Electrical Charge and it's properties.

Charge is the fundamental property of a material body due to which electrical forces arises.

When a piece of amber is rubbed with fur, it acquires the property of attracting light objects like bits of paper.

In the 17th  century, William Gilbert discovered that, glass, ebonite etc, also exhibit this property, when rubbed with suitable materials. The substances which acquire charges on rubbing are said to be ‘electrified’ or charged.

These terms are derived from the Greek word elektron, meaning amber. The electricity produced by friction is called frictional electricity. If the charges in a body do not move, then, the frictional electricity is also known as Static Electricity.

Charge is of two types:
Positive charge
Negative charge

Basic properties of electric charge

(i) Quantisation of electric charge

The fundamental unit of electric charge (e) is the charge carried by the electron and its unit is coulomb.  e  has the magnitude 1.6 × 10^−19  C. In nature, the electric charge of any system is always an integral multiple of the least amount of charge. It means that the quantity can take only one of the discrete set of values. The charge,  q = ne  where n  is an integer.

(ii) Conservation of electric charge

Electric charges can neither be created nor destroyed. According to the law of conservation of electric charge, the total charge in an isolated system always remains constant. But the charges can be transferred from one part of the system to another, such that the total charge always remains conserved.

(iii) Additive nature of charge

The total electric charge of a system is equal to the algebraic sum of electric charges located in the system. For example, if two charged bodies of charges +2q,  −5q are brought in contact, the total charge of the system is –3q.

Van de Graaff generator

Van de Graaff Generator

Van de Graaff Generator is a linear particle accelerator. In 1929, Robert J. Van de Graaff designed an electrostatic machine which produces large electrostatic potential difference of the order of 10 million volt.

Principle
The working of Van de Graaff generator is based on the principle of electrostatic induction and action of points.

Construction
A hollow metallic sphere A is mounted on insulating pillars. A pulley B is mounted at the centre of the sphere and another pulley C is mounted near the bottom. A belt made of silk moves over the pulleys. The pulley C is driven continuously by an electric motor. Two comb−shaped conductors D and E having number of needles, are mounted near the pulleys. The comb D is maintained at a positive potential of the order of 10 thousands volt by a power supply. The upper comb E is connected to the inner side of the hollow metal sphere.

Working
Because of the high electric field near the comb D, the air gets ionised due to action of points, the negative charges in air move towards the needles and positive charges are repelled on towards the belt. These positive charges stick to the belt, moves up and reaches near the comb E. As a result of electrostatic induction, the comb E acquires negative charge and the sphere acquires positive charge. The acquired positive charge is distributed on the outer surface of the sphere. The high electric field at the comb E ionises the air. Hence, negative charges are repelled to the belt, neutralises the positive charge on the belt before the belt passes over the pulley. Hence the descending belt will be left uncharged. Thus the machine, continuously transfers the positive charge to the sphere. As a result, the potential of the sphere keeps increasing till it attains a limiting value (maximum). After this stage no more charge can be placed on the sphere, it starts leaking to the surrounding due to ionisation of the air.The leakage of charge from the sphere can be reduced by enclosing it in a gas filled steel chamber at a very high pressure.
The high voltage produced in this generator can be used to accelerate positive ions (protons, deuteron)

Applications

It is used as an particle accelerator to generate energetic particle and x-ray beams in fields such as nuclear medicine, treatment of cancer and nuclear disintegration.

If a person insulated from the ground touches the sphere of a Van de Graaff generator, his or her body can be brought to a high electric potential. The hair acquires a net positive charge, and each strand is repelled by all the others. The result is a scene such as that depicted in the photograph.