Alexis Thérèse Petit (; 2 October 1791,

Vesoul Vesoul () is a commune in the Haute-Saône department in the region of Bourgogne-Franche-Comté located in eastern France. It is the most populated municipality of the department with inhabitants in 2014. The same year, the Communauté d'agglo ...

Haute-Saône Haute-Saône (; Arpitan: ''Hiôta-Sona''; English: Upper Saône) is a department in the Bourgogne-Franche-Comté region of northeastern France. Named after the river Saône, it had a population of 235,313 in 2019.Paris Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. S ...

) was a French

physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate cau ...

. Petit is known for his work on the efficiencies of air- and steam-engines, published in 1818 (''Mémoire sur l’emploi du principe des forces vives dans le calcul des machines''). His well-known discussions with the French physicist Sadi Carnot, founder of

thermodynamics Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of th ...

, may have stimulated Carnot in his reflexions on

heat engine In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state ...

s and

thermodynamic efficiency In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc. For a ...

. The

Dulong–Petit law The Dulong–Petit law, a thermodynamic law proposed by French physicists Pierre Louis Dulong and Alexis Thérèse Petit, states that the classical expression for the molar specific heat capacity of certain chemical elements is constant for tempe ...

(1819) is named after him and his collaborator

Pierre Louis Dulong Pierre Louis Dulong FRS FRSE (; ; 12 February 1785 – 19 July 1838) was a French physicist and chemist. He is remembered today largely for the law of Dulong and Petit, although he was much-lauded by his contemporaries for his studies into ...

Biography

Petit was born in

Haute-Saône Haute-Saône (; Arpitan: ''Hiôta-Sona''; English: Upper Saône) is a department in the Bourgogne-Franche-Comté region of northeastern France. Named after the river Saône, it had a population of 235,313 in 2019.École Polytechnique École may refer to: * an elementary school in the French educational stages normally followed by secondary education establishments (collège and lycée) * École (river), a tributary of the Seine flowing in région Île-de-France * École, Savoi ...

Paris Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. S ...

. He was then placed in a preparatory school where he actually served as a "''répétiteur"'' to help his own classmates digest the course material. He duly entered Polytechnique at the lowest permissible age, in 1807, and graduated "''hors-rang"'' in 1809 (which is to say that he clearly outranked all of his classmates). After graduation, Petit stayed at Polytechnique as a faculty member, first as ''répétiteur'' in analysis and mechanics (1809) then in physics (1810). He taught for some time at Lycée Bonaparte. At Polytechnique, he served as a substitute (1814) for Jean Henri Hassenfratz whom he would replace in 1815. He thus became the second professor of physics at Polytechnique and the youngest person ever to hold that position, at the age of 23. Petit and François Arago were brothers-in-law because they married two sisters. In 1814, the two men collaborated on a paper together. Petit first collaborated with

for the competition of the

Académie des sciences The French Academy of Sciences (French: ''Académie des sciences'') is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French scientific research. It was at th ...

about refrigeration (1815). Petit is now probably best known for the surprising

concerning the

specific heat capacity In thermodynamics, the specific heat capacity (symbol ) of a substance is the heat capacity of a sample of the substance divided by the mass of the sample, also sometimes referred to as massic heat capacity. Informally, it is the amount of heat t ...

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

s, which both men formulated together in 1819 and which

Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...

explained almost a century later. Petit also designed a special thermometer (using weights) to determine the thermal dilatation coefficients of several metals. Petit died from

tuberculosis Tuberculosis (TB) is an infectious disease usually caused by '' Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can also affect other parts of the body. Most infections show no symptoms, in ...

at the age of 28, shortly after the passing of his wife. He was succeeded by Dulong as professor of physics at the Polytechnique (1820).

Jules Jamin Jules Célestin Jamin (31 May 1818, Termes, Ardennes – 12 February 1886) was a French physicist. He was professor of physics at École Polytechnique from 1852 to 1881 and received the Rumford Medal in 1858 for his work on light. He improved Br ...

, a contemporary and fellow physicist provides biographical and temperament details:

Biography

References

Further reading