Fluid Dynamics

Energy

 

Nuclear Energy and Other Source of Energy

Energy

Energy has taken the place of basic need in our daily life. One way or another everything depends on upon energy. The source of energy is the one which can provide an adequate amount of energy in a convenient form over a long period of time.

Thus, we can say that energy required in every field. Usually, energy in the form of fuel and electricity is used to carry out all the activities whether in houses, offices or industries. All sources of energy can be divided into two main categories: renewable sources of energy and non-renewable sources of energy.

Renewable Sources of Energy

The sources of energy which are inexhaustible and are being continuously supplied by nature are known as renewables sources of energy.

For examplei) Wind ii) Flowing water iii) Ocean tidal energy iv) Interior of the earth v) Biogas Renewable Sources of Energy vi) The sun vii) Plants and vegetables

Non-renewable Sources of Energy

The sources of energy which are exhaustible and have been formed in nature long ago are known as non-renewable sources of energy.

Example: i) Coal ii) petroleum iii) fissionable materials like uranium iv) Natural0gas

These sources of energy are also known as conventional sources of energy.

Importance of Energy

Importance of energy in our everyday life are as follows:

1. we need the energy to develop a personality,
2. to sustain life energy is needed,
3. to cook food and performing daily activities like sitting, talking, walking, thinking etc,
4. to do different mechanical work life lifting a load, driving automobiles, carrying things from one place to another place etc.

Energy Crisis

The continuous use of a non-renewable source of energy causes the serious problem of energy scarcity. Such an energy scarcity is known as an energy crisis. Due to the overgrowth of modern civilisation1 and ever increasing population more energy has been used up. For example, the petroleum products, coal etc are being finished, forests are being reduced and the ground level has decreased. Overuse of non-renewable source of energy has brought this scarcity.

Conservation of Energy

In order to solve energy crises, we have to reduce the use of excess energy which is called conservation of energy. The following steps are to be taken to conserve the energy.

1. The renewable is created in sources of energy should be used in place of non-renewable one.
2. The use of firewood should be reduced as far as possible.
3. Deforestation should be stopped and forestation should be implemented.
4. Solar and electrical vehicles should be used in place of petroleum vehicles.
5. The world’s population must be controlled.
6. Awareness about the importance of energy conservation should people.

Transformation of energy

The process of changing or converting one form of energy into another form is known as transformation energy.

Some of the examples of transformation of energy are:

1. When we listen to music then electrical energy is converted into sound energy. In the case of watching TV, electrical energy is converted into sound energy and light energy.
2. When an electric current passes through an electric bulb, it glows and gives out light and heat. Electrical energy is converted into heat and light energy.
3. When we rub our hand, they become warm. In this case, mechanical energy is converted into heat energy.
4. When a pendulum swings, the potential energy is converted into kinetic energy and vice-versa.

Nuclear Energy

The energy released during a nuclear reaction is called nuclear energy. Nuclear energy can be obtained from two types of reaction: nuclear fission and nuclear fusion.

Nuclear fission

The process in which the heavy nucleus of a radioactive atom splits up into smaller nuclei when bombarded low energy neutrons is called nuclear fission. A tremendous amount of energy is released from nuclear fission which is converted into electric energy.

When uranium 235 atoms are bombarded with slow neutrons, the heavy uranium nucleus breaks up to produce two medium weight atoms, barium -139 and krypton-94, with emission of 3 neutrons.

₉₂U²³⁵+₀n¹→₅₆Ba¹³⁹+₃₆Kr⁹⁴+₀n¹+Tremendous amount of energy

₉₂U²³⁵+₀n¹→₅₆Ba¹³⁹+₃₆Kr⁹⁴+₀n¹+Tremendous amount of energy

Nuclear Fusion

The process in which two nuclei of light elements combine to form a heavy nucleus, is called nuclear fusion.The nuclear reactions which occur at extremely high temperature are called thermonuclear energy.

When deuterium atoms are heated to an extremely high temperature under extremely high pressure, then two deuterium nuclei combine together to form the heavy nucleus of helium, and a neutron is emitted.

₁H²+₁H²→₂He³+₀n¹+Tremendous amount of energy

₁H²+₁H²→₂He³+₀n¹+Tremendous amount of energy

Thermal Power Plant

A thermal power plant produces electricity by burning coal or oil.

Working of a Thermal Power Plant

Coal or oil is burnt in a furnace to produce heat energy. This heat energy is used to boil water in order to produce steam. The steam produced in the water reservoir is allowed to fall in the turbine rotates it with high pressure. The steam falling on the turbine rotates it with high speed. A generator or dynamo connected with the turbine through an axis rotates with high speed and produce electricity. The electricity so produce is transmitted to a distant place through transmission wires.

Hydroelectric Power Plant

The electricity produced by the flowing water is known as hydro-electric power.

For the construction of hydro-electric power plant, a dam or water reservoir is made over a river. The energy stored in a dam which has potential energy is allowed to fall on the water wheel or turbine. Flowing water with potential energy makes turbine rotate with an axle and the armature of the generator. The armature of the generator between two poles of a strong magnet. The rotation of the armature of the generator rotates within two poles of a strong magnet. The rotation of the armature of the generator between two poles of a strong magnet gives rise to electric current or electricity. This electricity is transmitted to the sub-stations through a transformer for further distribution to the houses factories.

Principle of Generation of Hydroelectricity

The potential energy of water stored in a dam is converted into kinetic energy of the falling water. The water falls on the turbine, so kinetic energy of the flowing water is converted into kinetic energy of the armature of the generator connected to the turbine. Then kinetic energy is converted into electrical energy known as hydroelectricity.

 

Advantage

1. It does not cause any environmental pollution.
2. Hydroelectricity is a cheap and renewable source of energy.
3. The hydroelectric power plant can be set up anywhere at a suitable place.

Disadvantage

1. Hydroelectric power is generated only near the rivers having water throughout the year. The electric power has to be carried to the sub-stations for distribution to the house and factories situated far off from the sites of hydroelectric power stations. This is done through the transmission wires, so a lot of money has to spend on this process.
2. A large area of fertile land submerged at the site of the dam constructed for tapping energy from the flowing water.
3. A large number of people residing near the site of a dam are dislocated. So, a lot of problems are to be faced in rehabilitating.
4. Large ecosystems are destroyed when land is submerged under the water reservoir of a dam.

Applications of Hydroelectricity

1. Hydroelectricity can be used in transportation.
2. It can be used for various household purposes such as cooling, heating, lighting etc.
3. It can be used to irrigate agriculture land.
4. Hydroelectricity can be used in industries to operate various machines.

Wind Energy

The wind possesses kinetic energy. The kinetic energy of the wind is known as wind energy.

Uses of Wind Energy

The kinetic energy of the wind can be used to

1. move the sail boats in lakes, rivers and seas.
2. operate water pumps to draw underground water.
3. run the flour mills to grind the grains.
4. produce electricity.

