Electrostatics

A plastic comb when brought near a piece of paper does not pick up small pieces of a paper. But if you comb your dry hair and bring the comb close to a small piece of paper, you will notice that the bits of paper are attracted towards the comb.

Why this happens? This happens because the comb gets charged or electrified when you comb your dry hair . The electricity (or charge) developed on a body on rubbing with another body is called frictional electricity or static electricity.

Charges

When we walk on a carpet made of insulating material such as rubber, nylon, wool or polyester, friction between soles of our footwear and the material of the carpet cause opposite charges to appear on them. When we touch the metal knob, the free charge on our body(generated due to friction) and free charge on the ground cause a discharge at a high voltage (several thousand volts to as much as 15,000 volts).

In early days a French chemist Charles Dufay observed that the charge acquired by a glass rod rubbed with silk is different from the charge acquired by an ebonite rod rubbed with fur or wool. Dufay termed the charge acquired by glass rod in first case as ‘vitreous’ and the charge acquired by ebonite rod on rubbing it with wool as ‘resinous’. Later on American scientist statesman Benjamin Franklin (1706-1790) introduced the terms positive in place of vitreous and negative in place of resinous, which is followed even today.

On rubbing, two materials acquire positive and negative charges equal in magnitude. The process of rubbing does not create electric charges. It results in only transfer of negative charges from one material to the other. The material, from which the negative charges have been transferred, gets an excess of positive charge and the one which receives the negative charge becomes negatively charged.

An uncharged body contains a large number of atoms each of which contains an equal number of protons and electrons. In some materials some of the electrons are bound rather loosely with their atoms. On rubbing, if some of the electrons are removed, the material which loses the electrons becomes positively charged and the material which has gained electrons becomes negatively charged. In the process of charging, positive charges in atoms are firmly bound and do not participate in the process of charging.

Like charges repel each other while unlike charges attract each other.

Conservation of charge states that the total amount of electric charge in an isolated system (where no charge can get into or out of the system) does not change with time. Within an isolated system interactions between different bodies of the system can cause transfer of charge from one body to another but the total amount of charge of the isolated system always remains constant.