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Tuesday, February 27, 2007

UNIVERSE - III : LONG ANSWERS

GIVE ANSWER IN DETAIL



*INDEX-TOPIC SEARCH


  1. Write an explanatory note on Solar System with a diagram.

    ANS:


    • Our solar system consists of the Sun, nine planets revolving around it, 91 satellites revolving around the planets, more than 1 lakh asteroids, comets, etc.
    • It is believed that the universe was formed due to Big Bang. After the big bang the sun was surrounded by a cloud of gases. These clouds were slowly converted into rocks due to contraction. These rocks kept on colliding with one another and during this process they either got broken or fused together to form bigger structures which we call planets. Thus, the present solar system came into existence.
    • The gravitational force of sun provides the centripetal force required to keep the planets in their orbits.
    • About 99% mass of the solar system comes is contained in sun. Sun is the main source of energy for planets. The amount of energy of sun reaching the earth is in desired proportion, essential for the origin and sustenance of life on earth.

  2. Write a note on Sun.

    ANS:

    • Sun is the center of our solar system.
    • About 99% of the mass of the solar system is contained in Sun. It mainly consists of hydrogen and helium along with elements like carbon, oxygen, magnesium, nickel, cobalt, etc.
    • It is the main source of energy for earth and other planets.
    • It is a sphere of hot gases with surface temperature of about 6000 K.
    • The 400 km thick, bright layer around the sun is known as 'photosphere'.
    • The layer beyond photosphere is invisible normally because the gases are very hot and less dense. But during solar eclipse, it becomes visible like a "crown". This layer is known as 'Corona'.



    • The temperature in the core region is about 1.5 crore (15 million) K. At this temperature, thermonuclear fusion process takes place in which four hydrogen nuclei fuse to form a nucleus of helium.
    • During this process some mass is lost which is converted into energy as per Einstein's mass-energy equation E = Δmc2, where Δm is the lost mass and c is the speed of light in vacuum.
    • The mass of sun is 1.99 x 1030 kg. This mass is called 'Solar Mass' (M0).

  3. Write a note on Mercury.

    ANS:

    • It is the smallest planet of our solar system.
    • It is nearest to the sun, i.e. its orbit is the smallest.
    • Its mass is about (1/18)th the mass of the earth.
    • Its gravitational field is weak.
    • Its atmosphere is thin and consists mainly of vapours of sodium and potassium.
    • Its day temperature is 427°C and the night temperature is (-173)°C.
    • Its surface has many craters formed by meteors. Some of the craters are volcanoes.
    • It has no satellite.
    • Life is not possible on this planet.

  4. Write a note on Venus.

    ANS:

    • Considering the distance from sun, it is the second planet of our solar system.
    • It is a neighbour of earth and its composition is much similar to that of earth.
    • It is the most brilliant planet.
    • Its atmosphere consists of clouds of carbon dioxide.
    • Unlike other planets, Venus spins in opposite direction from east to west. Due to this the sun appears to rise in the west on Venus.
    • The core of Venus contains metals like iron and nickel whereas the crust is made of rocks containing iron, magnesium and silicate rocks.
    • Its surface contains large mountains, valleys and volcanoes.
    • It has no satellite (moon).

  5. Write a note on Earth.

    ANS:

    • Considering the distance from sun, earth is the third planet of our solar system.
    • It is the only planet of our solar system where life exists because it has a thin layer of atmosphere which extends up to about 800 to 1000 km.
    • Atmosphere protects us from meteors as they burn out due to friction before reaching the earth's surface.
    • The ozone layer of atmosphere blocks ultraviolet rays of solar radiation from reaching the surface and protects us from its harmful effects.
    • The natural Green House Effect due to gases like carbon dioxide and methane keeps the atmosphere warm enough to sustain life.
    • Due to all these favourable circumstances life originated and sustained on the earth.
    • The surface of the earth is solid (rocky) and has very large proportion of silicon dioxide.
    • The core region is made up of molten (semiliquid) mass containing iron, magnesium, silicates, etc.
    • It has one natural satellite (moon).

  6. Write a note on Uranus.

    ANS:

    • William Herschel discovered Uranus in 1781 AD.
    • It is 64 times bigger than earth.
    • Its diameter is 3.7 times that of the earth.
    • Its core contains iron, magnesium and silicate rocks.
    • It has layers of hydrogen and helium surrounded by icy clouds of methane and ammonia.
    • It has 9 narrow rings of ash colour.
    • It has 21 moons.
    • It cannot be seen with naked eyes.

  7. Write a note on Pluto.

    ANS:[NOTE:(Only for information) Though we describe Pluto as a planet it has recently been removed from the list of planets due to certain reasons and at present it is no longer a planet. However, for us it is still a planet.]

    • It is cold, dark and yellowish.
    • Its density is similar to that of earth (therefore it is more like a terrestrial planet).
    • Its core contains silicate rocks surrounded by water, methane and carbon dioxide in the solid state.
    • Its outermost thin layer consists of nitrogen, methane and carbon dioxide.
    • Its atmosphere mainly consists of methane.
    • Its orbit is highly elliptical.
    • Pluto forms a binary system with its moon Sheron (actually 'Charon') and they revolve around their common centre of gravity.
    • It cannot be seen with naked eyes.

  8. Distinguish between Jovian planets and terrestrial planets.

    ANS:

  9. Write a note on Asteroids.

    ANS:

    • Asteroids are rocks with irregular shape(s) revolving around sun, which failed to assemble into planets (form planets) during the formation of the solar system.
    • They are found in the belt between the orbits of Mars and Jupiter.
    • There are about 1 lakh (0.1 million) asteroids.
    • Orbits of about 4000 asteroids are known.
    • Ceres is the largest asteroid known and it was the first asteroid to be discovered. Its diameter is about 1000 km.
    • Vesta is another asteroid which is luminous and has diameter of about 400 km. It was discovered in 1807 AD.
    • During their movement around sun, they sometimes collide with one another and get broken.
    • The debris of asteroids sometimes enters earth's atmosphere in the form of meteors which burn out due to atmospheric friction.
    • If a meteor is very large, it reaches the earth's surface. Then it is called a meteorite.
    • Asteroids contain silicon, nickel, chromium and calcium with possibly gold and platinum.

  10. Write a note on 'shooting stars'.

    ANS:

    • If a substance from space enters earth's atmosphere, it is called a meteor.
    • Such substances after entering earth's atmosphere burn due to friction and we see a streak of light in the sky.
    • Due to this meteors are called 'shooting stars', though they are not stars.
    • Maximum number of meteors are seen in the period between August and November.
    • If a large-sized meteor fails to burn completely after entering earth's atmosphere, it strikes the surface of the earth. Such an object is called a meteorite.

  11. Write a note on Comets.

    ANS:





    • In our solar system, there is a group of about a million celestial objects beyond Pluto which is called 'Cloud of Urt' [Wikipedia mentions it as : Cloud of Oort].
    • These objects revolve around sun in a highly elliptical orbit and are called Comets.
    • Comets are made up of dust and icy rocks.
    • When a comet comes closer to sun, the ice gets vapourised forming a luminous tail.
    • When the comet is nearest to sun, the tail is the longest and away from the sun.
    • As the comet moves away from sun, the tail starts becoming smaller and ultimately disappears.
    • Due to its tail a comet is called 'Tailed Star' despite the fact that it is neither a star nor is its tail permanent.
    • The orbits of more than 750 comets are known.
    • The most famous Halley's comet appears after a period of 76 years. It was last seen in 1986 AD and will be seen next in 2062 AD. It was named after Edmond Halley who had studied it in detail.
    • In 1997 AD, a brilliant comet 'Helibob'[Wikipedia mentions it as : Hale Bopp] was seen for about 19 months even with naked eyes.
    • A comet is basically made of water, carbon dioxide, ammonia, other frozen gases and dust particles.
    • A detailed study has shown that the tail of a comet contains molecules of CO and HCN. These molecules are essential for the formation of certain complex compounds required for the origin of life.
    • Some scientists believe that these molecules might have come to the earth from the tails of comets and resulted in the origin of life.
    • Ancient people believed that the arrival of a comet was responsible for war, epidemic and natural calamities like flood. Modern science has, however proved that the arrival of a comet is a normal event and there is nothing to panic about.

    [Only for Information : Recently a comet (McNaught) was seen in January 2007- See the Picture below.]


  12. Write a few sentences on 'Stars'.

    ANS:

    • Stars are self-luminous celestial objects which produce energy in the form of radiation due to the thermonuclear fusion process taking place within them.
    • Stars are hot gaseous spheres consisting mainly of hydrogen and helium.
    • Though stars appear to be permanent, they are born, they grow and they die.
    • The physical characteristics of a star change with time.
    • We observe stars of different colours. Stars appearing red have the lowest surface temperature and those appearing blue have the highest surface temperature. The colour and the surface temperature of a star depend on its phase.
    • Sun is the nearest star to us.

  13. Distinguish between star and planet.

    ANS:

  14. Write a note on Nakshatras.

    ANS:

    • An imaginary sphere covering the sky with the earth as its center is known as the Celestial Sphere.
    • The ecliptic of the celestial sphere is divided into 27 equal parts, each called a Nakshatra.
    • The angular region of a Nakshatra is 13° 20' [360/27].
    • Our moon covers angular displacement of about 13° 20' in one day along the ecliptic. Thus it remains for about a day in one Nakshatra.
    • Sun stays in one Nakshatra for about 13.5 days [365/27].
    • Nakshatras get their names on the basis of the imaginary figures(shapes) formed by luminous stars.
    • The Indian names of some Nakshatras are : Krittika, Bharani, Pushya, Magha, Ardra, Ashwini, Chitra, Vishakha, Swati, Rohini, etc. [The names given in the book : Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Saggitarius, Capricorn, Aquarius, and Pisces are in fact names of 12 Zodiac signs.]

