Energy

Energy is defined as a measure of the ability to do work or heat an object. Energy plays an essential role in everyday events as well as in scientific phenomena (it is one of the most quantitative physical properties in nature).

The term “energy” was introduced into philosophy by Aristotle to distinguish the “force” (δύναμις, dýnamis) proper to formless matter from the real capacity (Ancient Greek ἐνέργεια, enérgeia); the word is composed of “en” intensive particle and “ergon” capacity to act. The term “energy” was then first used by Kepler in his Harmonice Mundi of 1619 to denote a physical quantity. However, the term “energy” was not systematically introduced into scientific literature until the late nineteenth century.

A precise definition of energy is not easy to give; energy is not a concrete reality, but rather an abstract mathematical concept that expresses a link between possible processes and a temporal symmetry of physical laws. Therefore, there is no substance or fluid that corresponds to pure energy. As Feynman wrote: “It is important to realize that in physics today we do not know what energy is” [Richard Feynman, The Feynman Lectures on Physics, Vol. I, p. 4-1].

Energy is a broad physical quantity (the energy of two bodies is simply the sum of the energies of the bodies taken individually) that is central to the formulation of many theories, from classical mechanics to thermodynamics, from relativity to quantum mechanics.

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

Energy sources

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

A renewable resource is one that can be used repeatedly and replaced naturally (that can replenish itself at a rate similar to its use by humans). Renewable and non-renewable 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.

A non-renewable resource is a natural resource that is used up faster than it can be replenished by nature. It cannot be produced, grown, or generated on a scale that can sustain the rate at which it is consumed. Once used, it is not available for future use. Fossil fuels (such as coal, oil, and natural gas), types of nuclear power (uranium), and certain examples. Resources such as wood (if harvested sustainably) or metals (which can be recycled) are considered renewable resources. Non-renewable resources are also called exhaustible resources.

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.

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