern economic development and technical progress.

Until the beginning of the industrial revolution, except for isolated experiments, man used as energy sources, muscle strength, hydraulic energy and wind energy for the production of mechanical work, and vegetable fuels for obtaining heat. These were until then the only resources used to obtain energy, although in time these systems were perfected to achieve higher yields (more rational use of animals, improvement of sailing techniques, invention of the hydraulic wheel, etc.).

The principle of development for energy exploitation techniques was the appearance of the steam engine and the transformation of the hydraulic wheel into a turbine. Later, the construction of the first combustion engines gave the final push in obtaining mechanical energy from heat.

Today, the most used energy sources are waterfalls, fuels and nuclear fission, and, less importantly, solar, wind, geothermal and tidal energy.

The exploitation of hydraulic energy requires adequate topographic and hydrographic conditions and the construction of reservoirs and large-scale machinery in order to achieve acceptable energy yields (which can reach 80% or more of the potential energy of the Water). Generally always for the production of electrical energy.

The most important source of energy is constituted by fuels that have undergone exponential growth in recent years. The maximum performance is much lower than that obtained by hydraulic energy but its high concentration of energy explains the considerable development suffered. The heat obtained from the fuels is directly used or converted into mechanical energy (engines, turbines ...) and also to obtain electrical energy by electrochemical means (fuel cell).

The third most used type of energy is nuclear energy, which is generally obtained by fission of the nuclei of substances such as uranium, plutonium, thorium, etc ... The technology of nuclear fusion reactions is not yet sufficiently developed to allow industrial exploitation and is used only in experimental laboratories and military applications. The energy obtained in nuclear fission manifests itself in the form of heat, and reaches much higher temperatures than those obtained with conventional fuels, however, the yields that can currently be obtained are still very low (of the order of 30% ). Nuclear power has application basically in the production of electric power and in the propulsion of ships.

## History of the study of energy

The notion of energy appears for the first time from Joule and Carnot's research on the conversion of heat into mechanical work and thanks to Helmholtz's theory in which it links the work done by an isothermal system with internal energy and entropy. of this system. In this way, energy is not only linked to the concept of mechanical work, but also to electrical, chemical or calorific work. For this reason we can talk about electrical energy, chemical energy and heat energy.

At the same time it can be said that if a certain amount of work, or an equivalent part of this work, disappears in an isolated physical system, forming part of different forms of energy, the same amount of work must appear under other energy forms according to the definition of law of conservation of energy.

As a result of the investigations carried out by William Thomson and RJE Clausius in the middle of the s. XIX, the process of energy degradation became evident, that is, that the useful work that can be obtained in an isolated system not all energy is conserved but that there is a part of this energy that is degraded (entropy, second principle of thermodynamics). Later, and following the theory of A. Einstein on the equivalence between mass and energy (1905), the principle of conservation was extended, and currently there is no difference between the conservation of mass and the conservation of energy giving rise to The theory of relativity, conservation law.

In 1900, Max Planck gave the explanation of many phenomena hitherto unexplained by the theory of energy quanta (quantum mechanics), which was the contribution of a fundamental concept for the knowledge of energy.

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Last review: February 10, 2020