Coulomb

The coulomb is the standard unit of electric charge. One coulomb is defined as the amount of charge carried by one ampere of current in one second. One coulomb can also be defined as the amount of charge needed to create one volt of potential difference in a one farad capacitor. The symbol for the coulomb is an uppercase C.

History of the Coulomb

The coulomb was defined at an international conference in 1881, and is named after Charles-Augustin de Coulomb (1736-1806), a French physicist. He discovered Coulomb’s law which defines electrostatic force. The ampere was originally derived from the coulomb, until it was changed to a base unit itself.

Measuring Coulombs

Electric charge is measured with an electrometer which is a device that measures charge directly, unlike an electroscope which only measures relative charge. Older electrometers used valves but modern versions are solid-state, using field effect transistors that change their output based on the strength of the surrounding electric field.

Other Units

Electric charge density is expressed as the number of coulombs per cubic meter. Exposure to radiation, particularly x-rays and gamma rays, is often expressed as the coulombs of charge absorbed per kilogram of matter.

The coulomb is a large value, one ampere-hour of current transfers only 3600 coulombs of charge, so common values are measured in millicoulombs (one thousandth), nanocoulombs (one millionth), and picocoulombs (one billionth). Other units of electric charge that are sometimes used are the abcoluomb and the statcoulomb.