A device used to convert wind energy into the mechanical energy of the machine is called windmill.

Construction

It consists of a wheel rotates about an axle mounted on a pole. The wind energy is used to rotate the wheel about its axis.

Uses

1. The windmill to operate a water pump
2. The windmill for producing electricity

Windmill to operate a water pump

The axle of the windmill is connected to a crank rod. The free end of the water pump rod or piston is connected to U-bend of the crank rod.

Working: When the wind strikes the blades of the windmill, the kinetic energy of the wind is used to rotate the wheel of the windmill. As a result, the axle or shaft attached to the wheel rotates which turns the crank rod. The u-bend of the crank rod produces up and down motion of the piston of the water pump. Hence, the underground water is drawn to the surface of the earth.

Windmill for producing electricity (wind generator)

When the armature of a generator rotates between two poles of the strong magnet, then electricity is produced. Windmill rotates when the wind falls on the wheel of a windmill. So the armature of the generators rotates between two poles of a magnet along with the rotation of the wheel of the wind. Electric current is produced. This is how, the kinetic energy of the wind is converted into electric energy.

Advantages

1. Wind energy produces no smoke and no harmful gases.
2. Wind energy is free and running it is also very economical.
3. The source of wind energy is unlimited.

Bio-mass energy

During photosynthesis process, solar energy is converted into chemical energy. So, plants have stored chemical energy in the forms of biomass. The matter contained in the body of different plants and animal have biomass. The gas obtained from biomass is called biogas.

Advantages

1. It is a renewable source of energy.
2. It’s production is very economical.

Disadvantage

1. Due to the release of smoke, it causes air pollution.
2. A large amount of energy cannot be produced by biomass.

Application

1. The biomass energy can be used in the home for heating and cooling purposes.
2. It can be used for heating food in food industries.
3. The biogas like gobar gas can be used for lightening purposes.

Gobar gas or Bio Gas

Biogas is a mixture of various gases formed when the animal dung is mixed with water and ferment in the absence of air.

Biogas Plant

The arrangement of producing biogas from animal dung, human excreta, industrial and domestic wastes is known as biogas plant. The arrangement of producing biogas from animal dung, human excreta, industrial and domestic wastes is known as biogas plant. A biogas plant consists of a well shaped, underground tank T called digester, which is made of bricks, and has a dome-shaped roof D. The digester is a kind of sealed tank in which there is no oxygen which acts as a gas-holder for the biogas. On the left side of the digester tank is sloping inlet chamber I and on the right side is a rectangular outlet chamber O, both made of bricks. The inlet chamber is for introducing fresh dung slurry into the main digester tank whereas outlet chamber is for taking out the spent dung slurry after the extraction of biogas. The inlet chamber is connected to a mixing tank M while outlet chamber is connected to the overflow of tank F. There is a gas out let S at the top of the dome having a value V.

Working

Cow-dung and water are mixed in equal proportions in the mixing tank M to prepare the slurry. This slurry of dung and water is fed into the digester tank T through the inlet chamber I. the digester tank is filled with dung slurry up to cylindrical level, the dome being left free for the collection of biogas. It might take 50 to 60 days for the new-gas plant to become operative. Gradually when the biogas starts collecting in the dome, it exerts pressure on the slurry in the digester tank and forces the spent slurry to go into overflow tank F, through the outlet chamber O. the spent slurry is gradually removed by the overflow of the tank. The spent dung slurry left after the extraction of biogas is rich in iron and phosphorous compounds and forms good manure.

Advantages

1. Biogas is used for cooking food and heating water.
2. Biogas does not produce smoke during and hence there is no air pollution.
3. It is the cheapest source of energy.

Solar Energy

The energy emitted by the sun in the form of heat and light is known as solar energy. The hot air rises up and the cold air from the surrounding rushes to occupy its space. This is how the air moves. Moving air possesses kinetic energy which is also termed as the wind.

Sunlight makes photosynthesis possible through which plants prepare their food. The solar energy evaporates the water oceans, lakes and ponds. The evaporated water forms cloud. These clouds on cooling give rain. The rainwater is collected in the reservoir having potential energy. Finally, potential energy is converted into kinetic energy by letting water flow in higher speed. Kinetic energy rotates the turbine which is connected with the huge magnet. Current is induced by the magnet.

Even the energy of fossil fuels like coal, petroleum and natural gas also comes from solar energy. Due to high pressure, the biomass of dead animals and plants are converted into the fossil fuels like coal petroleum and natural gases.

The sun contains mainly light elements like hydrogen and helium. When the atoms of these elements fuse together at an extremely high temperature in the interior of the sun, a large amount of energy is radiated in the form of heat.

Solar Constant

The solar constant is defined as the energy received from the sun in one second by the unit square meter area of the outer edge of earth’s atmosphere exposed perpendicularly to the radiation of the sun at average distance between the earth and the sun. The upper part of the earth receives solar energy equal to 1.4-kilo joule per second per square meter (1.4 kJ/s m²). The amount is known as solar constant.

Thus, solar constant = 1.4 kJ/s m² or 1.4 kW/m²

Application of Solar Energy

1. It is used by green plants during photosynthesis.
2. The solar energy powers the flow of wind.
3. The solar energy causes rainfall by evaporating water on the earth’s surface.
4. It is used to produce electricity from the solar cells.
5. It is used for drying clothes and food grains.
6. It is used for transportation.
7. The solar energy sustains all lives on the earth.
8. It is used for heating water using the solar heater and various other purposes.

Solar Devices

The devices which convert solar energy into other forms of energy are called solar devices. Examples: solar cooker, solar furnace, solar water heater etc.

Solar Cookers

Construction:

It consists of a wooden box in which a metallic box painted black is fitted. The space between the wooden box and the metallic box is filled with an insulating material which minimizes the heat loss by conduction and radiance. The metallic box is covered by a thick glass sheet. A plane mirror is used to reflect the sun rays into the box shown in the figure. The uncooked food placed in the black container is put inside the box.

Working:

The plane mirror reflector is adjusted in such a way that maximum sunlight falls on it. The light reflected by the plane mirror falls on the thick glass sheet cover. The heat radiation coming from the sun and have a short wavelength and high energy pass through the glass sheet and is absorbed by the black container or any other object placed in the box and black surface of the box. The temperature inside the box is between 100 ^oC to 140 ^o C.

Advantages

1. The cost of cooking food in the solar cooker is very small as money is only spent upon solar cooker.
2. It can cook two or three dishes at a time.
3. It saves the costly fuel like wood, gas, kerosene oil etc.
4. Nutrition value of the food is preserved as there is no burning of fuel.
5. It can cook multiple dishes at a time.

