ELECTRIC CHARGE
There are only two kinds of electric charges. One is positive charge and other one is negative charge. It’s mention worthy that i) like charges repel and (ii) unlike charges attract each other.
CONDUCTORS AND INSULATORS
The substances which allow electricity to pass through them easily are called conductors. They have electric charges (electrons) that are comparatively free to move inside the material. Metals, human and animal bodies and earth are conductors. The substances which offer high resistance to the passage of electricity through them are called insulators. Most of the non-metals like glass, porcelain, plastic, nylon, wood are insulators. When some charge is transferred to a conductor, it readily gets distributed over the entire surface of the conductor. In contrast, if some charge is put on an insulator, it stays at the same place.
There is a third category called semiconductors, which offer resistance to the movement of charges which is intermediate between the conductors and insulators.
BASIC PROPERTIES OF ELECTRIC CHARGE
1. Charge has magnitude but no direction, similar to the mass. However, there is one difference between mass and charge. Mass of a body is always positive whereas a charge can be either positive or negative. Proper signs have to be used while adding the charges in a system. For example, the total charge of a system containing five charges +1, +2, +3, - 4 and -5, is +1+2+3-4-5= -2.
2. The total charge of the isolated system is always conserved.
3. All free charges are integral multiples of a basic unit of charge (e). i.e. Charge q = ne. where n is any integer, positive or negative. In the International System (SI) of Units, the value of the basic unit of charge is
$e=1.602192 \times 10^{-19} \ C$
WAYS OF CHARGING BODY
(A) Charging by friction:
When two bodies are rubbed together, a transfer of electrons take place from one body to another. The body from which electrons have been transferred is left with an excesses of positive charge, so get positively charged. The body which receives the electrons becomes negatively charged.
(B) Charging by electrostatic induction:
If a negatively charge rod is brought near the conductor mounted on insulated base as free electrons of conducting spheres close to rod experiences a force of repulsion and go to the other part of the sphere. Consequently the part of sphere close to rod becomes positively charge due to deficiency of electrons in that region.
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| CHARGING BY INDUCTION |
When the sphere is connected to the earth through a conducting wire, the some of the electrons of the spheres will flow to the ground.
When the negatively charged rod is moved away from the sphere, the electrons get redistributed on the sphere such that the same positive charge is spread all over the surface of the sphere.
(C) Charging by conduction:
Let us consider two conductors, one charged and other uncharged. If we bring the conductors in contact with each other the charge under its own repulsion will spread over both the conductors. Thus the conductors will be charged with the same sign.
