The word electricity originated about 600 B.C.; it comes from elektron, which was the ancient Greek word for amber. The true nature of electricity was not understood until much later.

the atomic structure of matter, consisting of a nucleus—neutrons and protons—surrounded by electrons. The fundamental electric quantity is charge, and the smallest amount of charge that exists is the charge carried by an electron, equal to

the amount of charge associated with an electron is rather small.This, has to do with the size of the unit we use to measure charge, the coulomb (C).

the definition of the coulomb leads to an appropriate unit when we define electric current, since current consists of the flow of very large numbers of charge particles.

The other charge-carrying particle in an atom, the proton, is assigned a plus sign and the same magnitude. The charge of a proton is

Electrons and protons are often referred to as elementary charges.

Electric current is defined as the time rate of change of charge passing through a predetermined area. Typically, this area is the cross-sectional area of a metal wire

The resulting current i is given by :

where we imagine q units of charge flowing through the cross-sectional area A in t units of time.

The units of current are called amperes, where 1 ampere (A) = 1 coulomb/second (C/s).