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Does energy ever disappear?

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April 26, 2018 10:03AM

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In physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves. Any particular conservation law is a mathematical identity to certain symmetry of a physical system. A partial listing of conservation laws that are said to be exact laws, or more precisely have never been shown to be violated:

The law of conservation of energy states that the total amount of energy in an isolated system remains constant. A consequence of this law is that energy cannot be created or destroyed. The only thing that can happen with energy in an isolated system is that it can change form, for instance kinetic energy can become thermal energy. Because energy is associated with mass in the Einstein's theory of relativity, the conservation of energy also implies the conservation of mass in isolated systems (that is, the mass of a system cannot change, so long as energy is not permitted to enter or leave the system).

5 months ago

Another consequence of this law is that perpetual motion machines can only work perpetually if they deliver no energy to their surroundings. If such machines produce more energy than is put into them, they must lose mass and thus eventually disappear over perpetual time, and are therefore impossible.

The law of conservation of mass/matter, also known as law of mass/matter conservation says that the mass of a closed system will remain constant, regardless of the processes acting inside the system. A similar statement is that mass cannot be created/destroyed, although it may be rearranged in space, and changed into different types of particles. This implies that for any chemical process in a closed system, the mass of the reactants must equal the mass of the products. This is also the central idea behind the first law of thermodynamics.

5 months ago

The law of "matter" conservation (in the sense of conservation of particles) may be considered as an approximate physical law that holds only in the classical sense, before the advent of special relativity and quantum mechanics. Another difficulty with conservation of "matter" is that "matter" is not a well-defined word in science, and when particles which all consider to be "matter" (such as electrons and positrons) are annihilated to make photons (which are often not considered matter) then conservation of matter does not hold, even in closed systems.

Mass is also not generally conserved in open systems, when various forms of energy are allowed into, or out of, the system (see for example, binding energy). However, the law of mass conservation for closed systems, as viewed over time from any single inertial frame, continues to hold in modern physics.The reason for this is that relativistic equations show that even massless particles such as photons still add mass to closed system.

5 months ago

Therefore, it allows mass (though not matter) to be conserved in all processes where energy does not escape the system.

The historical concept of both matter and mass conservation is widely used in many fields such as chemistry, mechanics, and fluid dynamics. In modern physics, only mass conservation for closed systems continues to hold exactly.

The conservation of relativistic mass implies the viewpoint of a single observer (or the view from a single inertial frame) since changing inertial frames may result in a change of the total energy (relativistic energy) for systems, and this quantity determines the relativistic mass.

The principle that the mass of a system of particles must be equal to the sum of their rest masses, even though true in classical physics, may be false in special relativity. The reason that rest masses cannot be simply added is that this does not take into account other forms of energy, such as kinetic and potential energy.