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In Physique, a law of conservation (nothing is lost, nothing is not created) expresses that a particular measurable property of a physical Système insulated remains constant during the evolution of this system. The list - nonexhaustive - enumerates laws of conservation which were never taken at fault:

Conservation of energy (mass including) in restricted Relativity;
conservation of the Momentum;
conservation of the Angular momentum;
Conservation of the electric charge;
Conservation of the load of color;
conservation of the Magnetic flux;
conservation of the baryon Number;
conservation of the lepton Number;
conservation of the product of symmetries $T, C, P$ .

Certain laws of conservation are only approximate, in the sense that they are slightly violated by certain interactions:

conservation of the isotopic spin (violated by the electromagnetic interaction and the weak interaction);
conservation of the parity of space $P$ (violated by the weak interaction);
conservation of the charge conjugation $C$ (violated by the weak interaction);
conservation of the savor of the leptons (violated by the weak interaction);
conservation of the savor of the quarks (violated by the weak interaction);
conservation of the produced $CP$ (violation by the weak interaction, highlighted on only one phenomenon).

On a purely historical basis, one can recall the law of

Conservation of the mass

who was at the base of chemistry and physics modern, law which was improved by the taking into account of the relativistic effects and the energy of mass, and for this reason, included in the law of conservation of energy.

The theorem of Noether expresses the equivalence which exists between the laws of conservation and the invariance of the physical laws with regard to certain transformations (typically called symmetry S ). This theorem applies only to the describable systems by a Lagrangian . There exists a similar theorem for the Hamiltonian Mécanique. For example, invariance by translation in time implies that energy is preserved, invariance by translation in space implies that the momentum is preserved, and invariance by rotation in space implies that the angular momentum is preserved.

Some remarks on these concepts

One speaks improperly about laws of conservation whereas it would be more precise to say “principle of conservation”. A law is an established relation. Example: the relation of Einstein between mass and energy.

A principle is a presumedly true result quite simply because any experiment forever shown that this result is false. Historically, several times, the conservation of energy seemed to be put at fault… and that led to discovered new forms of energies. It is the non-observance of the conservation of the momentum and energy at the time of the radioactive decay known as '' beta '' which in 1930 a particle called Neutrino supposed to carry the missing momentum and should have been waited until 1956 resulted in imagining to detect in experiments this particle which had neither load, neither mass, nor sufficient interaction to allow to detect it, so much so that it is told that its inventor W. Pauli would have stated “to have made a crime against physics by inventing a particle which one will be able to never detect”. Its existence was justified at the beginning only to perennialize the “laws of conservation of the momentum and energy”, but it gave place then to a theoretical formulation by E. Fermi of the weak interaction, which gave place to a whole whole of quantitative explanations.

Conversely, certain laws of conservation held for acquired were successively controuvées by the experiment: conservation of P and of C simultaneously, then, of the years afterwards, conservation of their product.

The concept of conservation is relatively simple to include/understand: If one puts in a volume something and that one closes the box well, one expects, when it later on is reopened, to find there what one put at it. This in physics is called a principle of conservation. If one does not find exactly what one put at it, it is that a part could enter or to leave in a form or another or whereas what misses (or is moreover) changed form and that one did not realize from there: for example, total energy is preserved, but none of its components, energies chemical, kinetic, electric, thermal, etc is preserved in general independently of the others.

This principle is so strong in physics that with each time it appeared not to be checked, that led to important discoveries: the Radioactivité has a time interpreted as the réémission of something which was received outside, and the final explanation came from equivalence mass-energy.

Energy in a volume of office is thus preserved, by principle, and if it decreases in volume, it is that a part left there… or that it was transformed into another form of energy.

See too

Antoine Lavoisier

Simple: Conservation law

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