  15. Write a note on galaxy.

    ANS:



    • Stars are found in big clusters called galaxies.
    • There are about 1011 galaxies in the universe and each galaxy contains about 1011 stars. Thus, there are in all about 1022 stars.
    • Galaxies have generally two types of shapes:(i)Spiral (ii) Elliptical. Some galaxies have irregular shape.
    • Most stars in elliptical galaxies are red indicating that they are very old. On the other hand the stars in spiral galaxies are bluish indicating that they are young. Thus, elliptical galaxies are older than the spiral galaxies.
    • The number of spiral galaxies is greater than that of elliptical galaxies.
    • The galaxy in which our solar system is situated is called the Milky Way. Milky Way appears as a milky belt stretching from North to South during dark night when the sky is clear.
    • Milky Way is spiral in shape when viewed from the top. When viewed from the sides, it is seen bulging at the center and tapered towards the ends.
    • The diameter of Milky Way is about 1 lakh (0.1 million) light years and the thickness of the middle (bulging) part is about 15000-20000 light years.
    • Sun (and therefore the solar system)is located at about 30000 light years away from the galactic center.
    • The sun completes one revolution around the galactic center in 22.5 crore (225 million) years at the speed of 250 km/s.

  16. Explain in short the theories regarding the Constitution of Universe.

    ANS:

    • Edwin Hubble showed that galaxies recede away from one another at a speed which is is directly proportional to the relative distance between them.
    • Based on this, it was concluded that the Universe is expanding continuously.
    • We can assume that the receding galaxies should have been together at one point in the past (where we can think of the Universe being of a definite size). At that time due to some huge explosion (Big-Bang), Universe came into existence (started expanding).
    • It has been estimated that the Big Bang might have taken place 13.7 billion years ago. Thus, the age of the Universe is 13.7 billion years.
    • According to another theory called 'Steady-State Theory', the Universe appears the same at all the time and remains unchanged. This theory cannot explain certain phenomena.

  17. Write a note on Construction and Working of a space shuttle.

    ANS:


    CONSTRUCTION:

    • A space shuttle appears like a huge plane.
    • It consists of three main liquid fuel rockets, a big liquid fuel tank which provides fuel to these rockets and two solid fuel rockets called booster rockets.
    • The outer surface of the space shuttle is made up of a special alloy to withstand extremely high temperatures.
    • It has a space to carry payloads and a robotic arm to perform functions in space.


    WORKING:

    • All five rockets ( two booster rockets and three liquid fuel rockets) are fired simultaneously.
    • The fuel of the booster rockets is used up before the shuttle reaches 45 km height and they get detached automatically and start coming down towards the earth.
    • The parachutes attached to them open up and control their speed so that they land slowly at a predetermined place in the sea. They are collected and reused after minor repairs.
    • The space shuttle continues to go up with the help of three rockets.
    • By the time the space shuttle attains its desired height the fuel tank becomes empty. It gets detached and during its journey towards earth it burns off due to friction.
    • Now the space shuttle starts revolving around the earth in its predetermined orbit.
    • A robotic arm released from its upper part performs required tasks.
    • A space shuttle can perform following tasks:

      • Launching a satellite in space.
      • Maintenance and relaunching of a satellite.
      • Bringing back a satellite from space.
      • Performing scientific experiments and observations in space.
      • Exchanging scientists in the space station and carrying equipment for them.


  18. Describe various orbits for artificial satellites.

    ANS:

    • Artificial satellites are kept in (i)Equatorial orbit (ii)Polar orbit.
    • The orbit which is parallel to the equator is known as Equatorial orbit.
    • If a satellite is placed at a height of 35786 km, its revolution period is 24 hours. Such asatellite is called 'Geostationary' as it appears stationary in space when observed from earth.
    • By proper arrangement of several geostationary satellites we can link all corners of the earth with one another.
    • Satellites in equatorial orbits do not require energy for revolutions.
    • The orbit which is parallel to the Meridian is called Polar orbit.
    • If a satellite is placed at a height of about 1000 km, its revolution period is about 2 hours. Such satellites complete several revolutions around earth in a day and they pass over a location on the earth at regular time interval.
    • Remote sensing satellites of India visit any location on the earth at the interval of 21 days whereas America's Landsat takes 16 days for that.
    • Like natural satellites, the orbits of artificial satellites are elliptical due to gravity effect.

  19. Mention uses of artificial satellites.

    ANS:The uses of artificial satellites are as follows :

    • In communication : For telecommunication (telephone), television transmission, radio networks and computer networks (internet).
    • In Education Field : For teleconferencing and conducting country-wide classrooms.
    • In Weather Forecasting : Getting prior information about monsoon, climatic changes, cyclones, etc.
    • In Oceanography : To study changes occuring under sea and the aquatic life.
    • In Geological Surveys providing information about water resources, mineral resources, changes in earth's crust, etc.
    • In Agriculture to estimate the crop yield and to detect disease spread in crops.
    • In defence to keep an eye on the activities of enemy country.

  20. Give details of Space Programmes conducted by ISRO.

    ANS:

    • For communication, INSAT [Indian National SATellite system] series of satellites consisting of INSAT-1,2,3 have been launched. INSAT-4 is ready for DTH (Direct To Home) service for TV transmission.
    • For commercial purpose IRS [Indian Remoye Sensing satellites] series of satellites (IRS-I, IRS-P, etc.)have been launched.
    • METSAT [METeorological SATellite] is used for weather forecast.
    • Resource satellites are used for oceanography.
    • 'Cartosat' is used for geographical/geological surveys.
    • For astronomical observation Rohini series has been launched.
    • ISRO has developed PSLV (Polar Satellite Launch Vehicle) and GSLV (Geosynchronous Satellite Launch Vehicle) for the launching of different satellites.

  21. Distinguish between polar orbit and equatorial orbit.

    ANS:
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Thursday, February 22, 2007

UNIVERSE - II : BRIEF ANSWERS

ANSWER IN BRIEF



*INDEX-TOPIC SEARCH


  1. Explain the theory of formation of our Solar System.
    ANS: It is believed that the universe was formed due to Big Bang. After the big bang the sun was surrounded by a cloud of gases. These clouds were slowly converted into rocks due to contraction. These rocks kept on colliding with one another and during this process they either got broken or fused together to form bigger structures which we call planets. Thus, the present solar system came into existence.

  2. Define Solar system and write the names of the planets in the ascending order of their distance from Sun.
    ANS:Our solar system consists of the Sun, nine planets revolving around the sun, 91 satellites revolving around planets, more than 1 lakh asteroids, comets, etc.

    The planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

  3. Explain : Corona.
    ANS:Sun is a sphere of hot gases.
    The 400 km thick, bright layer around the sun is known as 'photosphere'.
    The layer beyond photosphere is invisible normally because the gases are very hot and less dense. During solar eclipse, this layer becomes visible like a 'crown'. this is called 'Corona'.

  4. Mention the names and the characteristics of terrestrial planets.
    ANS:Mercury, Venus, Earth and Mars are terrestrial planets. Their characteristics are as follows:

    • Terrestrial planets have structure similar to that of the earth.
    • They have less number of satellites(moons).
    • Their atmosphere is very thin.


  5. Describe in brief how the atmosphere protects life on earth.
    ANS:The atmosphere on earth protects life in the following way:

    • Atmosphere protects us from meteors as they burn out due to friction before reaching the earth's surface.
    • The ozone layer of atmosphere blocks ultraviolet rays of solar radiation from reaching the surface and protects us from its harmful effects.
    • The natural Green House Effect due to gases like carbon dioxide and methane keeps the atmosphere warm enough to sustain life.


  6. Write a note on Mars.
    ANS:

    • Earth's neighbour mars is reddish in colour.
    • Its surface has large valleys, mountains and dry rivers.
    • Its atmosphere is extremely thin (1% of earth's atmosphere) and mainly contains carbon dioxide with traces of nitrogen and argon.
    • The 'Path Finder' mission of 1997 indicated the presence of flowing water on mars in the past.
    • At present the probability of existence of life on mars is negligible.
    • 'Phobos' and 'Demos' are its two moons (satellites).


  7. Write the names of jovian planets and mention their characteristics.
    ANS:Jupiter, Saturn, Uranus, Neptune and Pluto are Jovian planets. Their characteristics are:

    • They are beyond the orbit of mars.
    • Their composition is similar to that of Jupiter.
    • They are bigger in size but have less density.
    • They are mainly made up of hydrogen, helium and ammonia.
    • Their moons are bigger in size.
    • Most of them possess rings around them.


  8. Write a few points on Jupiter.
    ANS:

    • Considering the distance from sun, Jupiter is the fifth planet of our solar system.
    • It is about 1400 times bigger than earth and is the biggest planet of the solar system.
    • It is a luminous planet with some bright and hazy belts of brownish colour.
    • It can be seen with naked eyes.
    • It has 39 satellites (moons).


  9. Write a note on Saturn.
    ANS:

    • Saturn is the second largest planet of our solar system.
    • It is 850 times bigger than earth.
    • It is believed that saturn is completely made of hydrogen.
    • Its crust might be made up of fluid hydrogen whereas the core might be made up of solid hydrogen.
    • It has three beautiful luminous rings.
    • It has 30 satellites.


