Petit, who was born at Vesoul in France, entered the Ecole Polytechnique in 1807. He spent a period teaching physics in Paris, and received a doctorate in 1811. He was one of the professors of physics at the Ecole Polytechnique. Petit did some research with his brother-in-law D. F. J. Arago on the variation of refractive index with temperature. However, he is known solely for his work with his colleague Pierre Dulong (1785–1836) in which they established the law (Dulong and Petit's law) that the specific heat of a solid multiplied by the atomic weight is (approximately) a constant for different solids.