Energy, engine of the world, appear everywhere: in the celestial phenomena (mechanics of planets, radiation of the Sun) as in the human body (muscular energy), in the vegetable organizations (photosynthesis) as in the products of technology (trains, rockets, bombs), in the infinitely small (particles of high energy) as in the infinitely great (explosion of the supernovas).

Omnipresent, energy is not however concretely nowhere, because its existence is detectable only by its effects.

According to one of the great principles which govern the Universe, it completely preserves all while being transformed into an infinite variety of forms.

The term “energy” recovers one very a large number of significances. A gifted body of energy is, according to the etymology of the word, a body likely to provide work, movement.

It is necessary to await the invention of the steam engine by Thomas Newcomen, in 1705, and her physical interpretation one century later, so that is understood that mechanical work and heat are two forms of energy, before discovering others well of them (electrical energy, radiant, nuclear, etc).