  10. Write a few words about Neptune.
    ANS:

    • Neptune is bluish planet with two luminous and hazy rings.
    • Its core contains silicate rocks and ice.
    • The upper crust has methane ammonia and water in ice form.
    • It has 8 moons.
    • 'Triton' and 'Nerid' are its well-known moons.
    • It cannot be seen with naked eyes.


  11. 'Night-sky changes everyday.' Explain.
    ANS:

    • The earth takes 23 hours 56 minutes to complete one rotation about its axis whereas one day is made up of 24 hours.
    • Due to this the sun rises four minutes earlier than its rising time on the previous day.
    • During this 4 minutes there is 1° angular displacement of stars in the celestial sphere.
    • Thus, stars appear for about 6 months continuously (180° angular displacement) and then disappear for remaining 6 months.
    • This shows that the Night-sky changes every moment, though the change apparent only after a long period and for a short period(like 1 day) there is no noticeable change in the position of stars.


  12. Write a few points on Milky Way.
    ANS:

    • The galaxy in which our solar system is situated is called the Milky Way. Milky Way appears as a milky belt stretching from North to South during dark night when the sky is clear.
    • Milky Way is spiral in shape when viewed from the top. When viewed from the sides, it is seen bulging at the center and tapered towards the ends.
    • The diameter of Milky Way is about 1 lakh (0.1 million) light years and the thickness of the middle (bulging) part is about 15000-20000 light years.
    • Sun (and therefore the solar system)is located at about 30000 light years away from the galactic center.
    • The sun completes one revolution around the galactic center in 22.5 crore (225 million) years at the speed of 250 km/s.


  13. Expalin the principle of rockets.
    ANS:Rockets work on the principle of Newton's third law of motion.
    When the fuel in the rocket engines burns, gases produced during combustion come out of the nozzle with very high momentum and the rocket is pushed upward.

  14. Mention the properties of a rocket fuel.
    ANS:A rocket fuel should have the following properties:

    1. Its combustion rate should be high.
    2. It should not be explosive in nature.
    3. It should produce large amount of gases at high temperature and pressure.


  15. Mention the two major disasters of space shuttles.
    ANS:The two major disasters of space shuttles are :

    1. In 1986, the space shuttle Challenger exploded and seven astronauts on board died.
    2. In February,2003, the space shuttle Columbia met with an accident a few minute before its landing in which the astronaut of Indian origin, Kalpana Chawla, died along with others.


  16. What is an artificial satellite ? What is it equipped with ?
    ANS:A man-made automatic system launched in space with a special purpose and which revolves around the earth is called an artificial satellite.
    It is equipped with transponders, high-resolution cameras, radiometer, solar panels and necessary fuel.

  17. Describe the use of artificial satellites in communication.
    ANS:Following are the uses of artificial satellites in communication :

    • For telecommunication(telephone).
    • For television transmissions.
    • For radio networks.
    • For computer networks(internet).
    • For countrywide classrooms.
    • For teleconferencing.


  18. Explain the installation of artificial satellite by space shuttle.
    ANS:

    • A space shuttle carries a satellite to be launched in space with other instruments as payload.
    • After the space shuttle has attained its required height in space, it continues to revolve around the earth in an orbit.
    • Once it stabilizes properly in its orbit, a robotic arm in its upper part opens up.
    • It holds the satellite to be installed and releases it in the space in its orbit.
    • After the launching of satellite is over, the shuttle starts its return journey.


  19. Explain how the space shuttle is brought back from space.
    ANS:

    • After the completion of tasks in space the pilot directs the space shuttle towards the earth.
    • Though the outer surface of space shuttle is made up of special alloys to withstand extremely high temperatures, the speed of the space shuttle is controlled carefully while entering the earth's atmosphere.
    • The direction of the entry to the atmosphere is also critical.
    • Computers (on the earth) control the journey of space shuttle till it is a few meters above the earth's surface.
    • The pilot then takes control and makes the smooth landing of the space shuttle possible like an aeroplane.
    • Thus, a space shuttle is brought back to the earth.



  20. Describe the use of satellites for Remote Sensing.
    ANS:

    • Remote sensing is the method by which information about a substance, matter or a phenomenon can be obtained using scientific instruments without direct contact with them.
    • Remote sensing satellites can carry out geological survey of metallic ores in the earth's crust, water resources, agricultural resources, changes in forests, etc.
    • These satellites can be used to know about crop yield and diseases spread in crop.
    • They find use in oceanography and study of motion of fish.
    • The sensors of these satellites cover area of 10 square meter to 6400 square meter and send information to the earth station.



  21. ANS:
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Thursday, February 08, 2007

UNIVERSE - I :MCQs AND SHORT ANSWERS

*INDEX-TOPIC SEARCH

MULTIPLE-CHOICE QUESTIONS




SELECT THE CORRECT ALTERNATIVE:


  1. _________ suggested that Sun was at the center of our solar system and all the planets and other stars revolve around it.

    1. Plato
    2. Copernicus
    3. Ptolemy
    4. Galileo

  2. Who discovered the laws of planetary motion ?

    1. Newton
    2. Galileo
    3. Kepler
    4. Copernicus

  3. The sun is at a distance of _________ light years from the galactic centre.

    1. 25000
    2. 30000
    3. 35000
    4. 40000

  4. The sun consists of about _________ % of the mass of solar family.

    1. 85
    2. 89
    3. 94
    4. 99

  5. The surface temperature of the Sun is about _________.

    1. 5000°C
    2. 5000 K
    3. 6000°C
    4. 6000 K

  6. The mass of a star is equal to 10 solar mass, then its mass is _________ kg.

    1. 1.99 x 1030
    2. 1.99 x 1031
    3. 1.99 x 1032
    4. 1.99 x 1033

  7. _________ is the smallest planet of our solar system.

    1. Venus
    2. Mars
    3. Mercury
    4. Earth

  8. The mass of earth is _________ times the mass of mercury.

    1. 10
    2. 14
    3. 18
    4. 22

  9. The strength of the gravitational field of mercury is about _________ times that of the earth.

    1. 1/3
    2. 1/2
    3. 3
    4. 2

  10. The sun rises in the west on _________.

    1. Mercury
    2. Mars
    3. Pluto
    4. Venus

  11. _________ is not a Jovian planet.

    1. Earth
    2. Jupiter
    3. Saturn
    4. Neptune

  12. _________ is the largest planet of our solar system.

    1. Earth
    2. Jupiter
    3. Saturn
    4. Neptune

  13. There are _________ beautiful rings around Saturn.

    1. 2
    2. 3
    3. 4
    4. 5

  14. Saturn is about ________ times bigger in size compared to Uranus.

    1. 11
    2. 13
    3. 20
    4. 25

  15. The diameter of earth is de. Then the diameter of Uranus is _________ x de.

    1. 1.7
    2. 2.7
    3. 3.7
    4. 4.7

  16. Neptune has _________ moons.

    1. 5
    2. 6
    3. 7
    4. 8

  17. Triton and Nerid are moons of the planet _________.

    1. Neptune
    2. Mars
    3. Herschel
    4. Pluto

  18. Orbits of more than _________ asteroids have been determined.

    1. 100000
    2. 30000
    3. 12000
    4. 4000

  19. Asteroids revolve around _________.

    1. Earth
    2. Mars
    3. Jupiter
    4. Sun

  20. _________ is the largest asteroid.

    1. Ceres
    2. Vesta
    3. Sirius
    4. Viesta

  21. During which period are meteors seen in maximum number ?

    1. January to March
    2. April to June
    3. August to November
    4. November to February

  22. The tail of a comet is the longest when _________.

    1. It is nearest to the Sun
    2. It is farthest from the sun
    3. It is far from the earth
    4. It is near Saturn

  23. Which of the following is a star ?

    1. A tailed star
    2. A shooting star
    3. Sun
    4. Andromeda

  24. What is the period of revolution of Halley's comet ?

    1. 72 years
    2. 76 years
    3. 82 years
    4. 86 years

  25. The stars appearing _________ have highest temperature.

    1. red
    2. blue
    3. yellow
    4. white

  26. The stars appearing _________ have lowest surface temperature.

    1. red
    2. blue
    3. yellow
    4. white

  27. Each Nakshatra is an angular region of about _________.

    1. 15° 30'
    2. 14° 40'
    3. 13° 20'
    4. 12° 30'

  28. The moon undergoes an angular displacement of ________ in 1 day.

    1. 30°
    2. 20° 40'
    3. 15° 45'
    4. 13° 20'

  29. The moon stays in one nakshatra for about _________ day/s.

    1. 30
    2. 13.5
    3. 7
    4. 1

  30. The sun stays in one nakshatra for about _________ day/s

    1. 1
    2. 13.5
    3. 30
    4. 365

  31. Sun rises ______ minutes ______ than the time of its rise the previous day.

    1. 4, earlier
    2. 4, later
    3. 8.5, earlier
    4. 8.5, later

  32. What is the diameter of Milky way ?

    1. 1000 light year
    2. 10000 light year
    3. 100000 light year
    4. 100000 km

  33. 1 light year = _________ km.