Solar Heater

In a solar heater, plane reflector the solar radiation is reflected into a blank pipe containing water. If the solar radiation falls on the plane reflector, on the black pipe it reflects the solar radiation. The black pipe absorbs the heat and uses it for heating water in it.

 

Solar Furnance

It consists of a large number of movable plane mirrors and a parabolic reflecting surface. The parallel beam of light falling on the parabolic mirror is focused at a small area, F

Application

A solar furnace is used in melting metals of high melting point.

Solar cell

A device which converts sunlight into electrical energy is known as a solar cell.

Construction

The solar cell consists of an array of semiconductor device called diode. When solar energy falls on such a diode, a small potential difference is created across it. When the large numbers of diodes are connected in series, a large potential difference can be obtained. As the large numbers of series of diodes are connected in parallel, a large current can be obtained. Each diode is called solar cell and an array of such cells is called a solar battery or solar panel. For example, to run a water pump by the solar battery, the external battery is connected to run an electric motor.

Advantages

The main advantages of solar cells are

1. Solar cells have no moving parts, they require almost no maintenance, and work quite satisfactorily without the use of any light focusing device.
2. They can be set up in remote, inaccessible and very sparsely inhabited area where power transmission is difficult.

Uses of Solar Cells

1. They are used to operate electric bulbs and tubes in the remote villages and area.
2. They are used to supply electricity in artificial satellites.
3. They are used for operating traffic lights.
4. They are used in running light vehicles like tempo, cars, microbuses etc.
5. They are used in street lighting.

Fuel Energy

The material which produces energy while burning is called fuels. Petrol, wood, kerosene, diesel, etc are the examples of fuel.

Applications of Fuels

1. Fuels are used for generating electricity.
2. Fuels are used in transportation.
3. Fuels are used for cooking, lightening and heating process.
4. Fuels are used in industries for various purposes.
5. Fuels like petrol and diesel are used in running various grinding mills.

Geothermal Energy

The earth has three layers i.e., the core, the mantle and the crust. The core is the central part of the earth surrounded by the mantle. The outermost part of the earth which surrounds mantle is the crust. The mantle of the earth has molten mass called magma. This magma consists of molten rocks, gases and steam at very high temperature. Due to some geological changes, the hot magma rises up and is collected in the crust of the earth. The regions in the crust where the hot magma is collected are called hot spots. The heat energy stored in the hot spots of the earth’s crust is called geothermal energy.

Advantages

1. Geothermal energy causes no pollution, so it is environmentally friendly.
2. Geothermal energy can be converted continuously into electricity throughout the air.
3. The cost of converting geothermal energy into electricity is very less.

Tidal Energy

The rise and fall of the ocean due to the attraction of the moon is called tide. The rise in water is called high tide and fall in water is called low tide. The kinetic energy of water during tides is used to produce electricity.

Tides power plants are constructed near Bays. Tides rising water is allowed to fall on the turbine of the generator which produces electricity. Thus, the kinetic energy of the water is converted into electrical energy.

During low tides, gates of the dam are closed and hence the water level behind the dam rises. The raised water has potential energy. Again gates are opened and the water is allowed to fall back into the bay. This falling water is used to rotate the turbine of the generator. Hence, the electricity is produced.

Global Energy Consumption Pattern and Demands

The amount of energy consumed by a country depends on the living standards of its citizens and the degree of its industrialization. The average energy required for a man to maintain his daily activities require about 20 k cal per day. At present, the global energy consumption per capita per day is about 2000 times the 20 K calorie he needs for maintaining life. The figure shows the global energy consumption from 1900 to 1990.

At present, about 40% of demand for energy in the world is obtained from oils. Similarly, about 20% of the total energy consumption provided by coal, about 20% by natural gas, wool 10%, nuclear resources 7% and hydroelectric power 3%. Remaining required energy is fulfilled by the resources like solar energy, tidal energy, wind energy etc.

 

Energy Use in Nepal

In Nepal, existing energy sources are classified into two major groups:

1. Conventional Energy Sources
2. Alternative Energy Sources

Nepal is not an industrially developed country. So that biomass energy as the burning of wood, agricultural waste products, and animal waste are the important resources of energy. About 94% of the total energy consumption of the country is met from biomass, out of which 80% is met by burning wool and 8.5% from animal waste.

Pollution

Pollution is the addition of something in the ecosystem, which has a detrimental effect on the environment. Odum (1971) defined pollutants as an undesirable change in the physical-chemical and biological activities. Air containing dust, smoke and CO₂ makes air polluted. Likewise, the high concentration of chemical fertilizer such as nitrogen, phosphorous and potassium in the soil pollutes the land.

Types of pollution

Air pollution

An undesirable change in the physical, chemical or biological characteristics of air is called pollution. The substances which pollute the air called pollutants.

Sources of air Pollution

1. The major industrial exhausts which contain carbon monoxide, sulphur dioxide, an oxide of nitrogen, chlorine, fluorine etc.
2. Burning solid fuels such as firewood, agricultural wastes, coal etc are the causes of air pollution.
3. Chlorofluoro Carbon (CFC) and ammonia released by the refrigerators etc.
4. Different biocides such as pesticides, insecticides, fungicides etc.
5. Tobacco smoke.
6. Electric power plants, burning fossils fuels etc.

Control of Air Pollution

1. Control over the kinds of fuel used in cars, aeroplanes, power stations, etc.
2. The establishment of more smokeless zones.
3. Disposal of wastes by dilution processes.
4. The government should encourage scientific societies and newspapers to create awareness among people about pollution and environment.
5. The garbage in big cities should be treated with cobalt rays.

Effects of Air pollution

1. Air pollution causes respiratory tract infection (RTI) and asthma.
2. It deteriorates the cultural heritage and trees.
3. It brings various skin and eye allergy.
4. It is the main causes of global warming

Water pollution

Degradation in the quality of water is called water pollution. Water covers over the 3/4th part of the earth’s surface. It is a very important resource for people and the environment. Water pollution affects drinking water, rivers, lakes and oceans all over the world. In many developing countries, it is usually a leading cause of death, by people drinking from polluted water sources. Drainage and wastage from industries, laboratory, hospitals, and homes are the main factors that cause water pollution.

Causes of water pollution

There are various causes of water pollution. Main causes of water pollution are described below:

1. A mixture of the drain in water: In urban areas drainage and drinking water pipe are brought through the same route. If drainage pipe gets burst then drinking water pipe gets contaminated with drain water which pollutes the water.
2. Washing and cleaning near water resources: In rural areas, many people shares common tap where they wash their clothes and bath nearby water resources which pollute water.
3. Mixing of fertilizers in water: Farmers use different fertilizers on their farm which are carried to river by rainfall. Fertilizers get mixed with the river.
4. Wastage from different sectors: Wastage produced by hospitals, industries, and houses are thrown in the river. Due to lack of garbage disposal water gets polluted.
5. Mismanagement of drain: Drains are directly thrown into the water in urban areas. Sometimes people directly throw wastage in water.

