Matter is made of certain elementary particles. With the advancement in technology, we have discovered hundreds of elementary particles. Many of them are rare and of no concern to us in the present course. The three most common elementary particles are electrons, protons and neutrons having masses 9.1093897*10-31 kg, 1.6726231*10-27 kg and 1.6749286*10-27 kg respectively. Because of their masses these particles attract each other by gravitational forces.
Electrons are found to repel other electrons at 1 cm with a force of 2.3*10-24 N. This extra force is called the electric force. The electric force is very large as compared to gravitational force. The electrons must have some additional property, apart from their masses, which is responsible for the electric force. We call this property charge. Just as masses are responsible for the gravitational force, charges are responsible for the electric force. Two protons placed at a distance of 1 cm also repel each other with a force of 2.3*10-24 N. Thus protons also have charge. Two neutrons placed at a distance of 1 cm attract each other with a force of 1.9*10-60 N. Thus neutrons exert only gravitational force on each other and experience no electric force. The neutrons have mass but no charge. This study of electric forces exerted by these three particles is known as ELECTROSTATICS.
As mentioned above, the electric force between two electrons is the same as the electric force between two protons placed at the same separation. We may guess that the amount of charge on an electron is the same as that of a proton. However, if a proton and an electron are placed 1 cm apart, they attract each other with a force of 2.3*10-24 N. certainly, this force is electric, but is attractive and not repulsive. This charge on an electron repels the charge on another electron but attracts the charge of a proton. This although the charge on an electron and that on a proton have the same strength, they are of two different nature. It is, therefore, convenient to define one charge as positive and the other as negative.
The simplest way to experience electric charges is to rub certain solid bodies against each other. Long ago, around 600 BC, the Greeks knew that when amber is rubbed with wool, it acquires the property of attracting light objects such as small pieces of paper. This is because amber becomes electrically charged. If we pass a comb on dry hair, the comb becomes electrically charged and can attract small pieces of paper. An automobile becomes charged when it travels through the air. A paper sheet becomes charged when it passes though a printing machine. A gramophone record becomes charged when cleaned with a dry cloth.
The explanation of appearances of electric charge is simple. All material bodies contain large number of electrons and equal number of protons in their normal state. When rubbed against each other, some electrons from one body may pass on to the other body. The body that receives the extra electrons becomes negatively charged. The body that donates the electrons becomes positively charged because it has more protons than electrons. Thus, when a glass rod is rubbed with a silk cloth, electrons are transferred from the glass rod to the silk cloth. The glass rod becomes positively charged and the silk cloth becomes negatively charged.