    1. 9.46 x 1011
    2. 9.46 x 1012
    3. 4.96 x 1011
    4. 4.96 x 1012

  34. There are about ______ galaxies in the universe.

    1. 1011
    2. 1012
    3. 1022
    4. 1023

  35. There are about ______ stars in the universe.

    1. 1011
    2. 1012
    3. 1022
    4. 1023

  36. Who showed that the galaxies recede away from one another ?

    1. Plato
    2. Kepler
    3. Edwin Hubble
    4. Albert Einstein

  37. What is the approximate age of universe according to Big-bang theory ?

    1. 137 million years
    2. 137 billion years
    3. 13.7 million years
    4. 13.7 billion years

  38. A rocket works on the principle of ______.

    1. Newton's first law of motion
    2. Newton's second law of motion
    3. Newton's third law of motion
    4. First law of thermodynamics

  39. At what height do booster rockets get detached from the space-shuttle ?

    1. 45 km
    2. 100 km
    3. 300 km
    4. 1000 km

  40. ______ was the first satellite launched by India.

    1. Agni
    2. EDUSAT
    3. Rohini
    4. INSAT II

  41. The minimum velocity required for a satellite to remain in its orbit is ______.

    1. 8 km/s
    2. 8 km/h
    3. 800 km/h
    4. 800 m/s

  42. Remote sensing satellite of India visits any location on the earth at the interval of ______ days.

    1. 18
    2. 19
    3. 20
    4. 21

  43. Which is the series of satellites used by India for astronomical observation ?

    1. Agni
    2. Rohini
    3. INSAT
    4. Vijay

  44. Which of the following is a star ?

    1. Comet
    2. Asteroid
    3. Sun
    4. Phobos

  45. Which of the following is not a member of our solar system ?

    1. Shooting stars
    2. Artificial satellites
    3. Sun
    4. Asteroids

  46. Which planet has maximum number of satellites ?

    1. Mercury
    2. Jupiter
    3. Venus
    4. Saturn

  47. Which of the following is not an artificial satellite ?

    1. INSAT
    2. Rohini
    3. Phobos
    4. SROSS

  48. Which of the following is not a natural satellite ?

    1. Triton
    2. Demos
    3. Aryabhatta
    4. Sheron(Charon)

  49. How many satellites does Mercury have ?

    1. 30
    2. 21
    3. 8
    4. 0









ANSWERS TO MCQs



(1) B (2) C (3) B (4) D (5) D (6) A (7) C (8) C (9) A (10) D (11) A (12) B (13) B (14) B (15) C (16) D (17) A (18) D (19) D (20) A (21) C (22) A (23) C (24) B (25) B (26) A (27) C (28) D (29) D (30) B (31) A (32) C (33) B (34) A (35) C (36) C (37) D (38) C (39) A (40) C (41) A (42) D (43) B (44) C (45) B (46) B (47) C (48) C (49) D

SHORT QUESTIONS



ANSWER IN SHORT:

  1. Invention of what has made the advanced study of universe possible ?

         ANS :Invention of radio telescope has made the advanced study of universe possible.

  2. In whose memory has the observatory in space been named Chandra X-Ray Observatory ?

         ANS :Chandra X-Ray Observatory has been named in the memory of Dr. S. Chandrasekhara.

  3. How do radio telescopes provide accurate information about space ?

         ANS :Radio telescopes provide accurate information about space by receiving ultraviolet rays, X-rays, microwaves, etc. from space.

  4. What did Ptolemy believe about Universe ?

         ANS :Ptolemy believed that the earth was the centre of the universe and the sun, the moon, the planets and other stars revolved around it.

  5. Sun is the centre of the universe. True or false ?

         ANS :This is a false statement.

  6. Sun is the centre of our galaxy. True or false ?

         ANS :This is a false statement.

  7. Sun is the centre of the solar system. True or false ?

         ANS :This is a true statement.

  8. What is the name of our galaxy ?

         ANS :The name of our galaxy is Milky Way.

  9. What does our solar system consist of /

         ANS :Our solar system consists of the Sun, nine planets revolving around the Sun, ninety one satellites revolving around different planets, more than one lakh asteroids, meteors, comets, etc.

  10. Which force keeps the planets in their orbits ?

         ANS :The centripetal (gravitational) force between the Sun and the planets keeps the planets in their orbits.

  11. What is photosphere ?

         ANS :Photosphere is the 400 km thick, bright layer around the Sun.

  12. What is 'Corona'?

         ANS :A thin layer of gases around the photosphere of the Sun which becomes visible at the time of solar eclipse is called 'Corona'.

  13. What is the temperature in the core region of Sun ?

         ANS :The temperature in the core region of Sun is about 1.5 crore K.

  14. What happens during the thermonuclear fusion taking place in Sun ?

         ANS :During thermonuclear fusion taking place in sun, four hydrogen nuclei fuse to form a helium nucleus.

  15. Mention Einstein's mass-energy relation.

         ANS :Einstein's mass-energy relation is : E = Δmc2, where Δm is the change in mass and c is the velocity of light in vacuum.

  16. What is the mass of sun ?

         ANS :The mass of sun is about 1.99 x 1030 kg.

  17. Which two elements are chiefly the constituents of sun ?

         ANS :Hydrogen and helium are chiefly the constituent elements of sun.

  18. Mention the names of terrestrial planets.

         ANS :Mercury, Venus, Earth and Mars are the terrestrial planets.

  19. Mention the day and night temperatures of Mercury.

         ANS :The day temperature of mercury is about 427°C and its night temperature is about(-173)°C.

  20. What does the atmosphere of Venus contain ?

         ANS :The atmosphere of Venus contains carbon dioxide gas.

  21. Why does the sun rise in the west on Venus ?

         ANS :The sun rises in the west on Venus because unlike other planets Venus spins in the opposite direction (i.e. from east to west).

  22. Name the two planets which do not have natural satellites.

         ANS :Mercury and Venus do not have natural satellites.

  23. Which layer of the atmosphere protects us from ultraviolet rays of sun ?

         ANS :Ozone layer of the atmosphere protects us from ultraviolet rays of sun.

  24. What does the core region of earth contain ?

         ANS :The core region of earth contains molten mass consisting of iron, magnesium, silica, etc.

  25. What does the atmosphere of mars mainly consist of ?

         ANS :The atmosphere of mars mainly consists of carbon dioxide.

  26. Which mission of 1997 indicated the existenceof flowing water in the past(frozen now) on mars ?

         ANS :Path Finder mission of 1997 indicated the existence of flowing water in the past on mars.

  27. Give the names of the moons of mars.

         ANS :Phobos and Demos are the moons of mars.

  28. What do we mean by Jovian planets ?

         ANS :Those planets of our solar system which are beyond the orbit of mars are called Jovian planets as their composition is similar to Jupiter.

  29. How many times is the volume of Jupiter as compared to earth ?

         ANS :The volume of Jupiter is 1400 times that of the earth.

  30. How many times is Saturn bigger than the earth by volume ?

         ANS :Saturn is 850 times bigger than earth by volume.

  31. Who discovered Urqanus ? When ?

         ANS :William Herschel discovered Uranus in 1781 AD.

  32. How many moons does Uranus have ?

         ANS :Uranus has 21 moons.

  33. How many rings does Neptune have ?

         ANS :Neptune has two rings.

  34. What does the upper crust of Neptune contain ?

         ANS :The upper crust of Neptune consists of rocks having methane, ammonia and water in ice form (frozen state).

  35. Which planet has highly elliptical orbit ?

         ANS :Pluto has highly elliptical orbit.

  36. Which is the gas mainly present in the atmosphere of Pluto ?

         ANS :Methane gas is mainly present in the atmosphere of Pluto.

  37. How many moons does Pluto have ?

         ANS :Pluto has one moon.

  38. What are asteroids ?

         ANS :Asteroids are rocks revolving around the Sun, which failed to (form planets) assemble into planets during the formation of the solar system.

  39. Approximately how many asteroids are there between the orbits of mars and jupiter ?

         ANS :About 105 (1 lakh) asteroids are there between the orbits of mars and jupiter.

  40. On what basis is the size of an asteroid estimated ?

         ANS :The size of an asteroid is estimated on the basis of luminosity.

  41. What is the diameter of asteroid Ceres ?

         ANS :The diameter of asteroid Ceres is about 1000 km.

  42. What is the diameter of asteroid Vesta ?

         ANS :The diameter of asteroid Vesta is about 400 km.

  43. What are meteors ?

         ANS :Any substance of any size entering the earth's atmosphere from space is known as a meteor.

  44. What are meteorites ?

         ANS :A meteor of very large size which does not get burnt completely in earth's atmosphere and strikes the surface of the earth is called a meteorite.

  45. Which are the elements found in asteroids ?

         ANS :The elements found mainly in asteroids are silicon, nickel, chromium, calcium and probably gold and platinum.

  46. Why are meteors called 'shooting stars' ?

         ANS :Meteors are called 'shooting stars' because when they enter earth's atmosphere with tremendous speed, they burn due to friction and a streak of light is seen.

  47. What is 'Cloud of Urt'?

         ANS :The 'Cloud of Urt' is a group of about a million of celestial objects which are beyond the orbit of Pluto.

  48. What are comets ?

         ANS :Comets are celestial objects found beyond the orbit of Pluto, which are made of dust and icy rocks and which revolve around Sun in a highly elliptical orbit.

  49. Approximately how many comets are known ?

         ANS :About 750 comets are known.

  50. When was Halley's comet last seen ? When will it be seen next ?

         ANS :Halley's comet was last seen in 1986 AD and it will be seen next in 2062 AD.

  51. Which comet was seen in 1997 AD ?

         ANS :Heilbob was the comet seen in 1997 AD.