 

 

Control of Water Pollution

1. Renovating the existing sewer lines and connecting them to the central sewer line by branch and trunk lines.
2. All domestic sewage be centrally collected.
3. Industries may be forced to treat the effluents to a requisite quality standard and to be connected to the main sewer line.
4. Wastes from other sources like cattle fields and livestock farming should be managed properly.
5. All municipal and industrial waste water should be centrally collected and managed.

Effects of water pollution

1. Water pollution causes water-borne disease like diarrhoea, dysentery, and cholera.
2. It also brings various skin allergies if taken the bath with polluted water.
3. Acid rain deteriorates cultural heritages.
4. It has the negative impact on plants.
5. Aquatic animals cannot survive in polluted water.

Global Warming

In the atmosphere greenhouse effect is mainly due to the carbon dioxide layer. In the upper atmosphere, there is a protective layer of ozone. This ozone layer does not allow the ultraviolet radiation to enter into the earth’s atmosphere. Lower atmosphere contains carbon dioxide which traps the infrared radiation from different sources and maintains a suitable temperature. If there is an excess of carbon dioxide in the atmosphere, the temperature of the earth increases which is called global warming.

Acid rain

When the various gases like CO₂, SO₂, NO₂, combine with rainwater then their respective acids will be produced. These acids will react on the ground along with rain water in the cocktail form. This process is called acid rain. In the atmosphere, these gases react with water vapour and formed their corresponding acids like H₂CO3, H₂SO4, and others. When rainfall occurs these acids get mixed with rain and forms acid rain.

Effect of acid rain

1. The acidity of the soil increases which causes the reduction of productivity of the soil.
2. The acidity of water bodies increases which cause the decline in the population of aquatic organism.

 

 

Green House Effect

The artificial house which is made by glass or plastic inside which the temperature will be more than the outer environmental temperature is called greenhouse. The artificial greenhouse provides controlled environment for plants.

Examples of Green House Effect

1. The interior of a car parked in the sunshine with its window becomes very hot due to the greenhouse effect.
2. Rooms of a house in cold countries whose windows are closed and sunlight fall on then become warm due to the greenhouse.
3. Solar cookers and solar furnace are heated due to the greenhouse effect.

Advantages of artificial greenhouse

1. It provides the controlled environment for plants inside it. The plants which can be found in the desert can be grown in the Himalayan region inside it.
2. Those plants which are obtained in summer can be planted in winter inside it.

The Atmosphere of the Earth is Natural Green House

The water vapours and carbon dioxide in the atmosphere are good observers of infra-red radiation of longer wavelengths than that of the infra-red radiations of shorter wavelengths. The solar radiation passes easily through the atmosphere. The heat radiations passing through the atmosphere are observed by the earth’s surface and various objects like plants, water and rocks. The temperature of the earth increases. The earth’s atmosphere emits infrared radiation of longer wavelengths. These infrared radiations of longer wavelengths are absorbed by water vapours and carbon dioxide in the atmosphere. Due to this heat radiation, the warming of the earth’s atmosphere takes place.

Particle Physics

aParticle PhysicsThe branch of physics which deals with property, interaction and structure of elementary particles is called particle physics. Particle physics deals with smallest things in the universe whereas cosmology deals with the biggest thing in the universe.Elementary ParticlesThe particles which are structure less, indivisible and not regarded as made up of some other particles is known as elementary particles. In 1897, after the discovery t electron by JJ Thomson, it was assumed that atoms were considered as fundamental particles of all the matter. Thomson’s discovery of electron and Rutherford’s discovery of atomic nucleus and proton in 1811 made it apparent that atom where not fundamental in the sense that they have an internal structure.A brief description of some important particles is as follows:Electron The electron is the first fundamental particle to be discovered and it revolves around the nucleus of an atom in different orbits. Its charge is -1.6×10-19 C${\displaystyle -1.6\times {10}^{-19}C}$ and mass is ${\displaystyle }$9.1×10-31 C${\displaystyle 9.1\times {10}^{-31}C}$.ProtonProton was discovered by Rutherford in 1911. Its charge is +1.6×10-19 C${\displaystyle +1.6\times {10}^{-19}C}$ and mass is 1.6726×10-27${\displaystyle 1.6726\times {10}^{-27}}$ kg which is 1836 times the electronic mass.NeutronNeutron was discovered by Chadwick in 1932. It carries no charge but its mass is 1.6749×10-27${\displaystyle 1.6749\times {10}^{-27}}$ kg which is 1839 times the electronic mass. In free state the neutron is unstable, but it constituents a stable nucleus along with proton.PositronPositron was discovered by Anderson. Its charge and mass are same of the electron, the only difference is it is positively charged. Whereas electron is positively charged.AntiprotonIt was discovered in 1955. Its charge and mass are same as those of proton, the only difference is it is negatively charged whereas proton is positively charged.Antineutron It was discovered in 1956. It has no charge and its mass is equal to the mass of the neutron. The only magnetic moments will be in opposite direction.Neutrino and antineutrinoThe existence of these particles was predicted by Pauli while explaining the emission of β-particles from radioactive nuclei, but they were observed in 1956. Their rest mass and charge both are zero but they have energy and momentum. Both neutrino and antineutrino are stable particles. The only difference between them is their spins are in opposite directions.Pi-mesonsThe existence of these particles was predicted by Yukawa in 1935 as originator exchange forces between the nucleons, but they were actually discovered in 1947 in cosmic rays. Pi-mesons are of three types: 1. positive pi-mesons, 2. negative pi-mesons and 3. neutral pi-meson.PhotonsThese are the bundles of electromagnetic energy and travel with the speed of light. If the frequency of waves is 𝜈${\displaystyle 𝜈}$, then the energy of a photon is h𝜈${\displaystyle 𝜈}$ and momentum is h𝜈${\displaystyle 𝜈}$/c. its symbol is γ.Classification of Elementary Particles Elementary particles can be classified on the basis of different properties of particles. They can be classified on the basis of mass (massless, light, intermediate and heavy), charge (positive, negative, neutral), spin or statistics (Bosons and Fermions), interaction (Gravitational, strong, weak and electromagnetic), lifetimes (stable and resonance).The classification of massive elementary particles.

