Energy is the capacity of a body or system to change over time: whoever has more energy changes faster. It is measured in joules (J), and this unit bears the name of one of the most tenacious experimenters of the nineteenth century. The story of how it came to be understood that heat and mechanical work are two faces of the same quantity — energy — is a decisive episode in modern physics.
Joule, the brewer who discovered energy
James Prescott Joule (1818–1889) was not an academic, but the son of a wealthy Manchester brewer. His obsession with measurement precision, developed while controlling the temperature of beer, led him to build a famous apparatus: a system of rotating paddles driven by falling weights stirred water in an insulated container. By carefully measuring the heating of the water, Joule showed that a fixed amount of mechanical work always produces the same amount of heat: (Simonyi 2012, chap. 4).
His first report was met with complete indifference, except from one enthusiastic young man, William Thomson, the future Lord Kelvin, who, though sceptical, sensed the importance of the discovery (Simonyi 2012, p. 377).
Independently, the German physician Julius Robert Mayer (1814–1878) had reached the same conclusion already in 1842: he had noticed that the venous blood of sailors in the tropics was a brighter red, a sign that the body burned less food to keep warm. His paper was, however, rejected by the Annalen der Physik for being too philosophical in style (Simonyi 2012, pp. 376–377).
Joule’s measurement, a fixed conversion factor between calorie and joule, is what we call the mechanical equivalent of heat. Its deep meaning is that heat is not a substance in its own right (the old “caloric”), but a form of energy: the mechanical work spent by the falling weights does not disappear, it is found in full as the heating of the water. It is the first building block of the principle of conservation of energy.
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Topics: Lavoro ed energia Concepts: Energia termica da attrito · Conservazione dell’energia meccanica Skills: Bilancio energetico a bolle
Related exercises: Worked exercise — block on an inclined plane with spring and friction · Problem — Vertical projectile with drag · Problem — Conservation of energy with friction