  52. What is the composition of comets ?

         ANS :A comet basically consists of water, carbon dioxide, ammonia, other frozen gases and dust particles.

  53. Which molecules are believed to have come to earth from the tails of comets ?

         ANS :Molecules like CO and HCN are believed to have come to earth from the tails of comets.

  54. What did the ancient people believe to happen when comets arrived ?

         ANS :Ancient people believed that the arrival of a comet was responsible for war, epidemic or a natural calamity like flood.

  55. What are stars ?

         ANS :The self-luminous celestial objects which produce energy in the form of radiation due to the thermonuclear fusion process taking place within them are called stars.

  56. Mention the two main elements present in stars.

         ANS :Hydrogen and helium are two main elements present in stars.

  57. What is celestial sphere ?

         ANS :An imaginary sphere covering the sky with the earth as its centre is known as the celestial sphere.

  58. How many Nakshatras are there ?

         ANS :There are 27 Nakshatras.

  59. On what basis are Nakshatras given their name ?

         ANS :The Nakshatras are given their names on the basis of the imaginary figures formed by luminous stars.

  60. How much time does earth take to complete one rotation about its own axis ?

         ANS :Earth takes 23 hours and 56 minutes to complete one rotation about its axis.

  61. What is the angular displacement of a star in celestial sphere in one day ?

         ANS :The angular displacement of a star in celestial sphere in one day is about 1°.

  62. What is a galaxy ?

         ANS :Galaxy is a big cluster of stars in space.

  63. What is the name of our galaxy ?

         ANS :The name of our galaxy is Milky Way.

  64. What is the shape of our galaxy ?

         ANS :The shape of our galaxy is spiral.

  65. What is the thickness of the middle part of our galaxy ?

         ANS :The thickness of the middle part of our galaxy(Milky Way) is about 15 to 20 thousand light years.

  66. Define : Light year.

         ANS :Light year is the distance travelled by light in one year.

  67. Mention some Nakshatras.

         ANS :Ashwini, Swati, Vishakha, Pushya, Jyeshtha, etc. are some of the Nakshatras.

  68. What is the distance of Sun from the galactic center ?

         ANS :The distance of Sun from the galactic center is about 30000 light years.

  69. What is the speed of revolution of Sun ?

         ANS :The speed of revolution of Sun is about 250 km/s.

  70. How much time does sun take to complete one revolution around the galactic center ?

         ANS :Sun takes about 22.5 crore (225 million) years to complete one revolution around the galactic center.

  71. Mention two main shapes of galaxies.

         ANS :The two main shapes of galaxies are : Spiral and Elliptical.

  72. What is the colour of most of the stars in elliptical galaxies ?

         ANS :The colour of most stars in elliptical galaxies is red.

  73. What is the colour of most of the stars in spiral galaxies ?

         ANS :The colour of most of the stars in spiral galaxies is blue.

  74. Elliptical galaxies are older than spiral galaxies. True or false ?

         ANS :This is a true statement.

  75. Stars in spiral galaxies are young. True or false ?

         ANS :This is a true statement.

  76. The number of elliptical galaxies is more than that of spiral galaxies. True or false ?

         ANS :This is a false statement.

  77. On what does the rate of receding away of galaxies from one another depend ?

         ANS :The rate of receding away of galaxies from one another is directly proportional to the relative (intergalactic) distance between them.

  78. How many years ago did Big Bang take place ?

         ANS :The Big Bang took place about 13.7 billion years ago.

  79. What is the basic concept (belief) of Steady-state theory ?

         ANS :The Steady-state theory believes that the Universe appears the same at all times and remains unchanged.

  80. What is used as liquid fuel in rockets ?

         ANS :A mixture of liquid hydrogen and liquid oxygen is used as liquid fuel in rockets.

  81. Give an example of a solid fuel used in a rocket.

         ANS :A powdered mixture of aluminium and ammonium perchlorate (or ammonium nitrate) is used as solid fuel in a rocket.

  82. What is 'pay-load' ?

         ANS :'Pay-load' includes all scientific equipment, satellite, etc. that is carried by a rocket (or a spacecraft) to the space.

  83. What type of rocket is required to carry heavy payloads ?

         ANS :A multistage rocket is required to carry heavy payloads.

  84. What is the full form of PSLV ?

         ANS :The full form of PSLV is Polar Satellite Launch Vehicle.

  85. What is the full form of GSLV ?

         ANS :The full form of GSLV is Geosynchronous Satellite Launch Vehicle.

  86. What can a PSLV do ?

         ANS :A PSLV can project a satellite of 1000 kgmass in a polar orbit at 900 km height.

  87. What can a GSLV do ?

         ANS :A GSLV can project a satellite of 2500 kg mass in a geostationary orbit at 35786 km height.

  88. From where and when the EDUSAT launched ?

         ANS :EDUSAT was launched on September 20, 2004 from Dr. Satish Dhavan Satellite Launching Station, Shriharikota, in Andhrapradesh.

  89. What is the purpose of EDUSAT ?

         ANS :The purpose of EDUSAT is educational activity through satellite.

  90. How many rockets are there in a space shuttle ?

         ANS :A space shuttle has in all five rockets (engines).

  91. What is remote sensing ?

         ANS:Remote sensing is a method by which information about a substance, matter or a phenomenon can be obtained using scientific instruments without direct contact with them.

  92. Which type of information can be obtained by remote sensing satellites ?

         ANS:Informations like geological survey of metallic ores in the earth's crust, environmental changes, water resources, agricultural resources, diseases spread in crops, oceanic phenomena, etc can be obtained by remote sensing satellites.

  93. How is energy produced in stars ?

         ANS:Energy is produced by thermonuclear fusion process in stars.
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Friday, February 02, 2007

ENERGY SOURCES - III :LONG ANSWERS

GIVE ANSWER IN DETAIL



*INDEX-TOPIC SEARCH


  1. Explain renewable and nonrenewable sources of energy and mention the advantage of such classification.
    ANS:

    • Those sources of energy which will be available as long as the heat from sun reaches the earth are called renewable sources of energy.

    • The renewable sources are generally pollution-free.

    • Solar energy, hydel energy, wind energy, tidal energy, etc. are renewable sources of energy.

    • Those sources of energy whose formation takes millions of years and which are limited in supply are called nonrenewable sources of energy.

    • Nonrenewable sources create heavy pollution.

    • Coal, petroleum, natural gas, etc. are nonrenewable sources of energy.

    • The advantage of such classification of energy sources is that it helps us to decide about the conservation of available energy sources for future generation.

    • It also helps us in development of alternative sources of energy accelerating the scope for the development of appropriate technology.



  2. Explain the composition of sun's light.
    ANS:

    • Sun's light consists of electromagnetic waves of different wavelengths and reachea the earth in the form of radiation.

    • The waves having wavelength of 4000 A° to 8000 A° are called visible light because these waves cause sensation in our eyes.

    • The visible light consists of waves of different colours. The wavelength of violet light is about 4000 A° and that of red light is about 8000 A°.

    • Our eyes are not sensitive to waves with wavelength less than 4000 A° and greater than 8000 A°.

    • The radiations with wavelength more than 8000 A° are called infrared rays and those with wavelength less than 4000 A° are called ultraviolet rays.

    • X-rays and γ-rays are electromagnetic waves having wavelength less than that of ultraviolet rays.

    • A large part of these radiations is made up of visible light whereas about one-third of the light consists of infrared rays which provide us with heat.



  3. Write a note on solar cooker.
    ANS:



    • Solar cooker is a device that uses solar energy in the form of heat for cooking food.

    • As shown in figure, solar cooker consists of a container box whose body is made up of nonconducting plastic or fibrous material and insulated from outside to prevent heat loss.

    • A plane mirror is fixed on the top of the box inb such a way that it reflects sunrays into the box.

    • Solar cooker contains small containers painted black externally to absorb heat.

    • The box is covered with a glass-sheet to retain the heat inside due to greenhouse effect. This heat develops about 100-140°C temperature inside the box within 2-3 hours when placed in sunrays.

    • This type of cooker can be used to prepare food items like rice, dal, pulses, vegetables,etc.

    • ADVANTAGES

      1. There is no combustion of fuel.

      2. Maintenance is negligible.

      3. It is pollution-free.

      4. The natural taste of the food is maintained as it conserves all nutrients.

      5. During the preparation of food no personal attention is needed.



    • LIMITATIONS

      1. Food cannot be cooked on a cloudy day or in absence of direct sunlight

      2. It takes very long time for cooking.




  4. Write a note on solar water heater.
    ANS:



    • Solar water heater is a device which uses solar energy to heat water.

    • As shown in figure, a copper pipe with its external surface painted black is fixed in the form of a coil in a box similar to that of a solar cooker. The pipe in coiled form increases the surface area for heating.

    • The cold water is stored in an overhead reservoir tank kept at a higher level. This tank is connected to another tank kept at a lower level slightly above the heater.

    • One end of the copper pipe of the solar heater is connected to the bottom of the small tank and the other end is connected about halfway from the top of that tank.

    • This arrangement creates a pressure difference in the small tank and water keeps on circulating repeatedly through the coil of the heater.

    • The hot water being lighter than the cold water remains in the upper region of the tank and can be taken out for use as and when needed.



  5. Write a note on solar concentrators.
    ANS:

    • A solar concentrator is a device that receives energy from the sun on a large area and concentrates it onto a very small area.

    • The principle of its working is that a parallel beam of light incident on a concave mirror is focussed at the principal focus after reflection.