Characteristic Properties of Elementary ParticlesMass: The elementary has always the same rest mass. The magnitude of the rest mass serves as the principle label which identifies the particles uniquely.Charge: All known elementary particles have charge positive negative or zero. Further, the charge is always conserved in any collision process.Average lifetime: All known elementary particles except photon, electron, proton and neutrinos are unstable and undergo decay into elementary particles of similar mass. The decay probability of a particular particle is, however, independent of the length of the time has lived. Spin: Many elementary particles spin in a manner analogous to that of the earth on its axis, but with certain differences. The spin property forms a basis for the classification of elementary particles.Interactions: Four kinds of interactions between elementary particles are known: gravitational, weak, electromagnetic and strong.Particles and AntiparticlesA subatomic particle that has the same mass as another particle but opposite value of some other properties is called antiparticle. For example, the antiparticle of the electron is a positron, which has the same mass as that of the electron but has a positive charge equal to the proton's positive charge. The existence of antiparticles is predicted by relativistic quantum mechanics. When a particle and its antiparticle collide annihilation takes place. When an electron meets a positron the energy produced due to annihilation is ${\displaystyle }$2mc2${\displaystyle 2m{c}^2}$ where m is the mass of electron or positron and c is the velocity of light.particles Symbols antiparticles mass average lifes(s) (times me${\displaystyle m_{e_{}}}$) Proton p+ Antiprotons p¯ 1836 StableElectron e- positron e+ 1 stableNeutron n Antineutron n¯ 1839 1010Neutrino v Antineutrino v¯ 0 stablePion π+ π- 274 2.6 ×108${\displaystyle 2.6\times {10}^8}$ Pair-production and Pair-AnnihilationWhen an energetic γ-ray photon falls on heavy substance, it is absorbed by some nucleus of the substance, and its energy gives rise to the production of an electron and a positron. This phenomenon, in which energy is converted into mass, is called ‘pair-production’ and represented by following equation:

According to Einstein’s energy-mass relation, a body in the state of rest also has some energy, called its rest-mass energy. If the rest mass of the body be ${\displaystyle }$m0${\displaystyle m_0}$, then its rest-mass energy is E0=m0c2${\displaystyle E_0=m_0{c}^2}$The rest-mass of each of the electron and the positron is 9.1 × 10-31 kg.${\displaystyle 9.1\times {10}^{-31}kg.}$ so, the rest-mass energy of each of them is

For pair production, it is essential that the energy of γ-photon must be at least 2× 0.51 = 1.02${\displaystyle 2\times 0.51=1.02}$ MeV. If the energy of γ photon is less than this, it would cause photoelectric effect or Compton effect on striking the matter. If the energy of γ-photon is more than 1.02 MeV, then electron and positron are produced and the energy in excess of 1.02 MeV is obtain as a kinetic energy of these particles. The converse phenomenon of pair-annihilation is also possible. Whenever an electron and a positron come very close to each other, they annihilate each other by combing together and two γ-photons are produced. This phenomenon, in which mass is converted into energy, is called ‘pair-annihilation’, and s represented by following equation:

Fundamental Forces/InteractionsThere are four types of forces/interactions in the nature. They are:Strong Force:This force acts between hadrons/quarks and is mediated by mesons/gluons. This force is charge and mass independent and saturative. Its range is small and it is responsible for stability of nucleus.Electromagnetic Force:The force which acts in between all charged particles is called an electromagnetic force. It is stronger than gravitational force but weaker than strong force. This force is attractive for unlike charge and repulsive for like charge. It is responsible for the stability of atoms, binding atoms in a matter and chemical reaction.Gravitational Force:This force acts between the particles having mass and is always attractive. It is the weakest force of nature (1039${\displaystyle {10}^{39}}$ times weaker than strong force). It is mediated by graviton and is responsible for the stability of the universe.Weak Force:This force acts between leptons and hadrons. It is stronger than gravitational force but weaker than electromagnetic and strong force. This force is responsible for decay.LeptonsThe leptons are the lightweight elementary particles which do not have strong interactions. There are six types of leptons. They are electron (e-), the muon(µ-), the tau particle (τ-)${\displaystyle electron\left(e^{-}\right),themuon\left({\mu }^{-}\right),thetauparticle\left({\tau }^{-}\right)}$, electron neutrino (ve), muons neutrino (vµ) and tau neutrino (vτ${\displaystyle electronneutrino(v_e),muonsneutrino(v_{\mu })andtauneutrino(v_{\tau }}$). Each of the six particles has a distinct antiparticle. All leptons have spin and thus are fermions.The types of leptons are shown in the tableParticle Name Symbol Anti-particle Mass (MeV/c2${\displaystyle MeV/c^2}$) Lifetime (s) aElectron e- e+ 0.511 StableElectron neutrino ve ⏨𝜈e < 3× 10-6 Stable Muon µ- µ+ 105.7 2.20 ×10-6Muon neutrino vµ ⏨⏨vµ <0.19 Stable Tau τ- τ+ 1777 2.9 ×10-13Tau neutrino vτ ⏨vτ <18.2 Stable ${\displaystyle \begin{array}{l}Electrone-e^{+}0.511Stable\\ Electronneutrino{v}_e\overline{{𝜈}_e}<3\times {10}^{-6}Stable\\ \\ \\ Muon\mu -\mu +105.72.20\times {10}^{-6}\\ Muonneutrino{v}_{\mu }\overline{v_{\mu }}<0.19Stable\\ \\ \\ Tau{\tau }^{-}{\tau }^{+}17772.9\times {10}^{-13}\\ Tauneutrino{v}_{\tau }\overline{v_{\tau }}<18.2Stable\\ \\ \end{array}}$ HadronsHadrons are the strongly interacting particles. Each hadron has an antiparticle. There are two subclasses of hadrons: mesons and baryons. Properties of some hadrons are shown in the table below.Hadrons and their proportiesParticle Mass (MeV/c2) Charge RatioQ/e Spin Mean Lifetime(s) Quark ContentMesons π0 135.0 0 0 8.4 × 10-17 u ⏨u, d ⏨d${\displaystyle 8.4\times {10}^{-17}u\overline{u},d\overline{d}}$π+ 139.6 +1 0 2.60 × 10-8${\displaystyle 2.60\times {10}^{-8}}$ u⏨d${\displaystyle u\overline{d}}$π- 139.6 -1 0 2.60 × 10-8 d ⏨u${\displaystyle 2.60\times {10}^{-8}d\overline{u}}$K+ 493.7 +1 0 1.24 × 10-8 u ⏨s${\displaystyle 1.24\times {10}^{-8}u\overline{s}}$ K- 493.7 -1 0 1.24 × 10-8 s⏨u${\displaystyle 1.24\times {10}^{-8}s\overline{u}}$η0${\displaystyle {\eta }^0}$ 547.3 0 0 a≈10-18 u⏨u, d⏨d, u⏨s ${\displaystyle \begin{array}{l}\approx {10}^{-18}u\overline{u},d\overline{d},u\overline{s}\\ \end{array}}$Baryons P 938.6 +1 ½ stable uudn 939.6 0 ½ 886 uddΛ0${\displaystyle {\Lambda }^0}$ 1116 0 ½ 2.633 × 10-10 ${\displaystyle 2.633\times {10}^{-10}}$ uds∑+ 1189 +1 ½ 8.02 × 10-11${\displaystyle 8.02\times {10}^{-11}}$ uus∑0 1193 0 ½ 7.4 × 10-20${\displaystyle 7.4\times {10}^{-20}}$ uds∑- 1197 -1 ½ 1.48 × 10-10${\displaystyle 1.48\times {10}^{-10}}$ ddsΞ0 1315 0 ½ 2.90 × 10-10${\displaystyle 2.90\times {10}^{-10}}$ ussΞ- 1321 -1 ½ 1.64 × 10-10${\displaystyle 1.64\times {10}^{-10}}$ dssΔ++ 1232 +2 ½ ≈10-23${\displaystyle \approx {10}^{-23}}$ uuuΩ- 1672 -1 ½ 8.2 × 10-11${\displaystyle 8.2\times {10}^{-11}}$ sss∧+ 2285 +1 ½ 2.0 × 10-13${\displaystyle 2.0\times {10}^{-13}}$ udcMesonsMesons are intermediate mass particles. They are heavier than leptons but lighter than baryons. Mesons include pions, kaons, eta particles etc. They are all bosons with spin 0 and 1. There are no stable mesons.BaryonsBaryons include nucleons and hyperons. They are heavy particles and they have half-integer spin and, therefore, all are fermions. The only stable baryon is a proton.QuarksQuarks are fundamental constituents of all the hadrons. They are strongly interacting particles. No isolated existence of quark is discovered so far.Original Quark ModelAccording to this model all hadrons are a composite system of two or here fundamental constituents called quark. There are three types of quarks up, down and strange. Each quark has anti-quark of opposite charge, baryon number and strangeness.Main properties of quarks and anti quarks are given in the table:Name Symbol Spin Charge Baryon Number StrangenessQuarks Up u ½ +2e/3 1/3 0 Down d ½ -e/3 1/3 0 Strange s ½ -e/3 1/3 -1Anti Quarks Anti-up ⏨u${\displaystyle \overline{u}}$ ½ -2e/3 -1/3 0 Anti-down ⏨d${\displaystyle \overline{d}}$ ½ +e/3 -1/3 0 Anti-strange ⏨s${\displaystyle \overline{s}}$ ½ +e/3 -1/3 -1The composition of all hadrons could be completely specified by three simple rules.Mesons consist of one quark and one anti-quark.Baryons consist of three quarks.Antibaryons consist of three antiquarks.Composition of several baryons and mesons are given belowBaryons Quarks compositionP uudn uddλ0${\displaystyle {\lambda }^0}$ uds∑+ uus∑- dds∑0${\displaystyle {}^0}$ uds Ξ0${\displaystyle {}^0}$ ussΞ-${\displaystyle {}^{-}}$ dssΩ-${\displaystyle {}^{-}}$ sssΔ-${\displaystyle {}^{-}}$ dddΔ++${\displaystyle {}^{++}}$ uuuMesons Quarks compositionaπ0 uπ+ uπ- dK+ uK - sK0 u ${\displaystyle \begin{array}{l}{\pi }^{0}u\\ {\pi }^{+}u\\ {\pi }^{-}d\\ K^{+}u\\ K^{-}s\\ K^{0}u\\ \end{array}}$ The third quark is needed only to construct charged particle with strangeness.Charm and Other Recent Development Although quark model is successful in classifying particles into families, there were some discrepancies. so fourth quark was proposed by several physicists in 1967. The fourth quark was given the new property or quantum number called charm.In 1975 researcher reported strong evidence of the τ${\displaystyle \tau }$ lepton. This discovery led to the more elaborate quark model and purposed of new quarks called top (t) and bottom (b)The properties of overall quark with additional quark.

