Bihar Board Class 10 Science Solutions Chapter 14 Sources of Energy

 

Bihar Board Class 10 Science Chapter 14 Sources of Energy InText Questions and Answers

intext Questions (Page 243)

Question 1.
What is a good source of energy ?
Answer:
A good source of energy has following characteristics :

• It could do a large amount of work per unit volume or mass.
• It would be easily accessible.
• It would be easy to store and transport.
• It would be economical.

Question 2.
What is a good fuel?
Answer:
A good fuel has following characteristics :

• It should have high calorific value.
• It should have normal ignition temperature.
• It should not leave more ash and should not produce harmful gases on burning.
• It should be economical and easy to handle.

Question 3.
If you could use any source of energy for heating your food, which one would you use and why?
Answer:
I would use renewable and pollution free sources of energy to heat my food. Because, it would not disturb the ecological balance as well as it would be reproduced in nature by cyclic process when it would be used once.

Intext Questions (Page 248)

Question 1.
What are the disadvantages of fossil fuels?
Answer:
Following are the disadvantages of fossil fuels:

• Fossil fuels are formed under the earth’s surface by a long process which takes million yearSk to form fossil fuel and there are only limited reserves.
• Fossil fuels are non-renewable sources of energy.
• Burning of fossil fuels causes air pollution. The oxides of carbon, nitrogen and sulphur that are released on burning fossil fuels cause acid rain and other respiratory problems for human beings. .

Question 2.
Why are we looking at alternate sources of energy ?
Answer:
With the progress of technology, the demand of energy is increasing continuously. Our changing life style and use of more machines to do more and more of tasks easily and rapidly has increased the demand of energy. This demand of energy cannot be fulfilled by using traditional sources of energy. This is why we are looking at alternate sources of energy.

Question 3.
How has the traditional use of wood and water energy been modified for our convenience ?
Answer:
Wood and water are traditional sources of energy. Continuous supply of energy from these sources cannot be maintained as it requires enough trees to be planted. Therefore, following modifications are for our convenience :
(i) Wood is converted into charcoal by destructive distillation of wood which is a better fuel than wood. Use of charcoal is also ecofriendly as it does not produce smoke on burning: It has higher calorific value and leaves less residue on burning.

(ii) Use of water energy needs the construction of big dams. So these dams are being constructed at limited number of places. To improve the output of water energy, the technology is being developed to increase the efficiency of devices used to convert water energy into electricity.

Intext Questions (Page 253)

Question 1.
What kind of mirror – concave, convex or plain would be best suited for use in a solar cooker ? Why ?
Answer:
Concave mirror. Because, concave mirror can focus all the radiations coming from sun properly at a point and causes more heating.

Question 2.
What are the limitations of energy obtained from the oceans ?
Answer:
The energy potential from ocean is large but efficient commercial exploitation is difficult.

Question 3.
What is geothermal energy?
Answer:
The heat from inside earth heats up the water below the surface. This hot water can be used under favourable condition as source of energy. It is called geothermal energy.

Question 4.
What is the advantage of nuclear energy ?
Answer:
Nuclear energy is a non-conventional source of energy and provides large quantity of usable form of energy. This is one advantage of nuclear energy.

Intext Questions (Page 253)

Question 1.
Can any source of energy be pollution free? Why or why not?
Answer:
Exploiting any source of energy disturbs the environment. So, we can say that any source of energy cannot be pollution free. For example, if we use wood as source of energy it causes ecological imbalance as well as it causes air pollution due to the production of CO2? SO2 and NO2like harmful gases during burning of wood directly. Even use of solar energy causes excessive trapping of infrared radiation which causes global, warming.

Question 2.
Hydrogen has been used as rocket fuel. Would you consider it a cleaner fuel than CNG? Why or why not ?
Answer:
Hydrogen is a cleaner fuel than CNG because of the following reasons:

• Hydrogen has high calorific value than CNG.
• CNG is conventional source of energy but H2 is not.
• CNG is green-house gas but H2 is npt. .
• Burning of CNG gives out CO and CO2 gases but burning of H2 does not give any such harmful gases.

