# Electric charge

In physics, the electric charge is a property of the elementary particles that constitute matter, which manifests itself (even macroscopically) with electric forces. The unit of electric charge is the Coulomb (abbreviated C). The rate of flow of electric charge is called electric current.

The charges can exert both attractive and repulsive forces between them. Depending on the forces that can be exerted between the charges, there are two types of charges, conventionally called positive or negative. The Coulomb’s law allows to evaluate the force exerted between two point charges in dependence on the distance at which they are placed; this law also establishes that between two charges of the same type the forces are repulsive, while between two charges of different types they are attractive.

Electric charge is a conserved property; the net charge of an isolated system, the amount of positive charge minus the amount of negative charge, cannot change.

All the electric charges observed are multiple integers of a charge (which value is determined through very accurate measurements) is called elementary. The convention which assigns the sign to the charge is based on giving the electron and proton (which are two of the elementary constituents of matter and precisely carry elementary charges of a different type), respectively, negative charge and positive charge.

A single isolated charge can be called an “electric monopole.” Same positive and negative charges placed close to each other constitute an electric dipole. Two oppositely directed dipoles close to each other are called an electric quadrupole. It is possible to continue this process to any number of poles, but dipoles and quadrupoles are mentioned here because they find essential application in physical phenomena.

An object with an absence of net charge is referred to as neutral.

In the elementary particle physics, a physical principle called conservation of charge (contained in Maxwell’s Equations) is fundamental, never contradicted by experience, for which the algebraic sum of the charges possessed by interacting particles remains constant before and after the reaction.

## Electric charge density

The term electric charge is synonymous with the amount of charge. Physical quantities: linear charge density, surface charge density, volume charge density, are defined as the amount of charge distributed respectively on the unit of length, the surface, the volume of the body under examination. If this distribution is not uniform, the corresponding definitions must be given as a function of the body points, as limits of the ratio between the quantity of charge distributed on a specific geometric element and the measure of its surface, to tend to zero of the dimensions of the element itself.

### References

1. Images from Wikipedia: https://en.wikipedia.org/wiki/File:VFPt_plus_thumb.svg and https://commons.wikimedia.org/wiki/File:VFPt_minus_thumb.svg