ELECTRIC CURRENT AND CHARGE BASIC AND TUTORIALS

The earliest accounts of electricity date from about 2,500 years ago, when it was discovered that static charge on a piece of amber was capable of attracting very light objects, such as feathers. The word itself—electricit —originated about 60  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, however. Following the work of Alessandro Volta and his invention of the copper-zinc battery, it was determined that static electricity and the current that flows in metal wires connected to a battery are due to the same fundamental mechanism: 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:

qe = −1.602 × 10−19 C


As you can see, the amount of charge associated with an electron is rather small. This, of course, has to do with the size of the unit we use to measure charge, the coulomb (C), named after Charles Coulomb.

However, 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 positive sign, and the same magnitude.

The charge of a proton is

qp = +1.602 × 10−19 C


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; however, there are a number of cases we shall explore later in this book where the current-carrying material is not a conducting wire.


If we consider the effect of the enormous number of elementary charges actually
flowing, we can write this relationship in differential form:
i = dq/dt x C/s

The units of current are called amperes (A), where 1 ampere=1 coulomb/second. The name of the unit is a tribute to the French scientist Andr´e Marie Ampere.

The electrical engineering convention states that the positive direction of current flow is that of positive charges. In metallic conductors, however, current is carried by negative charges; these charges are the free electrons in the conduction band, which are only weakly attracted to the atomic structure in metallic elements and are therefore easily displaced in the presence of electric fields.