Intext Questions (Page 255)

Question 1.
Name two energy sources that you would consider to be renewable. Give reasons for your choices.
Answer:
Two renewable sources of energy are :
1. Hydro, energy : Hydro energy is the energy possessed by running water. The running water at high altitude is utilized and unused water is gone to oceans. Due to cyclic process we obtain running water at high altitude again. So hydro energy is a renewable source of energy.

2. Wind energy : Wind energy is trapped to perform various activities. The wind energy is caused in atmosphere by ; natural cyclic process. So it is a renewable source of energy.

Question 2.
Give the name of two energy sources that you would consider to be exhaustible. Give reasons for your choices.
Answer:
Coal and petroleum are the two exhaustible sources of energy because even they are reproduced in nature but it takes millions years to be reproduced in nature.

Bihar Board Class 10 Science Chapter 14 Sources of Energy Textbook Questions and Answers

Question 1.
A solar water heater cannot be used to get hot water on:
(a) a sunny day
(b) a cloudy day
(c) a hot day
(d) a windy day
Answer:
(b) a cloudy day.

Question 2.
Which of the following is not an example of a biomass energy source ?
(a) wood
(b) gobar gas
(c) atomic energy
(d) biomass
Answer:
(c) atomic energy.

Question 3.
Most of the sources of energy we use represent, stored solar energy. Which of the following is not ultimately derived from the sun’s energy ?
(a) geothermal energy
(b) wind energy
(c) fossil fuels
(d) biomass
Answer:
(a) geothermal energy.

Question 4.
Compare and contrast fossil fuels and the sun as source of energy.

Answer:
Fossil fuels :

• It is conventional source of energy.
• Sun is the ultimate source of all fossil fuels.
• Fossil fuels cannot be used unjudiciously because it is limited in quantity.
• Fossil fuels can be used as source of energy at anytime.
• Use of fossil fuel causes air pollution because during burning of fossil fuel harmfal gases are evolved.
• It costs to buy fossil fuel.

Sun :

• It is nonconventional source of energy.
• Sun is the ultimate and fina source of energy on earth.
• Sun is larger source of energy and can be used at large scale as according to our need.
• Solar energy can be used only in day time.
• Solar energy is a clean fuel and can be harnessed without causing . any pollution.
• It is cost free source of energy

Question 5.
Compare and contrast biomass and i hydroelectricity as a source of energy.
Answer:
Biomass :

• Biomass is a renewable and conventional source of energy.
• Biomass possesses chemical energy.
• Use of biomass as energy causes pollution of air.
• Use of biomass does not cause ecological imbalance.
• Biomass is relatively more economical source of energy than hydroelectricity.

Hydroelectricity :

• Hydroelectricity is also a renewable and conventional source of energy.
• Hydro energy possesses kinetic energy of running water at high altitudes.
• Hydro energy is pollution free – source of energy.
• Construction of dam for hydro-electricity causes ecological imbalance.
• Hydroelectricity is relatively costly source of energy.

Question 6.
What are the limitations of extracting energy from:
(a) the wind
(b) waves
(c) tides ?
Answer:
Source of energy Limitations
(i) Wind energy :
(a) Everywhere and every time the, blowing air is not available.
(b) To generate electricity the wind should have speed more than 15 km/h.

(ii) Wave energy :
(a) The wave is not available every time for generating electricity.
(b) It is costly to set up device to trap wave energy.

(iii) Tides :
(a) The locations for constructing dams for harnessing tidal energy are limited.
(b) Construction of dams for harnessing tidal energy is costly.

Question 7.
On what basis, would you classify energy sources as
(a) renewable and non-renewable
(b) exhaustible and inexhaustible ?
Answer:
(a) If the source of energy eaii be converted back to its original form after extracting energy, then it is said to be
renewable source, e.g., hydroelectricity. If the source of energy . cannot be replenished after use, it is called non-renewable source,
e.g., coal:

(b) Sources of energy that will not exhaust in significant time are called inexhaustible sources, e.g., sun, water. Sources of energy that will exhaust in reasonable time are called exhaustible source, e.g., coal.

