Alexis Thérèse Petit (; 2 October 1791 – 21 June 1820) 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 (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed b ...
, may have stimulated Carnot in his reflexions on
heat engine
A heat engine is a system that transfers thermal energy to do mechanical or electrical work. While originally conceived in the context of mechanical energy, the concept of the heat engine has been applied to various other kinds of energy, pa ...
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 he ...
. 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
Early life and studies
Petit was born in
Vesoul
Vesoul ( ) is a Communes of France, commune in the predominantly rural Haute-Saône department, of which it is the Prefectures in France, prefecture, or capital, in the Regions of France, region of Bourgogne-Franche-Comté located in eastern Franc ...
,
Haute-Saône
Haute-Saône (; Frainc-Comtou: ''Hâte-Saône''; 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. . At the age of 10, he proved that he was already capable of taking the difficult entrance exam to France's most prestigious scientific school of the time, the
École polytechnique
(, ; also known as Polytechnique or l'X ) is a ''grande école'' located in Palaiseau, France. It specializes in science and engineering and is a founding member of the Polytechnic Institute of Paris.
The school was founded in 1794 by mat ...
of
Paris
Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, largest city of France. With an estimated population of 2,048,472 residents in January 2025 in an area of more than , Paris is the List of ci ...
. 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 the 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).
Career
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
Dominique François Jean Arago (), known simply as François Arago (; Catalan: , ; 26 February 17862 October 1853), was a French mathematician, physicist, astronomer, freemason, supporter of the Carbonari revolutionaries and politician.
Early l ...
were brothers-in-law because they married two sisters. In 1814, the two men collaborated on a paper together.
Petit first collaborated with
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 ...
for the competition of the
French Academy of Sciences
The French Academy of 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 method, scientific research. It was at the forefron ...
about refrigeration (1815). Petit is now probably best known for the surprising
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 ...
concerning the
specific heat capacity
In thermodynamics, the specific heat capacity (symbol ) of a substance is the amount of heat that must be added to one unit of mass of the substance in order to cause an increase of one unit in temperature. It is also referred to as massic heat ...
of
metal
A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...
s, which both men formulated together in 1819 and which
Albert Einstein
Albert Einstein (14 March 187918 April 1955) was a German-born theoretical physicist who is best known for developing the theory of relativity. Einstein also made important contributions to quantum mechanics. His mass–energy equivalence f ...
explained almost a century later. Petit also designed a special thermometer (using weights) to determine the thermal dilatation coefficients of several metals.
Death
Petit died from
tuberculosis
Tuberculosis (TB), also known colloquially as the "white death", or historically as consumption, is a contagious disease usually caused by ''Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can al ...
at the age of 28, shortly after the passing of his wife. He was succeeded by Dulong as professor of physics at the Polytechnique in 1820.
Personality
Jules Jamin, a contemporary and fellow physicist provides biographical and temperament details of Petit and Dulong:
References
Further reading
*R Fox, Biography in Dictionary of Scientific Biography (New York 1970-1990).
*R Fox, The background to the discovery of Dulong and Petit's law, British J. His. Sci. 4 (1968–69), 1-22.
*J W van Spronsen, The history and prehistory of the law of Dulong and Petit as applied to the determination of atomic weights, Chymia 12 (1967), 157–169.
*
{{DEFAULTSORT:Petit, Alexis Therese
1791 births
1820 deaths
People from Vesoul
French physicists
Thermodynamicists
École Polytechnique alumni