Alexis Thérèse Petit (2 October 1791, Vesoul, Haute-Saône – 21 June 1820 in Paris) was a French physicist.
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, may have stimulated Carnot in his reflexions on heat engines and thermodynamic efficiency.
Petit was born in Vesoul, Haute-Saône. 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 of Paris. 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 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 entitled "Mémoire sur les variations que le pouvoir réfringent d’une même substance éprouve par l’effet gradué de la chaleur".
Petit first collaborated with Pierre Louis Dulong for the competition of the Académie des sciences about refrigeration (1815). Petit is now probably best known for the surprising Dulong-Petit law concerning the specific heat capacity of metals, which both men formulated together in 1819. Petit also designed a special thermometer (using weights) to determine the thermal dilatation coefficients of several metals.
Petit died from tuberculosis at the age of 29, shortly after the passing of his wife. He was succeeded by Dulong as professor of physics at the Polytechnique (1820).