Question 8.
What are the qualities of ideal sources of energy ?
Answer:
The qualities of ideal sources of energy are :

• It could do a large amount of work per unit volume of mass.
• It would be easily accessible.
• It would be easy to store and transport.
• It would be economical.

Question 9.
What are the advantages and disadvantages of using solar a cooker ? Are there places where solar cookers would have limited utility ?
Answer:
Advantages of use of solar cooker:

• It cooks food without causing any kind of pollution.
• It is economical to use solar cooker because nothing is to be paid, for using solar energy.
• It is easy to handle solar cooker and there is no chance of any kind of accident.
• The nutrients of food do not get destroyed.

Disadvaptages of use of solar cooker :

• Solar cookers cannot be used at night and in cloudy weather.
• It takes more time to cook food.
• The direction of solar cooker is to be changed continuously towards the direction of sun.
• Sun energy is not available uniformly all the time and at all the places.
• It cannot be used for making chapaties and for frying.

Yes, there are some places where solar cookers would have limited utility. At poles where sun is absent for half of the year the solar cooker has limited utility. The hilly areas where the sun shines for limited time periods and where inclined sun rays reaches, the use of solar cooker is difficult.

Question 10.
What are the environmental consequences of the increasing demand for energy ? What steps would you suggest to reduce energy consumption?
Answer:
Modernisation and industrialisation have increased the demand of energy. The consequences of increasing energy are listed below:
(i) The increase of energy demand can make excessive exploitation of sources of energy which disturbs the environmental balance.
(ii) Increasing demand’ of energy makes excessive use of conventional sources of energy which are limited in nature. Thus, energy crisis for future may be caused.

Bihar Board Class 10 Science Chapter 14 Sources of Energy Textbook Activities

Activity 14.1 (Page 242)

Question 1.
Name four sources of energy you use from morning when you wake up till you reach the school. Ans. The different forms of energy are as below:

• Heat energy to cook food.
• Electrical energy to lighten the room.
• Muscular energy to carry bag and to run bicycle.
• Sound energy to call friends.

Question 2.
Name the four forms of energy and their sources.
Answer:
Forms of Energy –

• Heat energy by burning LPG.
• Electricity.
• Muscular energy
• Sound from electric bell.

Source of Energy :

• Petroleum
• From hydro energy or from thermal power station
• Food material
• Hydroelectricity or from thermal power plant.

Question 3.
Can we call these ‘sources’ of energy ? Why or why not ?
Answer:
Yes, because they do work.

Activity 14.2 (Page 243)

Question 1.
What will be your choice of fuel if you live in forest?
Answer:
My choice will be wood as a fuel.

Question 2.
What will be your choice of fuel if you live in a remote mountain village or small island?
Answer:
My choice will be wood because wood is easily available in these areas.

Question 3.
What will be your choice of fuel if you live in Delhi? ‘
Answer:
My choice will be solar energy, LPG and electrical energy.

Question 4.
What will be your choice of fuel if you lived five centuries ago ?
Answer:
Wood.

Question 5.
Consider the various options we have when we choose a fuel for cooking our food.
Answer:
Various options for cooking fuel: LPG, solar cooker, kerosene, gobar gas, charcoal, wood, upala, saw dust, etc.

Question 6.
What are the criteria you would consider when trying to categorise something as a good fuel ?
Answer:
Criteria : The fuel should be

High energy-yield.

• Easily available.
• Environment friendly
• Easily transportable.
• Low cost.
• Storable.

Question 7.
Would your choice be different if you lived five centuries ago ?
Answer:
Certainly. Five centuries ago, LPG and kerosene were not used as cooking fuel.

Question 8.
How are the factors different in each case ?
(i) in a forest ?
(ii) in a remote mountain village or small island ?
(iii) in New Delhi ?
(iv) lived five centuries ago ?
Answer:
(i) Forest : Wood becomes first choice.
(ii) Remove village : Wood and upala are used most.
(iii) New Delhi : LPG is mostly used.
(iv) Five centuries ago : Wood is the option.

