We often observe a spark or hear a cracking sound,when we take off our woolen or synthetic clothes. Apart from this, sometimes we experience a sensation of a very light electric shock,while opening the door of the car after sliding from the seat . It happens because of the discharge of electric charges through our body , which got accumulated due to rubbing against the clothes or the material of the car seat.
In 600B.C., the Greek philosopher Thales observed that amber, when rubbed with wool, acquires the property of attracting objects like small bits of paper,dry leaves,dust particles,etc. There are many other substances like amber, which can also acquire the property of attracting such light objects on being rubbed with suitable substances. For example, a glass rod on being rubbed with silk ,an ebonite rod on being rubbed with fur , a plastic comb on being run through dry hair, etc . The amber, glass rod , ebonite rod or plastic comb are said to be electrified or charged with electricity. Wool ( in case of amber) ,silk (in case of glass rod) and fur (in case of ebonite rod) are also found to be electronically charged.
The electricity developed on objects , when they are rubbed with each other, is called frictional electricity.
The electric charges so developed cannot move from one part of the object to the other part. For this reason, frictional electricity is also known as static electricity or ELECTROSTATIC.
ELECTROSTATIC deals with the study of forces, potential and fields arising from static charges i .e charges at rest.
ELECTROSTATICS (COULOMB’S LAW ) PHYSICS
TWO KINDS OF ELECTROSTATIC CHANGES
TO show that electric charges are of two types,take a glass rod and suspend it with a thread after rubbing it with silk . If another glass rod rubbed with silk is brought near it, the two rods are found to repel each other. In the Same fashion, two ebonite rods each rubbed with fur are also found to repel each other. But when an ebonite rod rubbed with fur is brought near a glass rod rubbed with silk , the two rods are found to attack each other.The above observation lead to the following two conclusions.
1. The electric charge developed on glass rod (rubbed with silk) is different from the charge developed on ebonite rod(rubbed with fur).
2. The electric charge of same kind (charges developed on two glass rods or on two ebonite rods) repel each other, while those of different kinds ( charges developed on a glass rod and an ebonite rod) attract each other.
Historically, the electric charge developed on ebonite rod (or amber) was called resinous,while the charge developed on glass rod was called vitreous. The American scientist Benjamin Franklin introduced the concept of positive and negative charges in order to distinguish the two kinds of changes. He named vitreous as positive charge and resinous as negative charge. The property which differentiates between the two types of charges is called the polarity of the charge.
As describe earlier,when glass rod is rubbed with silk, silk is also found to be electricity charged. It is found that the nature of charge on silk is opposite to that on glass rod . Thus , when glass rod is rubbed with silk, the rod becomes positively charged and silk negatively charged. In the same manner,when an ebonite rod is rubbed with fur , ebonite rod becomes negatively charged and fur positively charged.
ELECTROSTATICS (COULOMB’S LAW ) PHYSICS
All the substances are made of small particles called atoms,which are made of three elem
entary particles namely electrons, protons and neutrons. Because of their masses, these particles exert gravitational forces on each other. However, in addition to gravitational force, protons and electrons exert an extra force on each other, which is quite large as compared to gravitational force. This extra force is called electric force. Therefore, apart from their masses, protons and electrons must possess some additional intrinsic property.
The additional property of protons and electrons, which gives rise to electric force between them, is called electric charge.
ELECTRIC charge is Scalar quantity. A proton possesses positive charge +e , while an electron possess an equal negative charge -e , where
e = 1.6 × 10 ^- 19 Coulomb.
Normally, an object contains equal no of protons and electrons and as such , it is electrically neutral. When the numbers of protons and electrons are not equal, the object possesses a net positive or negative charge.