Electrical Properties of Materials

3.3 Electrical Properties of Materials

From an electrical point of view, most materials can be classified into three groups; namely:

(a)                 Conductors.

(b)                 Semi-conductors.

(c)                 Insulators.

3.3.1 Conductors

According to atomic theory, an electron can escape from the parent atom and move about between the atoms. This is called a "free electron”.  Once an electron escapes from an atom, the atom becomes net positively charged. It may then attract a free electron to become a neutral atom again.

Metals are characterized by many free electrons. Materials having many free electrons easily allow an electric current to flow through them and are called conductors. Electrons are the electric charge carriers in solid conductors. Metals are normally good conductors.

Some of the conductors, in order of their conductivity, are silver, copper, gold, aluminium, tungsten and brass. Silver is the best conductor of all metals, but is only rarely used because it is very expensive. It is used in precision instruments and in special switches as a coating but not in wires. The most commonly used conductors are copper and aluminium because of their abundance.

Copper is used in flexible cables such as wiring whereas aluminium is used mostly in overhead transmission cables because it is lighter than copper. It is also cheaper.

When an electric field is applied it sets up an electric force.

Without an electric field applied to a conductor, the electrons move in all directions, so it cannot be said that there is an electric current. However, if a potential is applied across the ends of a conductor, the free electrons will tend to move in the same direction. This is what happens when a conductor is connected between the terminals of an electric cell.

The electromotive force (e.m.f.) of the cell is the field. The electrons are repelled from the negative terminal and attracted towards the positive terminal of the cell. Some electrons will leave the piece of metal and pass into the positive terminal. Those from the negative terminal will pass into the conductor. In this process, an electric charge will be transported or conducted through the conductor.

Other than metals, other conductors of electricity are electrolytes and some gases.

3.3.2 Semi-conductors

The atoms of some elements, such as carbon, silicon and germanium, do not have free electrons at low temperature. However, at room temperature, a few electrons acquire enough energy to escape from bonds and become free. As the temperature increases, more electrons becomes free; consequently rendering the substance a good conductor. Materials which are poor conductors at low temperatures and good conductors at higher temperatures are called semi-conductors. Electrons that are set free by the temperature rise are termed thermally generated electrons.  As an increase in temperature provides more charge carriers (electrons), the resistivity of semi-conductors decreases with increasing temperature.  Substances whose resistivity decrease with increase in temperature are said to negative temperature co-efficient.

3.3.3 Insulators

An insulator is a substance that does not conduct electricity (except at very high temperature). Many non-metallic compounds are insulators. The outstanding characteristic is that they contain no free electrons. In an insulator, the electrons are bound to their atom by very strong forces.

Some excellent electrical insulating materials are mica, porcelain, glass, rubber, dry paper, Bakelite, PVC.

It is important to observe that not all insulators are alike in their insulating qualities. The best of them contain no free electrons, while less perfect ones contain a limited number of free electrons. Insulating properties also depend on the temperature of the material.

Sheet mica is used for the heating elements of toasters, because it can withstand high temperatures without allowing any excess current leakage.

PVC Colour Coded Flexible Cable

Porcelain, is one of the best insulators available today and is used in high voltage transmission lines.Porcelain, like mica, has high insulating properties at high temperatures. It is mechanically stronger and can be moulded into large insulators. 

Porcelain

It is used for insulating the line support of electric transmission lines. PVC and rubber are excellent insulators where cable flexibility is required. They are mostly used in low and medium voltage rating cables. They have the disadvantage that their resistances decrease at temperatures above 80° C. They are pro­duced in many colours, making the colour coding of cables possible.  Cable colour coding is the process whereby given coloured insulations are used to signify cable application is specific areas.

Porcelain insulation on power transmission

Other insulating materials include wood, paper, cotton, mineral insulating material and air; but each material is suitable only for a specific use. Insulating materials are also used as dielec­tric materials in the construction of capacitors.  A capacitor is a device used to store an electric charge.