Wednesday, November 26, 2008



Energy is a fundamental concept of physics, with applications throughout the natural sciences. Everything in the known universe is composed fundamentally of energy.

Conversavation of Energy

Energy is subject to a strict global conservation law; that is, it can neither be created nor destroyed. Most kinds of energy (with gravitational energy being a notable exception) are also subject to strict local conservation laws, as well. In this case, energy can only be exchanged between adjacent regions of space, and all observers agree as to the volumetric density of energy in any given space. There is also a global law of conservation of energy, stating that the total energy of the universe cannot change; this is a corollary of the local law, but not vice versa. Conservation of energy is associated with the symmetry of the laws of physics, namely invariance with respect to time (via Noether's theorem).

Total Energy of a System

The total energy of a system can be subdivided and classified in various ways. For example, it is sometimes convenient to distinguish kinetic energy from potential energy. It may also be convenient to distinguish gravitational energy, electrical energy, thermal energy, and other forms. These classifications overlap; for instance thermal energy usually consists partly of kinetic and partly of potential energy.

Transfer of Energy

The transfer of energy can take various forms; familiar examples include work, heat flow, and advection. The word "energy" is also used outside of physics in many ways, which can lead to ambiguity and inconsistency. The vernacular terminology is not consistent with technical terminology. For example, the important public-service announcement, "Please conserve energy" uses vernacular notions of "conservation" and "energy" which make sense in their own context but are incompatible with the technical notions of "conservation" and "energy" that are used in the law of conservation of energy.

Special Relativity

In classical physics energy is considered a scalar quantity, having no direction in space. In special relativity energy is not a Lorentz scalar, but rather one component of the energy-momentum 4-vector, such that energy is associated with the timelike direction. In other words, energy is invariant with respect to spacelike rotations, but not invariant with respect to boosts.

Astronomy and cosmology

The phenomona of stars, nova, supernova, quasars and gamma ray bursts are the universe's highest-output energy transformations of matter. All stellar phenomena (including solar activity) are driven by various kinds of energy transformations. Energy in such transformations is either from:
  1. Gravitational collapse of matter, usually molecular hydrogen, into various classes of astronomical objects (stars, black holes, etc.)

  2. or from nuclear fusion of lighter elements, in this case, primarily hydrogen.

Dark Energy

Dark energy is believed to make up 70% of the universe. Light elements, primarily hydrogen and helium, were created in the Big Bang. These light elements were spread too fast and too thinly in the Big Bang process through nucleosynthesis to form the most stable medium-sized atomic nuclei, like iron and nickel. This fact allows for later energy release, as such intermediate-sized elements are formed in our era. The formation of such atoms powers the steady energy-releasing reactions in stars, and also contributes to sudden energy releases, such as in novae.

Black Holes

Gravitational collapse of matter into black holes is also thought to power the very most energetic processes, generally seen at the centers of galaxies. Some believe black holes lead to an alternate web of universes as mass entering the black holes accelerates beyond the boundaries of Relativistic Physics. This is a moment where the universe contracts to an infinitesimal point, relatively speaking and then expands equally and oppositely in a completely different dimension of space and time.

Cosmologists are still unable to explain all cosmological phenomena purely on the basis of known conventional forms of energy, for example those related to the accelerating expansion of the universe. Dark Energy is necessary to describe certain cosmological observations, particularly regarding black holes, where light travels faster than the speed of light.


Energy comes from the Greek ενέργεια, where εν- means "in" and έργον "work". The compound εν-εργεια in Epic Greek meant "divine action" or "magical operation"; it was later used by Aristotle with the meaning of "activity, operation" or "vigour", and by Diodorus Siculus for "force of an engine."

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