    • A much higher temperature is obtained using such heating devices.

    • A solar concentrator is generally made of a large spherical or parabolic reflector mounted on a stand such that it receives maximum amount of solar radiation. For this we can rotate a reflector in such a way that it faces the sun throughout the day.

    • A simple device using concentrators can attain the temperature in the range of 180-200°C. Such devices are used domestically to heat water and cook food.

    • For commercial applications, a large number of small plane mirrors is arranged in such a way that all of them concentrate the solar radiation over a small area. Such solar concentrators are useful in generating electricity.

    • In a solar tower, a concentrator kept at a height of 50-70 m from the ground is used to vaporise water and the steam rotates the turbines of a generator.

    • The solar furnace created at Mount Louis in France attains the temperature as high as 3000°C with the help of more than about 3500 small mirrors.



  6. Write a note on Solar Cells.
    ANS:



    • Solar cell is a device that converts solar energy directly into electrical energy.

    • Earlier, it was observed that when solar rays fall on a thin wafer of selenium, electricity is generated.

    • A solar cell made of selenium wafer converts only 0.7% of solar energy into electrical energy which is a very small output and hence impracticable for generating electricity.

    • The first solar cell made in 1954 could convert about 1% of incident solar energy into electricity.

    • Modern solar cells most commonly made from semimetal silicon can have efficiency of upto 25%. Silicon is easily available and is eco-friendly.

    • A typical solar cell consisting of a 2 x 2 cm square piece of pure silicon can produce about 0.7 watt electricity with about 10% efficiency.

    • This electricity is quite small and to get large amount of electricity several solarcells are connected together in series. This arrangement is called Solar Panel.

    • ADVANTAGES:

      1. Solar cells provide eco-friendly environment.

      2. Solar cells can be used anywhere as a self-generating source of electricity.


    • LIMITATIONS:

      1. The availability of the special grade silicon required for making solar cells is very limited.

      2. The cost of connecting material used in solar panels is very high as it is generally silver.

      3. The current produced by solar cells is direct current (DC) and it has to be converted to alternating current (AC) for some devices. This involves high cost and loss of efficiency.

      4. The conventional method of storing solar energy in storage batteries is not efficient enough.



    • USES:

      1. In artificial satellites.

      2. In radio wireless transmission.

      3. In TV transmission.

      4. In traffic signals and research centres.

      5. In calculators and watches.

      6. In solar cars.





  7. Write an explanatory note on WIND ENERGY.
    ANS:

    • Sun's energy is the main factor responsible for the movement of air. Moving air is called wind and it possesses kinetic energy.

    • Windmills, invented 250 years ago by Persians, are once again gaining popularity.

    • Modern windmills convert wind energy into either mechanical energy or electrical energy.

    • A windmill consists of a fan-like structure mounted at some height on a strong support. Its blades are so designed that when wind strikes them, a pressure difference is created between them. This difference produces a turning effect and makes them rotate.

    • The height of the windmill, the number of blades and their shape, etc. are decided on the basis of average wind velocity and local environmental factors.

    • The rotational motion of the windmill is utilised to obtain mechanical work like in a water-lifting pump.

    • If a turbine is connected to the windmill, electricity can be generated but the electricity generated by one windmill cannot be used commercially.

    • To generate a lrage amount of electricity, a wind energy farm is established where there are several windmills erected in a large area.

    • In Gujarat, wind-energy farms are located at following places : Lamba near Porbandar, Okha, Mandavi and Dhank.

    • The largest windfarm in India is near Kanyakumari in Tamilnadu and it generates 300 MW electricity.

    • The greatest advantage of wind-energy farms is that the electricity is generated from a renewable source and it is pollution-free.

    • LIMITATIONS

      1. Wind energy farms can be established only at places with continuous flow of wind.

      2. The average wind velocity should be 16 km/h to enable its working.

      3. It requires a large area of land and cost of installation is very high.

      4. It creates noise pollution.





  8. Write an explanatory note on hydel (hydroelectric) energy.
    ANS:

    • The electricity generated by moving the turbines of a generator using the energy of flowing water ( or falling water) is called hydroelectric energy or hydel energy.

    • During rain or snowfall, the potential energy of water molecules is converted into kinetic energy. When ice at higher altitudes melts, its gravitational potential energy is converted to kinetic energy and it flows as a stream of water or as a river.

    • If we store rainwaters by constructing a dam, the water has higher potential energy. When this water is allowed to fall from a height, its potential energy is converted to kinetic energy.

    • This kinetic energy of flowing water can be used to rotate the turbine blades along with the armature of electric generator to produce electricity.

    • The hydroelectric power station at Ukai on the river Tapi in Gujarat generates 300 MW of electricity. A hydroelectric power station has been constructed on river Narmada at Sardar Sarovar dam.

    • Mini hydroelectric power plants can be constructed in hilly regions or at small dams if the water falls through the height of at least 10 meters.

    • ADVANTAGES:

      1. Once completed, the hydroelectric power station needs only maintenance and proper management which is not much costly.

      2. A dam constructed for hydroelectric power station can also be used for irrigation and it prevents floods.


    • LIMITATIONS:

      1. The cost of installation (construction) of a hydroelectric power station is very high.

      2. The requirement of a large area for constructing the dam leads to destruction of forests and causes a great ecological imbalance.




  9. What is OTEC ? Describe the working of OTEC plants.
    ANS:



    • OTEC stands for Ocean Thermal Energy Conversion process.

    • Oceanic waters absorb very large amount of solar energy during daytime. This creates sizeable temperature difference between water at the surface and water in depth. This temperature difference can be used to convert thermal energy into electrical energy.

    • PRINCIPLE: If a source of energy is at higher temperatureand the sink (where unused energy is released) is at lower temperature, then a fraction of the heat energy obtained from the source can be converted to electrical energy by running a turbine.

    • The hot water at the surface of the ocean works as source and comparatively cold water at the depth acts as the sink.

    • The temperature difference between these two is used to produce electricity.

    • A temperature difference of at least 20°C is the primary requirement for OTEC. This difference ois available at the depth of about 700 - 900 m. Such places are found between 20°N and 20°S latitudes where such temperature difference is maintained throughout the year.

    • The benefit of using ocean thermal energy is that this energy is available round the clock whereas solar energy is available only during the daytime.



  10. Explain Tidal Energy OR How can electrical energy be generated using tidal energy of the sea (ocean) ?
    ANS:

    • The level of water near the seacoast changes twice a day. This everyday movement of water along the seashore is known as tides. A high tide occurs on every new moon day and on a full moon day. During the tide the water level rises by a few meters.

    • The energy possessed by the rising and falling water is known as tidal energy.

    • As shown in the figure, a dam is constructed across a narrow opening of a sea. Due to tide, the water moves in and out through the openings and flows over the turbines fixed inside the walls of the dam. The movement of turbines generates electricity.

    • Tides are not uniform and the rise and fall of water is not large enough to generate electricity on a large scale. Also, there are a very few sites where we find a narrow opening suitable for a dam.

    • Wind energy is an indirect form of solar energy. Wind possesses tremendous kinetic energy and causes waves in the oceans. The height of such waves is great when the wind velocity is high. Such waves can be used to generate electricity by rotating turbines using their energy.

    • The greatest limitation of this method is that the location of turbine and other equipment is in the sea. They require a lot of maintenance and replacement. This makes them costlier.



  11. Write a note on Geothermal energy.
    ANS:

    • Geothermal energy is the energy obtained from the heat of the earth.

    • Geothermal energy does note depend directly or indirectly on solar energy.

    • The interior region of the earth below its crust is very hot and it consists of molten rocks called 'magma'. Magma is in semisolid state.

    • The geological changes push this semiliquid magma towards upper surface and it gets collected at some depth below the surface of the earth. These places are called 'hot spots'. Hot spots are considered as the source of geothermal energy.

    • When the underground water comes in contact of such hot spots, it turns into steam which is trapped in the space between rocks and becomes compressed due to high pressure.

    • This steam can be collected by introducing after drilling holes. These pipes reach upto the depth of hot spots. The steam coming up through the pipes can be used to run turbines.

    • Sometimes the steam finds its way through the cracks in the crust and comes up to the surface in the form of geysers. The steam from such geysers has the temperature of about 150 -200°C.

    • Geothermal energy is an ecofriendly source of energy and the cost of electricity generated from it is almost half of that of the electricity generated nusing conventional methods.

    • ADVANTAGES:

      1. This energy can be used for 24 hours throughout the year.

      2. The cost of production is comparatively low.

      3. It is eco-friendly, i.e. pollution-free.


    • A large number of geysers is found in USA and New Zealand but we have limited places in India for geothermal energy.

    • In Gujarat, we have geysers at places like Unai, Tulsi Shyam in Saurashtra, and Lasundra and Tuva villages in Godhra District.



  12. Describe with diagram the process of preparation of charcoal.
    ANS:



    • AIM: To obtain charcoal from wood.

    • APPARATUS-MATERIAL: Hard-glass test tubes, stand, Bunsen burner, bent glass tubes, pieces of wood, water, matchstick, etc.

    • PROCEDURE: Arrange the apparatus as shown in the figure. Put some pieces of wood in the horizontal glass tube. Take some water in the other (vertical) glass tube. Slowly heat the test tube containing pieces of wood with the help of the burner.

    • OBSERVATIONS:

      1. A black liquid begins to drip in the test tube containing water and settles at the bottom. This thick black semifluid is tar.

