(Salford, United Kingdom, 1818 - leaves, id., 1889). British physicist, is due to the mechanical theory of heat, and in whose honor is the unit of energy in the international system called July.
James Prescott Joule
James Prescott Joule was born in a family dedicated to the manufacture of beer. From shy and humble character, received private lessons in his own home of physics and mathematics, being their teacher, the British chemist John Dalton; He combined these classes with his professional activity, working alongside his father at the distillery, which came to direct. Dalton encouraged him towards scientific research and conducted his first experiments in a laboratory close to the maker of beers, forming at the same time at the University of Manchester.
Joule studied aspects of magnetism, especially those relating to the magnetisation of iron by the action of electric currents, which led him to the invention of the electric motor. He also discovered the phenomenon of magnetostriction, appearing in ferromagnetic materials, in which its length depends on its state of magnetization.
James Prescott Joule
But more fruitful research of Joule is the relative to the various forms of energy: with his experiments verified that the flow of an electric current through a conductor, it experiences a temperature increase; starting from there deduced that if the electrical supply is an electrochemical battery, energy would come from the processing carried out by chemical reactions, which would turn it into electricity and this would be transformed into heat. If a new element is inserted in the circuit, electric motor, mechanical energy is originated. This takes you to the enunciation of the principle of conservation of energy, and although there were other renowned physicists who contributed to the establishment of this principle as Meyer, Thomson and Helmholtz, was Joule who provided him with greater strength.
In 1840, Joule published heat for photovoltaic electricity production, in which established the law that bears his name and which affirms that the heat from the passage of electric current in a conductor is proportional to the product of the resistance of the conductor by the square of the current. In 1843, after many experiments, obtained the numerical value of the mechanical equivalent of heat, which concluded that 0,424 was equal to one calorie, which allowed the conversion of mechanical and thermal units; This is very similar to the value currently like 0,427. Thus the relationship between heat and work, already advanced by Rumford, who served as a cornerstone for the further development of the statistical thermodynamics was firmly established. In these works, Joule was based on the law of conservation of energy, discovered in 1842.
While in 1848 had already published an article restrain theory kinetics of gases, where for the first time estimated the speed of the gas molecules, abandoned its line of research and chose to become Assistant to William Thomson (Lord Kelvin), and, as a result of this collaboration, was the discovery of the Joule-Thomson effect, according to which it is possible to cool a gas expansion if necessary to separate work takes place in the gas molecules. This then enabled the liquefaction of gases and led to the internal energy of a perfect gas law, according to which the internal energy of a perfect gas is independent of its volume and temperature dependent.