Particle Name		Symbol	Spin	Charge	Baryon Number	Charm ness	Bottom ness	Top ness
Quarks	Charm	c	1/2	+2e/3	1/3	1	0	0
	Bottom	b	1/2	-e/3	1/3	0	1	0
	Top	t	1/2	+2e/3	1/3	0	0	1
Quarks	Anti-Charm	⏨c${\displaystyle \overline{c}}$	1/2	+2e/3	-1/3	-1	0	0
	Anti-Bottom	⏨b${\displaystyle \overline{b}}$	1/2	-e/3	-1/3	0	-1	0
	Anti-Top	⏨t${\displaystyle \overline{t}}$	1/2	+2e/3	-1/3	0	0	-1

After the discovery of top quark according to the standard model all the matter is composed of six strongly interacting particles the quarks (u, d, s, c, b, t) and six weakly interacting particles the leptons (e, µ, τ, ne, nµ, nτ) together with their antiparticles. Universe All matter, energy, and space that exists, is called the universe. The branch of physics which deals with the study of the universe is called astronomy. The branch of astronomy, which deals with physical processes connected with the celestial bodies and the intervening region of space, is called astrophysics. The study of the origin, evolution, and nature of the universe is called cosmology.The Solar SystemThe solar system, in which we live, is part of the universe which includes the planets, asteroids, and comets revolving round the sun, in an elliptical orbit.The SunSun is a star which mainly contains extremely hot hydrogen gas and radiates energy in all directions. The temperature of the outer region is photosphere which is about 6000 K and has a diameter of 1.4 x 109${\displaystyle {}^9}$ m. the diameter of the invisible part called chromospheres, is much greater than this. The mean distance between the sun and earth is 1.496 x 1011${\displaystyle 1.496x{10}^{11}}$ m. and is called one astronomical unit (A.U.). The mass of the sun is 2⨯ 1030${\displaystyle 2⨯{10}^{30}}$ kg which is called one solar mass.The planetsThere are eight planets revolving round the sun in their elliptical orbits. They are Mercury, Venus, Earth, Mars, Jupiter, Saturn and Neptune. Mercury is the nearest to the sun while Neptune is the farthest one.CometsComets are astronomical bodies, moving round highly elongated elliptical orbits. It consists of frozen gases, like ammonia, methane, water and nuclei of solid particles. When moving near the sun, a comet has a head and a tail. Substances like water in the comet get vaporised and the radiation pressure forces the vapour away in the shape of the tail.AsteroidsMinor planets revolving in elliptical orbit around the sun, mostly between the orbits of mars and Jupiter, in the same plane as that of the earth are called asteroids. The diameter of the asteroids varies from 1.6 km to 1000 km. Among the ten thousand asteroids Ceres is the largest with the size of 1000 km, Pallas about 600 km and Vesta is about 540 km. Meteor, Meteoroids, and MeteoritesMeteor is an object orbiting the sun which when enters the earth’s atmosphere is vigorously accelerated, due to gravity and becomes incandescent. Meteors are collectively called as meteoroids. When these objects come close to the earth’s surface they are heated to high temperatures due to friction and look like bright lines of fire and are called shooting stars. Some of the meteoroids survive while passing through the earth surface and they hit the surface. They are called meteorites. The starsA star is a self-luminous celestial body which converts nuclear energy into heat and light through nuclear fusion. There are billions of stars in the universe which are not uniformly distributed but collected together in the groups, called galaxies.Stellar EvolutionBirth of star: The inter-steller space contains an enormous amount of dust particles and gas, which come closer and closer due to the gravitational force of attraction and a cloud is formed. Within the cloud large clumps are formed, which attracts more mass and they get heat up due to contraction. The temperature of the central core heats up with the occurrence of nuclear fusion of hydrogen. During fusion of hydrogen atoms helium atom is produced with the release of energy. The gravitational force of attraction towards the centre of the star, due to its own mass is balanced outward pressure due to the heat generated due to the nuclear fusion.Death of a star:When the central core is completely converted into helium, there is no more production of heat. There is no outward pressure to balance the inward gravitational pull. So, star contracts and temperature increases causing the outer layer of the star to expand. Due to expansion outer layer is cooled and after certain time star looks red. The red star is called red giant. Due to further contraction, the temperature may rise to such high value, that fusion of helium will take place, forming heavier elements. Once again, the fuel will be exhausted and then there will be a violent explosion, called a supernova. Due to this explosion, a large portion of star’s envelope is thrown into interstellar space and this is the death of a star.White dwarf:For stars like the sun, the gravitational compression will leave the core, composed of protons and electrons, flying around a gas-like phase, called the electron gas. The electron gas is able to withstand the inward gravitational pull. Under this situation, the star is called white dwarf. White dwarf starts cooling and changes its color from white to yellow and yellow to red. Finally, it becomes a black dwarf, without emission of any radiation.Black hole:If the original mass of the star is greater than 5 solar mass, the gravitational pull inward will be so high that the core contracts to a radius R given by R = 2GM

