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Energy is defined as a measurement of the ability to do work or to heat an object. Energy plays an essential role both in everyday events and in scientific phenomena (is one of the most quantitative property of physics in nature).

The SI unit of energy is the joule [J], which is the energy transferred to an object by the work of moving it a distance of 1 meter against a force of 1 newton.

\[1\;\textrm{J}=\dfrac{1\;\textrm{kg}\cdot \textrm{m}^2}{\textrm{s}^2}\]

The term energy comes from the late Latin energīa, in turn, derived from the Greek ἐνέργεια (enérgeia). The word is composed of “en” intensive particle, and “ergon” ability to act. The term was introduced by Aristotle in philosophy to distinguish the δύναμις (dýnamis) the “power” proper to the shapeless matter, from the real capacity (ἐνέργεια).

The term “energy” was used for the first time to indicate a physical quantity by Kepler in his Harmonice Mundi of 1619. However, the term “energy” was introduced systematically in scientific literature only since the late nineteenth century.

A precise definition of energy is not simple to provide; energy is not a concrete reality but rather an abstract mathematical concept that expresses a link between the possible processes and a temporal symmetry of physical laws. There is, therefore, no substance or fluid corresponding to pure energy. As Feynman wrote:

It is important to realize that in physics today, we have no knowledge of what energy is.

Richard Feynman, The Feynman Lectures on Physics, Vol I, p. 4-1.

Energy is an extensive physical quantity (the energy of two bodies is simply the sum of the energies of the bodies taken individually), which has a central importance in the formulation of many theories, from classical mechanics to thermodynamics, from the theory of relativity to quantum mechanics.

A body can increase or decrease its energy as a result of an interaction with other bodies: the variation of energy then reflects the changes that have occurred in its microscopic properties.

The law of conservation of energy

In physics, the law of conservation of energy is one of the most important conservation laws observed in nature. The conservation principle has guided the discovery of new forms of energy and has allowed us to discover new types of physical processes and even new particles.

The principle of conservation of energy reflects the temporal symmetry of the physical laws with respect to time translations; that is, that these do not change over time.

The law of conservation of energy states that the total energy of an isolated system remains constant, it is said to be conserved over time.

This law means that energy cannot be created or destroyed, but is merely changed from one form into another or transferred from one object to another at different stages. So we can conclude that in the entire system, the total energy remains the same, but only the transformation takes place.

For example the electricity available in an electric oven is converted to a thermal form which goes into the object in the oven.

At the beginning of the 20th century, some nuclear decays were discovered with the emission of electrons that did not seem to satisfy the principle of energy conservation. To solve the problem in 1924, Niels Bohr put forward the idea that at the atomic level energy was not strictly conserved, proposing a theory that turned out to be wrong. Wolfgang Pauli in 1930 and Enrico Fermi in 1934 postulated the existence of new interactions and a new particle never observed before, which was able to transport energy and which was missing in the experiments. In this way, guided by the principle of conservation of energy, they were able to discover the neutrino, a particle with no electric charge, actually observed experimentally in 1959.

Classically, conservation of energy was distinct from conservation of mass; however, special relativity showed that mass could be converted to energy and vice versa by \(E = mc^2\), and science now takes the view that mass-energy is conserved.

Types and forms of energy

Essentially the total energy of a system can be subdivided into potential (stored) energy or kinetic (working) energy, or combinations of the two in various ways. While these two categories are sufficient to describe all forms of energy, it is often convenient to refer to particular combinations of potential and kinetic energy as its form.

Energy sources

World energy resources are the estimated maximum capacity for energy production given all available resources on Earth. Energy sources can be categorized as renewable and non-renewable.

Renewable and nonrenewable energy sources can be used as primary energy sources to produce useful energy such as heat or used to produce secondary energy sources such as electricity.


See also