ELECTROSTATICS

When a glass rod is rubbed with silk, a few electrons from the atoms of glass migrate to silk. Thus there becomes a deficiency of electrons in the glass and an excess of electrons in the silk. Hence, the glass rod becomes positively-charged and the silk becomes negatively-charged. Similarly, when we rub an ebonite rod with cat-skin, a few electrons from the skin migrate into the ebonite. Hence, the ebonite rod due to excess of electrons becomes negatively-charged and the skin due to deficiency of electrons becomes positively-charged.

CONDUCTORS,INSULATORS AND SEMICONDUCTORS

Electrically most of the materials can be placed in one of two classes: conductors and insulators.

Conductors are those which electric current can easily flow. Metals, human body, earth, mercury, and electrolytes are  conductors of electricity.

In metals, only negative charge is free to move. Positive charge is immobile. The actual charge carriers in metals are 'free electrons'. In metals, the outer electrons of the atom leave the atoms and become free to move throughout the volume of metal. In electrolytes both positive and negative charges move. The charges in an electrolyte are carried by ions and it is the ions that move in an electrolyte.

Those substance in which electric charge cannot flow is called insulators. Glass, hard-rubber, plastics, dry wood are insulators. Insulators have practically no free electrons.

There are substances which regarding their electrical conductivity are intermediate between conductors and insulators. These are called semiconductors. Silicon and germanium are known to be semiconductors.

There conductivity can however, be greatly increased by adding traces of impurities in them.

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