# Charles Augustin de Coulomb and the Electrostatic Force

Charles-Augustin de Coulomb (1736 – 1806)

On June 14, 1736, French physicist Charles Augustin de Coulomb was born. He is best known for developing Coulomb’s law, the definition of the electrostatic force of attraction and repulsion, but also did important work on friction. The SI unit of electric charge, the coulomb, was named after him.

#### Engineering and Mechanics

Coloumb began to focus more and more in engineering and mechanics, which provided him with a firm foundation on which his later theoretical efforts were built. He submitted his first treatise to the Academy of Sciences in Paris in 1773, and many more would follow on topics ranging from mathematical solutions of engineering problems to studies of friction, elasticity, electricity and magnetism. While stationed in Cherbourg, Coulomb wrote a treatise on the compass that won him a prize from the Paris Academy in 1777. The treatise was also significant because it dealt with the torsion balance (and, in general, the torsion of thin threads), which would later become the basis of his experiments to measure even weak forces. After a visit to Rochefort in the late 1770s, Coloumb was promoted to a Captain and employed at La Rochelle, the Isle of Aix and Cherbourg. In this period, Coulomb discussed his experiments with electrostatic forces and explained the inverse square law that they led Coulomb to posit.

Coulomb electric balance, torsion balance

#### The Inverse Square Law

Similar to Isaac Newton’s inverse square law of gravitational force, Coulomb’s law states that the electric force between charged objects inversely depends upon the distance between the objects. That is, like gravity, the electric force acts in a line between two objects and decreases with the square of the distance between them. The primary difference between the law for gravity and that for electric force is that gravitation is influenced by the mass of the objects, whereas Coulomb’s law depends upon the charge of the objects involved. When the objects in question are both positively or both negatively charged, the forces between them are repulsive, but attractive forces arise between objects carrying opposing charges [1].

#### A Wooden Fortress

In 1779, he was sent to Rochefort to help the Marquis de Montalembert, who was pursuing new ideas in fortress construction that deviated from the classical line of Vauban,[9] to build a fort entirely of wood. At the Rochefort shipyards, he conducted experiments that led to his treatise Theorie des machines simples, in which he developed his theory of frictional forces. It earned him the Grand Prize of the Paris Academy of Sciences in 1781 and led to his admission to the Academy, which made Coulomb independent.

#### Later Years

After the outbreak of the French Revolution in 1789, Coulomb lost or resigned many of his posts. In 1791 he left the Corps de Genie. After the Academy of Sciences was dissolved in 1793, he retired to the countryside in Blois, where he owned a house and continued his experiments. It was not until 1795 that he returned to Paris with the founding of the Institut de France, which replaced the Academy, and his election as a member of the Institute. Napoleon brought him back into the civil service from 1802 to 1806 as supervisor of the country’s education system, promoting the establishment of new grammar schools (Lycée) throughout France. Charles-Augustin de Coulomb passed away on August 23, 1806.

Walter Lewin, 8.02x – Lect 1 – Electric Charges and Forces – Coulomb’s Law – Polarization, [11]