ELECTRICAL CONDUCTIVITY

                                   ELECTRICAL CONDUCTIVITY

Electrical conductivity is a basic property of material. Due to this property one material can conduct electricity. Some materials are good conductor of electricity that means electrical current can pass through them very easily; again some materials do not allow electrical current to flow through them. The material through which current easily, called good conductor of electricity in other words, the electrical conductivity of these materials is high. On the other hand the materials do not allow the electrical current to flow through them are called insulators. There are some materials whose electrical conductivity is not as high as conductor and also not as poor as insulator, they have an intermediate conductivity and these type of materials are known as semiconductors.

In an atom, electrons revolve in the orbits around the nucleus. The electrons are revolving in different orbits. Some orbits are closer to the nucleus some are away from the nucleus. The electrons closer to the nucleus posses lower energy than those farther from nucleus. This phenomenon can be compared to a mass m, which possessing increasing potential energy as its distance above the earth is increased. If a mass is lifted from earth surface, its potential energy increases with increase in height of lifting. Similarly in atom, if the distance between electrons and nucleus increases, the potential energy of the electrons is increased. Thus it can be said that the position occupied by an electron in an atom signifies a certain energy level of that electron.

Again due to opposite charges in electron and nucleus, there will be an attraction force between them. Naturally this attraction force becomes weaker as the distance between nucleus and electrons increase. Hence the electrons min the outer most orbit of an atom experiences least attraction force. So the outermost atom can easily be detached from the parent atom.

Let’s explain the details with band theory

When a number of atoms are brought together, the electrons of one atom experience forces of other atoms. This effect is most pronounced in outer most orbits. Due to this force, the energy levels, which were sharply defined in an isolated atom, are now broadened into energy bands. Due to this phenomenon generally two bands result, namely valance band and conduction band.

Valance Band

The outermost orbital of an atom, where electrons are so tightly bounded that, they can not be removed as free electron

Conduction Band

This is the highest energy level or orbital in outer most shell, in which electrons are free enough to move.

Band Gap

There is one energy gap separates these two bands, - the valance band and conduction band. This gap is called forbidden energy gap.

Electrical Conductivity of Metal

In metals, the atoms are so tightly packed that electrons of one atom experience sufficiently significant force of other closed atoms. That result, the valance band and conduction band in metals come very closer to each other may even overlap. Consequently, by receiving very small amount of energy from external heat or electrical energy source, the electrons readily ascend to higher levels in the metal. Such electrons are known as free electrons. These free electrons are responsible for current flows through a metal. When external electric source is connected to a piece of metal, these free electrons starts flowing towards higher potential terminal of the source, causing current to flow in the metal. So metal is good electrical conductor. In metal density of free electrons in conduction band is much higher than other materials, hence metal is referred as very electrical conductor. In other words electrical conductivity of metal is very good.

Electrical Conductivity of Semiconductor

In semiconductor the valance band and conduction band are separated by a forbidden gap of sufficient width. At low temperature, no electron possesses sufficient energy to occupy the conduction band and thus no movement of charge is possible. But at room temperature it is possible for some electrons to give sufficient energy and make the transitions in conduction band. The density of electrons in conduction band at room temperature is not as high as in metals, thus can not conduct electrical current as good as metal. The electrical conductivity of semiconductor is not as high as metal but also not as poor as insulator. That is why, this type of material is called semiconductor.

Electrical Conductivity of Insulator

Practically electrical conductivity of insulator is nil. The atoms in the insulator molecules are electrically stable enough. The outer most shell of these atoms are completely filled with electrons. In such material where forbidden gap is very large and as a result the energy required by the electron to cross over to the conduction band is practically large. Insulators do not conduct electricity easily. That means the electrical conductivity of insulator is very poor.

PIYUSH PUSHKAR