CONDUCTOR AND INSULATOR

All the materials are classified into two types- conductor and insulators- based on their ability to conduct charges.

a)   CONDUCTOR

Materials through which electricity (charge) can pass easily is called conductors. Metals, mercury, acidic water, human body, earth etc. are conductors of electricity. These can be charged by friction but the charges they gain don’t remain localized and spread over the whole surface.

 In metals outermost electrons in the atoms are almost free and may move throughout the metal. They are not bound to any particular atom. They can be treated as free electrons. They are the actual carries of electricity in metal. For example, 1c.c. of copper contain approximately 10^23 free electrons! That is why metals are good conductor of electricity. Silver is a very good conductor, so are copper, gold and aluminum.

INSULATOR

There are some materials which don’t allow   electricity to pass through them. Those are called insulators. Glass, wood, ebonite, silk, rubber, sculpture, air etc are insulator or non conductor of electricity. These can gain charges by friction but the charges cannot move to other parts and remain localized.

SURFACE DENSITY OF CHARGES

We saw that the charges distribute themselves over the outer surface of a conductor. But their distribution is not uniform over the surface. The distribution of charges on a surface is characterized by surface charge density at a point on the surface. It is defined as the amount of charge per unit area of this surface surrounding the point. If q is the amount of charge and a is the small area over which the charge is spread, the surface density of charge is given by

σ= q/a

It depends on the shape of the surface of conductor. If the surface has large curvature at some points, the accumulation of charge there is also large. For uniform curvature of the surface, the charge distribution is also uniform. It is also greatly affected by the presence of other conductor in the neighborhood.