Today In Science History – October 11 – James Prescott Joule

James Joule

James Joule (1818-1889) English physicist
Credit: Henry Roscoe, 1906

October 11 marks the passing of James Prescott Joule. Joule was an English physicist who helped formulate the first law of thermodynamics.

He showed heat energy and mechanical work were related and could be converted from one form to another. He also showed the total energy of a system is conserved. This lead to the first law of thermodynamics which states: The total energy of an isolated system remains constant. Energy can be converted from one form to another, but energy cannot be either created or destroyed.

Joule’s experimental results showing the relationship between mechanical energy and heat were initially criticized by other scientists of the time. The prevailing theory of the nature of heat prior to Joule’s work was known as the caloric theory. Caloric was a substance that transported heat. When a hot object touches a cold object, caloric is transferred from the hot object to the cold object until they reach equilibrium. Some theorists held ‘cold’ was the lack of caloric and others believed there was a second substance known as frigoric. In any case, the total amount of caloric in a closed system was believed to be constant. Joule’s results showed if there was a set source of caloric in a closed system, the increase in temperature of a system resulting from mechanical work violated that theory by creating caloric. Joule’s work was the beginning of the end of caloric theory. One holdout from this theory is the unit of energy known as the calorie.

Joule also showed electrical energy and heat energy were interchangeable by relating the heat dissipated by a resistor to the electrical current passing through it. The heat generated was proportional to the square of the current passing through the resistor and directly proportional to the resistance itself. This relationship is known today as Joule’s First Law.

Later in life, he worked closely with William Thomson (Lord Kelvin) to establish the absolute temperature scale. They also discovered the Joule-Kelvin effect where an expanding gas is cooled by the expansion.

Joule never attended a university and began many of his achievements from a home laboratory where he spent his time after his work running the family brewery. After the brewery was sold off, he spent the rest of his life pursuing his scientific hobby. He was a major contributor to the new scientific field of thermodynamics and was honored by naming the SI unit of energy, joule, after him.

About Todd Helmenstine

Todd Helmenstine is the physicist/mathematician who creates most of the images and PDF files found on Nearly all of the graphics are created in Adobe Illustrator, Fireworks and Photoshop. Todd also writes many of the example problems and general news articles found on the site.