c2${\displaystyle R=\frac{2GM}{c^2}}$, where M is the mass of the star and c is the velocity of light. After meeting above criteria, a black hole is formed. The gravitational pull, within this, is so strong that even a light photon cannot escape from it.Neutron StarIf the mass of the star is between 1.4M and 5M the core may end up as a neutron star. When the mass is greater than 1.4 solar mass, due to gravitational compression, the electrons are forced into the nuclei by the process called the inverse 𝛽${\displaystyle 𝛽}$-decay. In this process, the electrons and protons combine to form neutrons. Since the core contains only neutrons, it is called a neutron star.Supernova:When the neutron star is formed, the compression of the core produces a tremendous amount of gravitational energy. Due to the release of energy, the outer layers of the star explode. The phenomenon of the brightness of the star sharply increasing for some time and then decreasing is called a supernova.Galaxy:Galaxy is a large collection of a group of stars and is the building block of the universe. The galaxies are of different shapes, like a spiral, elliptical or irregular. Normal galaxies emit an only little amount of radio waves, but radio-galaxies give out million times more radio waves than normal galaxies. Our galaxy is a milky way and Andromeda is the nearest galaxy to us. Redshift and Expanding UniverseOn the basis of observations on the galaxies the distant galaxies are receding from us and also away from each other at a very high speed. According to Doppler effect in light, if source of light is receding from an observer, its wavelength appears to increase 𝜆0=𝜆sc+v

c-v${\displaystyle {𝜆}_0={𝜆}_s\sqrt{\frac{c+v}{c-v}}}$, that is the light emitted by it appears to have a longer wavelength than when it was at rest. Thus, a receding source of light should show a shift towards longer wavelength region in the lines of its spectrum. This phenomenon is called the red shift.If a radiation of particular wavelength l emitted by a star/galaxy is observed through a electroscope, then due to velocity v of the star/galaxy with respect to the earth, the wavelength recorded or observed will be l0 which is quite different from the real wavelength l. Then the red shift is denoted by z and is defined by z=𝜆0-𝜆

𝜆 = ±v

c$$z=\frac{{𝜆}_0-𝜆}{𝜆}=\pm \frac{v}{c}$$ 0r, z= Δ𝜆

𝜆 = ±v

c$$0r,z=\frac{\Delta 𝜆}{𝜆}=\pm \frac{v}{c}$$ Where positive sign means star/galaxy is receding away from the earth and negative sign means moving towards the earth. Hubble’s lawThis law states that the speed of recession of the galaxy is proportional to its distance from us. If v is the speed with which a galaxy recedes and r is its distance from earth,v= Hr${\displaystyle v=Hr}$ where H is called Hubble constant. Its value is 2.3 X 10-18 s-1.${\displaystyle 2.3X{10}^{-18}{s}^{-1}.}$ The Big BangHubble’s law suggests that at some time in the past, all the matter in the universe was far more concentrated today. It was then blown apart on an immense explosive called big bang, giving all observable matter more or less the velocities that we observe today.According to Hubble law, the matter at a distance away from us is travelling with speed v=Hr${\displaystyle v=Hr}$The time needed to travel a distance r is given by, t= r

Hr =1

H${\displaystyle t=\frac{r}{Hr}=\frac{1}{H}}$ By this hypothesis, the big bang occurred about 14 billion years ago. It assumes that all speeds are constant after the big bang; that is it neglects any changes in the expansion rate due to gravitational attraction or other effects.The events that went on during succeeding time after the big bang are summarised as follows:1. 14 billion years ago initial expansion began. This event was referred to be singularity because at this time the volume of the universe was zero and density of mass-energy was infinite.2. At t = 10-43${\displaystyle {}^{-43}}$ s the temperature of the universe was about 1032${\displaystyle {}^{32}}$ K, the average energy per particle was about 1019${\displaystyle {}^{19}}$ GeV. The entire universe was much smaller than a proton.3. At t = 10-35${\displaystyle {}^{-35}}$ s the temperature had decreased to about 1027${\displaystyle {}^{27}}$ K and average energy to about 1014${\displaystyle {}^{14}}$, The universe has undergone a tremendously rapid inflation increasing the size by a factor of about 10 30${\displaystyle {}^{30}}$.4. At t = 10-32${\displaystyle {}^{-32}}$ s the universe was a mixture of quarks, leptons and mediating bosons.5. At t = 10-6${\displaystyle {}^{-6}}$ s the temperature was about 1013${\displaystyle {}^{13}}$ K and the typical energies were 1GeV. At this time, quarks began to bind together to form nucleons and antinucleons.6. At t = 1 min, the universe has now cooled enough so that protons and neutrons in colliding, can stick together to form the low mass nuclei H2, He3, He4, and Li7${\displaystyle H^2,H{e}^3,H{e}^4,andL{i}^7}$ the predicted relative abundance of these nuclides are just what we observe in the universe today.7. At t = 300,000 years, the temperature was about 104 ${\displaystyle {}^4}$ K and electrons can stick together to bear nucleus when they collide forming atoms.Atoms of hydrogen and helium under the influence of gravity begin to clump together, starting the formation of galaxies and stars. Early supernovas spewed out the various elements heavier than helium that later became incorporated in stars and in their satellite planets.Critical DensityThe average density of matter in the universe determines whether the universe continues to expand indefinitely or not. The particular density needed just to stop the expansion of the universe is called critical density.Expression for critical densityConsider a large spherical volume of the universe with the radius R and mass M containing many galaxies as shown in the figure. Let the mass of our universe m and is located at the surface of the sphere. According to the cosmological principle, the average distribution of the matter within the sphere is uniform. Let r be the density of matter inside this sphere.The total energy E of the galaxy is the sum of its kinetic and potential energies, that is E = 1