      2. If a lighted matchstick is brought near the outlet of the bent tube fitted to the test tube containing water, the gas released there starts burning. This is coalgas.

      3. In the end, a black residue is left in the test tube being heated. This is charcoal.




  13. Write a note on biogas.
    ANS:



    • Biogas is produced by the decay of biomass and it consists of 65-75% methane, 30-40% carbon dioxide as well as traces of H2,H2S, and N2.

    • Because biogas is generally obtained from the solid wastes of animal dung, sewage, crop residues, agro-wastes and poultry droppings, it is traditionally called 'Gobar gas'.

    • Methane is an excellent fuel. The calorific value of biogas is 35-40 kJ/g.

    • Two types of biogas plants are commonly used in India.They are :

      1. Fixed-Dome Type

      2. Floating Gas-Holder Type


    • The floating gas-holder type plants have been prepared by the Khadi and Village Industries Commission (KVIC) and are used where biogas is obtained from the animal dung. It consists of a dome like structure, made of steel, that floats up as the pressure of biogas evolved increases.

    • The fixed-dome type structure has a longer life. Its dome can be made from bricks. Thus, the cost of the plant is reduced. This type of plant is more suitable for obtaining biogas from human excreta and other bio-wastes.

    • PROCESS:

      • A slurry of dung, other biowastes and water is prepared in a mixing tank.

      • This slurry is fed into the digester which is a closed underground tank made of bricks.

      • The digester is sealed from the top.

      • The anaerobic micro-organisms decompose the biomass in the digester and produce biogas which is a mixture of fuel gases like methane, hydrogen, hydrogen sulphide with carbon dioxide and nitrogen.

      • After a few days, sufficient gas is produced which is supplied to the places of use.


    • ADVANTAGES:

      1. It can be supplied to the consumers through pipes.

      2. Its main component methane is an excellent fuel and burns without smoke.

      3. It does not produce ash during burning.

      4. It can be used as a fuel for domestic heating, cooking, lighting as well as to generate electricity.

      5. It can be supplied to industries as a fuel.

      6. The slurry left behind in the digester serves as an excellent manure for farmers.

      7. It provides us with a scientific method of disposal of organic wastes.




  14. Explain Hydrogen as a fuel.
    ANS:

    • Hydrogen, when burnt in presence of oxygen, produces a lot of heat energy and water with explosion.

         H2 + 1/2 O2 → H2O + Heat

      Due to its high calorific value it can be considered as an alternative source of energy.

    • Hydrogen is obtained as a by-product in many industrial processes. It is also produced during electrolysis of water.

    • ADVANTAGES:

      1. Its combustion produces water. Therefore, it is free from pollution.

      2. Its calorific value is 150 kJ/g which is the highest among known fuels.

      3. It is easily available as the main by-product of some industrial processes.


    • USES:

      1. For the production of methanol, ammonia and hydrocarbons.

      2. As a fuel in space-ships.

      3. As a source of energy in high temperature flames.


    • LIMITATION:The combustion of hydrogen is highly explosive in nature. Thus, it cannot be used as a source of energy both for domestic as well as industrial applications.



  15. Write a note on Coal.
    ANS:

    • Cual is a conventional source of energy.

    • Coal mainly consists of carbon with traces of other elements like hydrogen, oxygen, nitrogen, phosphorus, pottasium, etc.

    • It is available as deposits below the surface of the earth.

    • India's first coal mine was dug in Raniganj of West Bengal in 1854 AD.

    • Based on the carbon content, coal is classified as peat, lignite, bituminous and anthracite.

    • Peat contains about 27% of carbon (which is the lowest).

    • Lignite contains about 28-30% carbon.

    • Bituminous coal contains about78-87% carbon.

    • Anthracite contains about 94-98% carbon.

    • Anthracite is the best quality coal which does not produce any residue(ash) after burning.

    • As a fuel coal burns in presence of oxygen to form CO2 and produces a lot of heat.

    • It is widely used as domestic as well as industrial fuel and for generating electricity in thermal power stations.

    • Coke (a form of carbon) is produced from coal by its destructive distillation.

    • Coke is used as a reducing agent in the extraction of metals from their ores and for preparation of steel.



  16. Describe fractional distillation of petroleum with neat diagram and write the names of the products obtained.
    ANS:

    • Petroleum (crude oil) is a complex mixture of many hydrocarbons. It cannot be used in crude state. Its components are separated by fractional distillation process.

    • As shown in figure, the fractional distillation tower is about 31 m high cylindrical structure with 3 m diameter. It is made of iron.

    • It consists of trays made of specially designed bricks. These trays are kept at regular intervals from bottom to the top of the tower. They are porous and have bubble caps.

    • The crude oil is heated to about 400-430°C temperature before being introduced into the tower from its base.

    • All the hydrocarbons are vaporised at this temperature and as these vapours rise up, they get cooled down. The component having the highest boiling point gets condensed first and becomes liquid. Similarly all the components condense gradually into liquid form and get collected over different trays kept at different levels.

    • The residue left as tar and bitumen is collected at the bottom of the tower.



  17. Write a descriptory note on the products (components) obtained during fractional distillation of petroleum.
    ANS:

    • PETROLEUM GASES:
        >
      • Petroleum gases are collected at the top as they do not condense.

      • It is a mixture of hydrocarbons like methane, ethane, propane and butane.

      • Butane is an easily combustible gas and is a very good domestic fuel.

      • Butane is converted into liquid under pressure and stored incylinders as LPG (Liquefied Petroleum Gas) to use it as domestic fuel.

      • When we release the pressure in the cylinder, this liquid butane gets converted into gas and reaches the burner through pipes.

      • LPG is highly inflammable gas and catches fire immediately.

      • It is difficult to detect its leakage as it is odourless. To avoid disaster due to leakage a small amount of foul smelling mercaptan is added to it. The smell of mercaptan helps us take precautionary measures to avoid accident.


    • PETROL:

      • It is obtained in the temperature range of 40 to 200°C.

      • Its proportion in petroleum is about 45% and is also known as 'gasoline'.

      • The hydrocarbon molecules in it contain 5-10 carbon atoms.

      • Its calorific value is 47 kJ/g.

      • It is used as a fuel in automobiles.


    • KEROSENE:

      • It is obtained in the temperature range of 200-300°C.

      • The hydrocarbon molecules in it contain 10-14 carbon atoms.

      • Its calorific value is 48 kJ/g.

      • It is used domestic fuel for cooking and in lanterns. Highly refined kerosene is used as a fuel in jet planes.


    • DIESEL:

      • It is obtained in the temperature range of 300-350°C.

      • The hydrocarbon molecules in it contain 14-20 carbon atoms.

      • Its calorific value is 45 kJ/g.

      • It is used in heavy vehicles like trucks, buses, tractors, railway engines, steamers as well as in water pumps and generators.

      • It was invented by Rudolph Diesel.


    • LUBRICATING OIL:

      • It is obtained in the temperature range of 350-400°C.

      • The hydrocarbon molecules in it contain more than 20 carbon atoms.

      • It is used in the preparation of grease and wax.


    • PETROLEUM WAX:It is obtained in semifluid state above 400°C and is used in the preparation of candles.

    • ASPHALT:The thick, black, viscous liquid left as residue is called asphalt(bitumin) and is used in the preparation of roads and as a water repellant.



  18. Write a note on Natural Gas.
    ANS:

    • Usually natural gas is found with petroleum.

    • It mainly consists of methane which is a very good fuel along with some hydrogen.

    • Natural gas has the advantage that it can be transported to both domestic as well as industrial consumers through the pipelines.

    • It can be compressed to liquid state when it is called Liquefied natural Gas (LNG) or Compressed Natural Gas (CNG) and it can be transported through tankers easily.

    • Its combustion produces comparatively less pollution as only carbon dioxide and water are formed after combustion.

    • In India natural gas is found in Khambhat, Tripura, Jaisalmer, Bombay High and KG (Krishna-Godavari) basins. India's reserves of natural gas are estimated to be about 100 billion cubic meter.

    • In Gujarat, the power station at Dhuvaran uses natural gas to generate electricity.

    • The uses of natural gas are:

      • As a fuel in industries and power stations.

      • In the manufacture of artificial fertilisers like ammonia and urea.

      • As a source of hydrogen gas.

      • As a source of methane for use as domestic fuel.





  19. Write a note on Conditions of Burning.
    ANS:Burning or combustion is an exothermic process in which a substance combines with oxygen and a lot of heat is produced. The following are the conditions which must be satisfied for burning to take place.


    1. IGNITION TEMPERATURE: Ignition temperature is the minimum temperature to which a substance must be heated in presence of oxygen to start burning. A substance does not burn until it reaches its ignition temperature. Thus, a substance must be heated upto its ignition temperature to start burning.

    2. OXYGEN: Adequate supply of oxygen is necessary for efficient combustion. If the supply of oxygen is sufficient, the process is called COMPLETE COMBUSTION and if the supply is insuuficient then it is called INCOMPLETE COMBUSTION. During complete combustion the fuel burns with blue flame and maximum amount of heat energy is released. During incomplete combustion, heated unburnt carbon particles give out yellow light and the complete energy of the fuel is not released.

      Stoves using kerosene are designed such that they get sufficient oxygen from their surroundings. On the other hand, kerosene lamp or lantern used for light is designed such that it does not get sufficient oxygen and gives yellow light.

    3. FUEL SUPPLY: For combustion to continue properly, a minimum level of the fuel supply must be maintained.



  20. Mention the characteristics of the Ideal Fuel.
    ANS:The characteristics of an ideal fuel are:

    • It should be available easily and in enough quantity.

