Energy [physics]

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 properties of physics in nature).

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

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