2mv2 + (-GMm)

R(1)= 1

2mv2- GMm

R$$\begin{array}{c}E=\frac{1}{2}m{v}^2+\frac{(-GMm)}{R}\\ =\frac{1}{2}m{v}^2-\frac{GMm}{R}\end{array}$$ If E is positive, our galaxy has enough energy to escape from the gravitational attraction of the mass inside the sphere, in this case the universe should keep expanding forever. If E is negative, our galaxy cannot escape and the universe should eventually pull back together. The cross over between these two cases occurs when E = 0, and 𝜌 = 𝜌c.${\displaystyle 𝜌={𝜌}_c.}$ 1

2mv2 = GMm

RBut, M = 4

3𝜋R2𝜌c $$\begin{array}{c}\frac{1}{2}m{v}^2=\frac{GMm}{R}\\ But,M=\frac{4}{3}𝜋R^2{𝜌}_c\end{array}$$ If v be the speed of our galaxy relative to the centre of the sphere, then by Hubble’s law,v= HR${\displaystyle v=HR}$so, equation (1) beccomes 1

2m(HR)2 = Gm

R(4

3𝜋R3𝜌c)pc = 3H2

8𝜋G= 6.3× 10-27 kgm-3$$\begin{array}{c}\frac{1}{2}m(HR{)}^2=\frac{Gm}{R}(\frac{4}{3}𝜋R^3{𝜌}_c)\\ p_c=\frac{3H^2}{8𝜋G}\\ =6.3\times {10}^{-27}kg{m}^{-3}\end{array}$$ The mass of a hydrogen atom is 1.67 x 10-27${\displaystyle {}^{-27}}$ kg so this density is equivalent to about four hydrogen atom per cubic meter. Dark MatterDark matter is the non-luminous material distributed throughout the universe that cannot be directly detected by observing any form of electromagnetic radiation but whose existence is suggested by gravitational effects on the visible matter. According to present observations of a structure larger than the galaxy, big bang cosmology, dark matter and dark energy account for the vast majority of the mass in the observable universe.The galaxies near the Milky Way appear to be rotating faster than the rotation rate expected from the amount of visible matter that appears to be in these galaxies. Many astronomers believe that 96% of the matter in a typical galaxy is invisible. Some astronomers argue that the cluster galaxies are bound together from billions of years by the gravity due to the presence of enough mass which include up to 96 % dark matter and energy (72% dark energy, 24% dark matter).Black HoleA black hole is a region of space in which the gravitational pull is so strong that nothing can escape. General relativity predicts that if a star of mass more than solar masses have completely burned its nuclear fuel; it should collapse into configuration known as black hole. The resulting object is independent of the properties of matter that produced it and is completely described by its mass and spin. The most striking feature of this object is the existence of a surface called horizon, which completely encloses the collapsed matter. The horizon is an ideal one way membrane i.e. particles and light can go inward through the surface but not outward. As a result, the object is dark i.e. black and hides from view a finite region of space. The escape velocity, v=√(2GM/R) ${\displaystyle v=\surd (2GM/R)}$shows that a body of mass M will act as a black hole if its radius R is less than or equal to a certain critical radius.Karl Schwarzschild in 1916, derived an expression for the critical velocity from Einstein’s general theory of relativity, known as Schwarzschild radius Rs. This is given asc=2GM

Rs ${\displaystyle c=\sqrt{\frac{2GM}{R_s}}}$or, Rs=2GM

c2 ${\displaystyle or,R_s=\frac{2GM}{c^2}}$ If a spherical, non-rotating body of mass M has a radius smaller than Rs, then nothing –not even light can escape from the surface of the body. The body is then a black hole. Any other body within a distance Rs from the centre of the black hole is trapped by the gravitational attraction of the black hole and cannot escape from it. The surface of sphere with radius ‘Rs’, surrounding a black hole is called event horizon. We cannot see events occurring inside it. All that can be known about a black hole is its mass (from its gravitational force on other bodies), its electric charge (from electric forces on other charged bodies). Since light cannot escape from a black hole, then how can we know about black holes? The answer is that any gas or dust near to the black tends to be pulled into an accretion disc that swirls around and into the black hole, rather like a whirl pool. The friction within the accretion disc’s material causes it to lose mechanical energy and spiral into the black hole. As it moves inward, it is compressed together and this causes heating of the material, just as air compressed in a bicycle pump gets hotter. Temperature in excess of 10 6${\displaystyle {}^6}$ K can occur in the accretion disc so that the disc emits x-rays. Astronomers look for these x-rays emitted before the material crosses the event horizon to signal the presence of a black hole. Gravitational WaveGravitational waves are 'ripples' in space-time caused by some of the most violent and energetic processes in the Universe. Albert Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity. Einstein's mathematics showed that massive accelerating objects (such as neutron stars or black holes orbiting each other) would disrupt space-time in such a way that 'waves' of undulating space-time would propagate in all directions away from the source. These cosmic ripples would travel at the speed of light, carrying with them information about their origins, as well as clues to the nature of gravity itself.

The strongest gravitational waves are produced by events such as colliding black holes, supernovae (massive stars exploding at the end of their lifetimes), and colliding neutron stars. Other waves are predicted to be caused by the rotation of neutron stars that are not perfect spheres, and possibly even the remnants of gravitational radiation created by the Big Bang. A gravitational wave is an invisible (yet incredibly fast) ripple in space. Gravitational waves travel at the speed of light (186,000 miles per second). These waves squeeze and stretch anything in their path as they pass by.Einstein predicted that something special happens when two bodies—such as planets or stars—orbit each other. He believed that this kind of movement could cause ripples in space. These ripples would spread out like the ripples in a pond when a stone is tossed in. Scientists call these ripples of space gravitational waves.Gravitational waves are invisible. However, they are incredibly fast. They travel at the speed of light (186,000 miles per second). Gravitational waves squeeze and stretch anything in their path as they pass by.How are gravitational waves detected?When a gravitational wave passes by Earth, it squeezes and stretches space. LIGO can detect this squeezing and stretching. Each LIGO observatory has two “arms” that are each more than 2 miles (4 kilometers) long. A passing gravitational wave causes the length of the arms to change slightly. The observatory uses lasers, mirrors, and extremely sensitive instruments to detect these tiny changes.