Electric charge

The electric charge is a type of physical property of matter, scalar and signed, responsible for one of the fundamental interactions of matter, the electromagnetic interaction, and the source of the electromagnetic field. There are two types of electric charges: positive and negative (commonly carried by protons and electrons respectively). Like charges repel each other and unlike charges attract each other. An object with an absence of net charge is referred to as neutral.

Electric charge, like any other physical quantity, is measurable, that is it is possible to establish how much electric charge a charged body has according to a charge measurement unit. In the International System the unit of measure of electric charge is the coulomb (symbol C), from the name of the French physicist Charles Augustin Coulomb (1736-1806). The coulomb is defined from the electric current (constituted by the orderly movement of electric charges) and is equal to the charge that passes in 1 second through the section of a circuit crossed by a current of 1 ampere (A), the unit of measure of current.

Precise measurements of electrical charges were carried out by the French physicist Charles Coulomb in the 1780s using a device called a torsional balance measuring the force generated between two electrically charged objects. The results of Coulomb’s work led to the development of a unit of electrical charge named in his honor, the coulomb. If two “point” objects (hypothetical objects having no appreciable surface area) were equally charged to a measure of 1 coulomb, and placed 1 meter apart, they would generate a force of about 9 billion newtons, either attracting or repelling depending on the types of charges involved.

For the moment we are interested in defining the value of the smallest existing electric charge, that of electron, which is indicated with \(e\) and is worth:

\[e=1.6021\cdot 10^{-19}\;\textrm{C}\]

All the other electric charge, existing in nature or artificially produced, are multiple of the electron charge, that for this reason is also called elementary electric charge (the electric charge of electron therefore is extremely small and it is for this reason that has not been chosen as unit of measure of electric charge, the amount of charge produced by a lightning is for example 10 C, that would correspond to 1020 times the electron charge).

Charge conservation

Charge conservation is the principle that the total electric charge in an isolated system never changes. The net quantity of electric charge, the amount of positive charge minus the amount of negative charge in the universe, is always conserved. Charge conservation, considered as a physical conservation law, implies that the change in the amount of electric charge in any volume of space is exactly equal to the amount of charge flowing into the volume minus the amount of charge flowing out of the volume.

In analogy with what we have seen for mass and energy, there is also a law of conservation of electric charge: in an isolated physical system, the total amount of electric charge remains unchanged over time, even when electric charges are exchanged between the parts of the system (i.e. in an isolated system there is no creation of electric charges, but they pass from one body to another, preserving themselves in quantity). When we rub a glass rod with a wool cloth, the rod is positively charged, but the excess electrons pass on the cloth, which is negatively charged: the total electric charge of rod-cloth system is conserved, that is remains unchanged.

The law of conservation of charge must be valid also at atom level: therefore, when in an atomic or nuclear reaction there is production of a negatively charged particle, must be produced also a positive charge.

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