Conservation of energy  

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Train wreck at Montparnasse (October 22, 1895) by Studio Lévy and Sons.
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Train wreck at Montparnasse (October 22, 1895) by Studio Lévy and Sons.

The nineteenth century law of conservation of energy is a law of physics. It states that the total amount of energy in an isolated system remains constant over time. The total energy is said to be conserved over time. For an isolated system, this law means that energy can change its location within the system, and that it can change form within the system, for instance chemical energy can become kinetic energy, but that energy can be neither created nor destroyed. In the nineteenth century, mass and energy were considered as being of quite different natures.

Since Albert Einstein's theory of special relativity showed that energy has an equivalent mass (see mass in special relativity), and mass has an equivalent energy, one speaks of a law of conservation of mass-energy as an updated version of the nineteenth century law. All particles, both ponderable (such as atoms) and imponderable (such as photons), respectively have both mass equivalents and energy equivalents. The difference between ponderable and imponderable particles is that ponderable particles cannot ever be accelerated to move at lightspeed, while imponderable particles always move at lightspeed (at least as to their phase velocity; they can exist as standing waves in a cavity with extremely reflective walls).

The total mass and the total energy of a system may both be respectively defined in special relativity, but for each, its conservation law holds. Particles, both ponderable and imponderable, are subject to interconversions of form, in both creation and annihilation. Nevertheless, in an isolated system, conservation of total energy and conservation of total mass each holds as a separate law.

A consequence of the law of conservation of energy is that no intended "perpetual motion machine" can perpetually deliver energy to its surroundings.


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Unless indicated otherwise, the text in this article is either based on Wikipedia article "Conservation of energy" or another language Wikipedia page thereof used under the terms of the GNU Free Documentation License; or on original research by Jahsonic and friends. See Art and Popular Culture's copyright notice.

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