Conservation of Energy

Conservation Of Energy

The law of conservation of energy, first formulated in the nineteenth century, 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 twentieth century, the definition of energy was broadened. It was found that particles that have rest mass are equivalent to amounts of energy (see mass-energy equivalence). There particles were found subject to annihilation in which matter particles (such as electrons) can be converted to non-matter (such as photons of electromagnetic radiation), or even into potential energy or kinetic energy. Matter could also be created out of kinetic or other types of energy, in the process of matter creation. Thus, matter (defined as ponderable matter particles) was found not to be conserved.

In such a transformation process within an isolated system, neither the mass nor the energy changes over time, although the matter content may change. Therefore, conservation of energy, and conservation of mass, each still holds as a law in its own right (indeed they are restatements of the same law, when mass and energy are recognized to be equivalent). When stated alternatively, in terms of mass and of energy, they appear as the apparently distinct laws of the nineteenth century.

A consequence of the law of conservation of energy is that no intended "perpetual motion machine" can perpetually deliver energy to its surroundings. Any delivery of energy by such a device would result in delivery of mass also, and the machine would lose mass continually until it eventually disappeared.