Conductors and inslulators
The ultimate particles from wich any substance is made are called atoms. Some substance, called elements, contain only one sort of atom while others, called compounds, contain two or more types of atom combined together. The atoms themselves are built up of electric charges and differ only in the number of charges which they contain. Figure 1.1 shows a simplified picture of an atom which consists of: (a) The nucleus wich is the central part of the atom and is made up of positive electric particles and neutral particles combined. The greater part of the mass of the atom is concentrated in the nucleus. (b) The electrons which are negative electric particles that circle the nucleus in orbits.
Figure 1.1 structure of atom.
The number of electrons in an atom is normally equal to the number of positive charges in the nucleus,but it is possible to remove or add electrons to the orbits. An atom with an excess of electrons is said to be negatively charged while an atom wich is deficient in electrons is positively charged.In some materials the electrons in the outer orbits are easily moved;they tend to transfer themselves from atom to atom and so can wander freely about inside the material. Such a movement of electrons constitutes an electric current, and those materials in which electric currents can flow freely are called conductors.Some typical conductors are silver,copper,brass and carbon.In order materials the electrons are tightly bound to their own particular atoms,with the result that electric currents cannot flow freely in them;these materials are known as inslulators.Some typical inslulators are rubber,paper,ebonite,glass, and polythene.1.13 When electricity flows from one place to another the amount of charge transferred is equal to the number of electrons which have been moved.As the electron is such a small quantity of electricity,a unit called the coulomb (abbreviation C,symbol Q)is used for practical measurements. One coulomb is equal to 6.24*10-18 electrons (i.e. 6240 000 000 000 000 000 electrons).