    • Its ignition temperature should be well above the room temperature and in accordance with the necessity.

    • It should have high calorific value.

    • It should burn completely and leave minimum amount of volatile material.

    • Its storage and transportation should be easy and safe.

    • It should be economical (less costly).

    • It should not produce poisonous gases on combustion and thus cause minimum pollution.



  21. Describe an experiment to find out calorific value of wax.
    ANS:

    • AIM: To determine the calorific value of wax.

    • APPARATUS & MATERIAL: Beaker, tripod, thermometer, water, candle, etc.

    • PROCEDURE:Take a candle and record its mass(weight). Take fixed quantity (say, 100g) of water in the beaker and measure its initial temperature. Heat the water with the help of the candle and extinguish the candle after some time when the temperature of water has risen sufficiently. Record the final temperature of water as well as the mass of the extinguished candle.

    • OBSERVATIONS:

      1. Initial mass of the candle = W1 gram

      2. Mass of water = m gram

      3. Initial temperature of water = t1 °C

      4. Final temperature of water = t2 °C

      5. Final mass of the candle = W2 gram


    • CALCULATION:

      1. Quantity of wax used = W = W1 - W2 gram

      2. Increase in the temperature of water = t = t2 - t1 °C



      Now, Heat absorbed by water Q = m x s x t
              [where s is the sp.heat of water]

          ∴ Calorific value of wax = Q/W = (m x s x t)/t

    • CONCLUSION:The calorific value of wax is _______ cal/g.



  22. Explain Nuclear Fission.
    ANS:

    • During an attempt to produce transuranic elements, Prof. Fermi and his coworkers observed that when neutrons are bombarded on a nucleus, β-particles are emitted.

    • Hahn and Strassman found that 92U235 on bombardment by a neutron produces 56Ba139 which emits β-radiation and gets converted into 57La140.

    • The splitting of a heavy nucleus into daughter elements spontaneously or as a result of bombardment by a neutron is called NUCLEAR FISSION.

    • There are two categories of nuclear fission:(1) Prompt fission (2) Delayed fission.

    • If an atom (like uranium) is bombarded by a high-energy neutron, its nucleus splits immediately. This is prompt (rapid) fission.

    • In delayed (slow) fission, the bombarded neutron is absorbed by the target nucleus (uranium) and excites the nucleus which splits later on.

    • During nuclear fission, the heavy nucleus obtained after fission has atomic mass 130 - 149 u and the light nucleus has the atomic mass 85 - 104 u.

    • If the nucleus of a heavy element (or its isotope) splits without being bombarded by any particle, the fission is called 'Spontaneous fission'.

    • During nuclear fission, some neutrons are released. For example, in the fission of 92U235, on an average 2 - 3 neutrons are released.

    • During fission, a large amount of energy is released. Let us take the following example:

      92U235 + 0n1 → 35Mo95 + 57La139 + 20n1

      Here the sum of the atomic masses on the left hand side is

      235.124 u + 1.009 u = 236.133 u

      and the sum of the atomic masses on the right hand side is

      94.946 u + 138.955 u + 2.018 u = 235.919 u

      the difference in the total mass of reactants and products is called 'MASS DEFECT'. The mass defect in the above example is

      236.133 u - 235.919 u = 0.214 u

      This lost mass (mass defect) is converted into energy. According to the Einstein's mass-energy equation when 1 u mass is converted, 931.48 MeV energy is produced.

      Thus, in the above example, 0.214 u mass is converted to produce 200 MeV energy which is equivalent to 3.2 x 10-11 Ws (joule) of energy.


    • The fission of 1 kg of 92U235 produces 109 W-day or 1000 MW-day energy. If only 30% of this energy is converted into electrical energy, we get 300 MW-day energy for which 2500 tons of coal are needed.

    • Electrical energy is produced by using chain reaction in which the neutrons produced in previous reaction are used to split uranium nuclei in subsequent steps. This type of fission is carried out in special type of vessels called Nuclear Reactor.




  23. What is Chain Reaction ? Describe the ideal conditions to support a chain reaction.
    ANS:



    • If in a reaction the products of one step result in producing the subsequent step of the reaction then that reaction is called 'Chain Reaction'.

    • During the fission of 92U235, two or three neutrons are released. If the energy of these neutrons is utilised to cause fission of other nuclei, then again two or three neutrons are released per nuclei. Thus, the process of fission continues as chain reaction.

    • The reaction mentioned above will go beyond control if all the neutrons released in one step are utilised to cause fission in the subsequent step. Because the number of neutrons released will go on increasing, the amount of energy produced will also go on increasing and reach an uncontrollable stage.

    • The following processes can take place after neutrons are produced in the fission reaction.:

      1. At least one of the neutrons (released during previous step) hits the other 92U235 to cause fission to release more neutrons ( & energy).

      2. The neutrons may be captured by the uranium nucleus (parent nucleus) without fission to take place.

      3. The neutrons may be captured by the other nucleus (daughter nuclei) either in fission material or in the surrounding containers even without causing fission.

      4. The neutrons may not interact with one another and the nucleus and may escape from the system.


    • The fission reaction would stop if the last three processes dominate. However, the fission reaction will continue if the first process is repeated at least once.

    • Thus, the ideal condition to support the chain reaction will be to lower the energies of released neutrons to thermal neutron energy range of 0.04 eV.

    • This can be achieved by using special substances called moderators like graphite and heavy water which take out some of the energy of the neutron.



  24. With the help of a neat diagram describe a nuclear reactor.
    ANS:



    • A nuclear reactor is used for generation of electric power, production of fissile materials, nuclear research and for the production of man-made radioactive isotopes.

    • As shown in figure a nuclear reactor consists of a core of reaction vessel called pressure chamber that can withstand very high pressures.

    • The core consists of fuel rods (uranium rods) used as the fuel as well as a moderator like heavy water or graphite.

    • The water is pumped into the pressure chamber to remove the heat generated during fission. This heat converts water into steam.

    • This steam with high pressure and temperature is used to rotate the turbines of the generator and electric power is generated.

    • The steam loses its heat in the process and cools down. It is allowed to cool further and the water thus obtained is reused in the process.

    • The different fractions (products) obtained are : petroleum gases, petrol(gasoline), kerosene, diesel, lubricating oil, petroleum wax, tar, etc.



  25. Explain : Nuclear Fusion.
    ANS:

    • The process in which two nuclei combine to form a heavy nucleus is called Nuclear Fusion.

    • For example, two nuclei of heavy hydrogen (deuterium), 1H2 fuse to form one nucleus of helium, 2He3.

    • During nuclear fusion, the mass of the products formed is slightly less than the mass of the reactants. This is called 'Mass Defect' and the 'missing mass' is converted into energy according to the Einstein's mass-energy equation, E = mc2.

    • When two deuterium nuclei combine, following two reactions are possible:

      1H2 + 1H2 → 2He3 + 0H1 + 3.3 MeV

      1H2 + 1H2 → 1H3 + 1H1 + 4.0 MeV


    • The main advantage of the above reaction is that we obtain about 33 g of deuterium from 1 cubic meter (1000 lit) of seawater. Thus, seawater contains about 1015 tons of deuterium.

    • 1 gallon of deuterium releases energy equivalent to that released by 600 gallons of petrol.

    • The following reaction involving tritium is possible at low temperature and it gives more energy.

      1H3 + 1H2 → 2He4 + 0n1 + 17.6 MeV


    • The proportion of tritium in seawater is less but we can obtain it by bombarding lithium isotope with a neutron.

      3Li6 + 0n1 → 1H3 + 2He4

      3Li7 + 0n1 → 1H3 + 2He4 + 0n1


    • In thermonuclear fusion process, a fusion fuel is heated to 106 K when ionization takes place to form plasma.

    • Plasma is a mixture of moving charged particles with equal number of negative and positive charges.

    • It is difficult to control thermonuclear fusion. However, attempta are going on to develop a process of controlled fusion to obtain energy.

    • Uncontrolled nuclear fusion is used to prepare weapons of mass destruction like hydrogen bomb.

    • As compared to nuclear fission, the nuclear fusion has following advantages:

      1. Fuel for nuclear fusion is cheap and easily available.

      2. There is no requirement of disposing off of nuclear wastes.

      3. It is pollution free.




  26. Write a note on Nuclear Hazards.
    ANS:

    • A nuclear blast can cause death of millions of people in a few seconds.

    • It can damage huge structures like buildings.

    • Heat produced during such blast can spread into large area destroying life.

    • There are mainly two types of dangers from nuclear radiation : (1) Pathological danger (2) Genetic danger.

    • Pathological effects cause diseases like cancer which may prove fatal.

    • Genetic effects cause damage to the children of next generation in the form of various physical disabilities.

    • Leakage of radiation from nuclear reactors adversely affect all spheres of the environment.

    • The accidental leakage from a nuclear reactor in Chernobyl (Ukraine) in 1986 was so serious that about 50 tons of radioactive material had spread over a large area that thousands had died and about 2 lakh people had to be shifted to safer place. The radiation in this explosion was 200 times the radiation spread by the atomic bomb in Hiroshima and Nagasaki.

    • The disposal of nuclear wastes, rods of nuclear fuel is a great problem as it spreads radiation continuously. Such material is at present buried very deep in the earth where it is unlikely to come in contact of underground water.



  27. DISTINGUISH BETWEEN NUCLEAR FISSION AND NUCLEAR FUSION.

    ANS:
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