# Electric Charge

If an atom contains its usual number of electrons, the atom or groups of similar atoms display no electrical properties. Such an atom or atoms are termed electrically neutral. If electrons are lost or captured by the atoms, they become electrically charged. Positively or negatively charged atoms or combinations of atoms are called ions (from the Greek ion: to wander).

Long before electrons were discovered, it was known in the field of static electricity that there were two types of electric charge known as positive and negative. Rubbing a hard rubber rod with wool, for example, causes electrons to be transferred from the wool to the rubber, giving the latter a negative charge while the lost electrons from the wool cause it to be positively charged.

The smallest possible charge that there can be (elementary charge e) is the charge on one electron. This charge is regarded as negative as a result of some arbitrary decisions made during the history of man's study of electricity.

 Electrons carry a negative elementary charge of e = -1.602·10-19 C.

Where the "C" is the unit of charge the coulomb, named after the French physicist. In formulae the charge is very often represented by the letter "Q". One coulomb of charge comes about as the result of 1/e = 6.25·1018 electrons.

Charged objects are attracted or repelled by forces that depend on the nature of the charges.

 Objects with the same charge repel one another while objects with differing charges attract each other.

The magnitude of the force F depends on the magnitude of the two charges and on the distance between the objects. For two point charges Q1 and Q2, separated by a distance r the following relationship, Coulomb's law, applies. The force thus decreases with the square of the separation of the two charges. The value e0 = 8.85·10-12 As/Vm is a constant called the absolute permittivity of free space (vacuum) and er is called the relative permittivity or dielectric constant, the value of which depends on the material in the space separating the charges. If either of the charges has a charge of 0, no force is apparent. The direction of the force (attracting or repelling) depends on the polarity of the charges.

The following animation illustrates the relationship. Give the two balls various charges by dragging a charge onto either ball with the mouse and see what happens. 