2.2 Law of conservation and conversion of energy

The law of conservation and conversion of energy is the fundamental general law of nature. This law states that

Energy does not vanish nor does it appear anew, it only passes from one form of energy into another in various physical and chemical processes.

In other words, for any isolated system (i.e. a thermodynamic system which exchanges no heat, work, or substance with the surroundings) the amount of energy in the system remains constant.

The law of conservation of energy was known long ago in mechanics, as applied to mechanical (kinetic and potential) energy. After the equivalency of work and heat had been elaborated by the works of Joule and other scientists, the law of conservation of energy was spread to cover other kinds of energy and, in accordance with its content, it became known as the law of conservation and conversion of energy.

At the present time, different kinds of energy are known, the energy of thermal motion of microparticles of a body, the kinetic energy of the body as a whole, the gravitational field energy (in particular, the potential energy of a body lifted above the ground), magnetic field energy, the energy of electromagnetic radiation, nuclear energy, etc. The law of conservation and conversion of energy relates unambiguously all kinds of energy in the process of their mutual conversion. It is also referred to as the first law of thermodynamics. It should be noted once more that the first law of thermodynamics is of a general nature.