# Charles-Augustin Coulomb

#### 1736 - 1806

Charles-Augustin de Coulomb, 1736 - 1806, French physicist, whose name is the unit for electrical charge.

• Nationality
• French

• Subject
• Physics

• Fields
• electrostatics, friction, geotechnical engineering

• Posts
• He was one of the first members of the French National Institute.

Inspector of Public Instruction, 1802.

• Publications
• Recherches théoriques et expérimentales sur la force de torsion et sur l'élasticité des fils de metal, 1784 (Theoretical research and experimentation on torsion and the elasticity of metal wire).

Premier Mémoire sur l’Électricité et le Magnétisme, 1785, a paper which describes his torsional balance, for measuring the electric force between two charged balls.

Second Mémoire sur l’Électricité et le Magnétisme, 1785, which describes the "determination according to which laws both the Magnetic and the Electric fluids act, either by repulsion or by attraction", and lays down his inverse square relationship.

Troisième Mémoire sur l’Électricité et le Magnétisme, 1785, which describes the loss of charge of a body over time, and the influence of humidity.

Coulomb published another 4 'Mémoires' on electricity and magnetism, on 'electrical fluid' properties and magnetism.

• Laws
• Coulomb's Law

A coulomb (C) is the S.I. derived unit for electrical charge = \$6.241⋅10^{18}\$ electrons.

• Theories
• Coulomb discovered and described by equations the force between electric charges in terms of the square of their separating distance, and the same for magnetic poles. This relationship is now known as Coulomb's Law.

Torsional force on a wire.

• Equations
• \$F = k{Q_1⋅Q_2}/{r^2}\$

The force that exists between two charged particles is equal to the product of the two charges, \$Q_1\$ and \$Q_2\$, divided by the square of the distance, r, between them, multiplied by k, Coulomb's constant: \$k = 1/{4πε_0} = 8.99 × 10^9 N m^2 C^{-2}\$

where \$ε_0 = 8.55 × 10^{-12} N^{-1} m^{-2} C^2\$, the permittivity of free space.

• Experiments/Discoveries
• Torsional force: using his own design for a torsional balance, Coulomb discovered that the moment of the torque is, for wires of the same metal, proportional to the torsional angle, the fourth power of the diameter and the inverse of the length of the wire.

Coulomb was an engineer and experimentalist, and made significant discoveries concerning electricity and magnetism. André-Marie Ampère was inspired by Coulomb in his discoveries of the laws of electrodynamics.

Torsional balance, Charles-Augustin de Coulomb

He designed his own apparatus, most famous of which is the torsional balance, which provides unprecedented accuracy in the measurement of forces exerted by charged balls on each other.

Coulomb's work is commemorated in the S.I. derived unit for charge:

A coulomb is defined as the number of electrons which pass a point in a circuit per second needed to create 1 ampere of current:

\$\$1 C = 1 A⋅s\$\$

The fundamental S.I. unit for current, ampere is therefore one coulomb of charge per second:

\$\$1 A = 1 C/s\$\$

A coulomb is also the charge on a capacitor with capacitance of one farad charged to one volt.

\$\$1 C = 1 F⋅ 1 V \$\$

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