STATIC ELECTRICITY
Electricity by Friction
The electrical effects produced by friction are well known. A hard rubber comb can attract small bits of paper after it has been used on a dry hair. This happens because the comb, after rubbing with hair, becomes charged with electricity. The same phenomenon is noticed when a plastic pen is rubbed on a coat sleeve. The friction of textiles can also produce electrification. If after a dry day, one takes off terylene clothes in a dark room, one can see electric sparks and even hear their crackling sound.
Electricity produced by friction between two dissimilar objects is known as static electricity. Depending on the nature of the objects, one acquires a positive charge and the other an equal negative charge. For example, if a glass rod is rubbed with silk, the rod acquires positive charge and the silk an equal negative charge. On the other hand, when an ebonite rod is rubbed with flannel, the rod acquires negative charge and the flannel an equal positive charge. It is found that like charges repel and unlike charges attract.
Electrification by friction can be explained on the basis of transfer of electrons (negatively charged particles of an atom). When a glass rod is rubbed with silk, some electrons from the rod attach themselves to the silk. Thus by losing, electrons, glass becomes positively charged and by gaining the same number of electrons silk acquires an equal negative charge.
When a hollow metallic conductor is charged with static electricity it is found experimentally that the charge resides entirely on the outside of the conductor; the inner surface remains uncharged.
If a car is struck by lightning, persons sitting inside are shielded from the electricity and not harmed at all since the charge remains on the outer surface and may are to the ground through the lowest metallic part of the car.
If a pear-shaped conductor is charged, it is found that concentration of charge on and near the pointed end is much greater. If the charge on the conductor is increased, the pointed end starts losing charge. It can be shown that a pointed end not only enables a conductor to lose charge, it can also act as a collector of charge. The lightning conductor is based on this principle.
Lightning Conductor Lightning is a gigantic electric discharge occurring between two charged clouds or between a charged cloud and the earth. Lightning conductors are used to protect tall buildings from lightning damage. A lightning conductor is a thick copper strip fixed to an outside wall of the building. The upper end of the strip is in the form of several sharp spikes reaching above the highest part of the building and the lower end is connected to a copper plate buried in the earth. When charged clouds pass overhead, the lightning conductor accepts any discharge which may occur and conducts it harmlessly to earth.
Insulators, Conductors, Superconductors and Semiconductors
When a brass rod is held in the hand and rubbed with fur, it also gets charged like an ebonite rod, but the charge cannot be detected because it is conducted through brass and hand to earth. The charge on ebonite can be detected because it cannot flow through ebonite and hand to earth. Thus brass is a conductor of electricity whereas ebonite is an insulator of electricity. In fact all substances can be arranged in order of their ability to conduct electrical charges. Nearly all metals are good conductors and most non-metals are poor conductors or insulators. Metals conduct electricity because they have a large number of conduction or free electrons. Insulators have no free electrons.
The resistance of metals to flow of electricity reduces with decreasing temperature. At temperatures near absolute zero, metals have almost zero resistance and become superconductors.
Scientists have recently discovered that certain ceramics can be made to behave as superconductors at relatively high temperatures of above 100 K. Currently, a lot of research is going on in the field of high temperature superconductivity and scientists are hoping to achieve it at room temperatures. If this is achieved there will be a great technological revolution.
Certain materials, such as silicon and germanium, have electrical resistivity intermediate between those of conductors and insulators. These materials are termed as semiconductors. They are good insulators in their pure crystalline form but their conductivity increases when small amounts of impurities are added to them. After the addition of impurities, they become n-type and p-type semiconductors.
Transistors :- Transistors used in radios, televisions, computers and other devices are composed of both n-type and p-type semiconductors. They require very little power and in normal use last almost indefinitely.
Integrated Circuits (IC) :- An integrated circuit can be defined generally as an arrangement of multifunction semiconductor devices. It consists of a single-crystal chip of silicon, nearly 1.5 mm² in cross-section, containing both active and passive elements and their interconnections.
