Gold Leaf Electroscope

What is a Gold Leaf Electroscope?

Gold leaf electroscope is an instrument used to test the presence of charge on a body. It works on the principle that like charges repel each other.

The gold leaf electroscope has a very very thin piece of gold foil (called gold leaf) fixed at the top to a piece of copper.
The copper has a large round top, called the cap. The whole thing is put inside a glass case, to stop air blowing the delicate gold leaf around.

The piece of copper goes through insulation in the top of the glass case, so that any charge on the gold leaf cannot escape.

Charge can be transferred to the electroscope by wiping the charged object across the cap. The charge flows over the conducting copper and gold, and the gold leaf rises up because it is repelled by the same charged copper rod.

http://www.gcsescience.com/pse4-electrostatic-gold-leaf-electroscope.htm

Electric field lines and it's properties

Working of microwave oven.

Microwave oven

It is used to cook the food in a short time. When the oven is operated, the microwaves are generated, which in turn produce a non− uniform oscillating electric field. The water molecules in the food which are the electric dipoles are excited by an oscillating torque. The frequency of oscillating electric field is set at 3GHz which is the natural frequency of oscillation of water molecule. So resonance occurs and water molecules start oscillating with larger amplitude. Hence few bonds in the water molecules are broken, and heat energy is produced. This is used to cook food evenly.

Electrostatic shielding

It is the process of isolating a certain region of space from external field. It is based on the fact that electric field inside a conductor is zero.

During a thunder accompanied by lightning, it is safer to sit inside a bus than in open ground or under a tree. The metal body of the bus provides electrostatic shielding, where the electric field is zero. During lightning the electric discharge passes through the body of the bus.

Electric dipole and electric dipole moment.

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.

Lightning conductor

This is a simple device used to protect tall buildings from the lightning.

It consists of a long thick copper rod passing through the building to ground. The lower end of the rod is connected to a copper plate buried deeply into the ground. A metal plate with number of spikes is connected to the top end of the copper rod and kept at the top of the building.

When a negatively charged cloud passes over the building, positive charge will be induced on the pointed conductor. The positively charged sharp points will ionize the air in the vicinity. This will partly neutralize the negative charge of the cloud, thereby lowering the potential of the cloud. The negative charges that are attracted to the conductor travels down to the earth. Thereby preventing the lightning stroke from the damage of the building.

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.