Activity 14.3 (Page 244)

• Take a table tennis ball and make three slits into it.
• Put semicircularfins cut out of a metal sheet into these slits.
• Pivot the tennis ball on an axle through its centre with a straight metal wire fixed to a rigid support. Ensure that the tennis ball rotates freely about the axle.
• Now connect a cycle dynamo to this.
• Connect a bulb in series.
• Direct a jet of water or steam produced in a pressure cooker at the fins (Fig. 14.3). What do you observe ?

Answer:
The electricity is produced, which is a source of energy.

Questions Based On the activity

Question 1.
In above activity which form of energy is converted into electrical energy?
Answer:
In this activity energy stored in vapour (mechanical energy) is converted into electrical energy by using dynamo.

Question 2.
Why is tennis ball rotated ?
Answer:
Due to force exerted by the vapour pressure there is rotation in tennis ball.

Question 3.
Name the energy which is finally converted into electrical energy.
Answer:
Mechanical energy is finally changed into electrical energy in this activity.

Question 4.
Write the series of conversion of One form of energy to another form of energy.
Answer:
Heat energy → steam (vapour pressure) → rotational energy in tennis → electrical energy.

Activity 14.5 (Page 249)

• Take two conical flasks and paint one white and the other black. Fill both with water.
• Place the conical flask in direct sunlight for half an hour to one hour.
• Touch the conical flasks. You could also measure the temperature of water in the two conical flasks with a thermometer.

Question 1.
Which one is hotter ?
Answer:
Painted conical flask is hotter.

Question 2.
Can you think of ways in which findings should be used in your daily life ?
Answer:

• We use white clothes in summer.
• We use dark clothes in winters.
• The base of the cooking utensil is black.

Activity 14.6 (Page 249)

Question 1.
Which kind of surfye will absorb maximum radiation from sun ?
Answer:
Black surface is good absorber of all kind of radiations coming from sun.

Question 2.
Which kind of radiation of sun causes heating effect ?
Answer:
Heating effect is caused by infrared radiations.

Question 3.
Why are solar heating devices painted in black ?
Answer:
The solar heating devices are painted in black because black colour is the best absorber of heat. ,

Question 4.
Write one property of ultraviolet radiation.
Answer:
These radiations cause chemical effect.

Question 5.
What is maximum temperature achieved by solar cooker ?
Answer:
A box type solar cooker can attain a temperature of maximum 140°C and a solar concentrator type cooker can attain . a temperature of 180°C to 200°C.

Question 6.
Write advantages of solar energy.
Answer:
Solar energy is abundant and pollution free source of energy.
It is economical for remote areas.
Its maintenance is very less and work satisfactorily.

Question 7.
What are the limitations of solar energy ?
Answer:
It is expensive. Special grade silicon for making solar cells is limited. Its efficiency is low.

Activity 14.7 (Page 252)

Question 1.
Discuss in class the question of what is the ultimate source of energy for biomass, wind and ocean thermal energy.
Answer:
The ultimate source of energy for’biomass, wind and ocean thermal energy is Sun. The green plants utilise sunlight to prepare food by photosynthesis. The Sun is the ultimate source ; of energy for all biotic things and any subsequent energy obtained from them.

The heat coming from Sun causes uneven heating of land and soil which causes wind and energy from it. The heat of sun also causes uneven heating of ocean water which is utilised to produce energy. Thus, Sun comes out to be t the ultimate source of energy for biomass, wind and ocean thermal energy.

Question 2.
Is geothermal energy and nuclear energy different in this respect ? Why ?
Answer:
Yes. Geothermal and nuclear energy are two forms of energy which are not directly or indirectly related to solar energy.

Question 3.
Where would you place hydro-electricity and wave energy ?
Answer:
These two forms of energy are also related to sun energy indirectly.

Activity 14.8 (Page 253)

Question 1.
Gather information about various sources and how each one affects the environment.
Answer:

Question 2.
Debate the merits and demerits of each source and select the best source of energy on this basis.
Answer:
The Fossil Fuels : Coal and Oil

Advantages :

• Very large amount of energy can be generated cheaply.
• Transporting it from place to place is easy.
• Engines using fossil fuel are very efficient. That means, only a very small amount of energy is lost.
• Fossil fuels are easily obtainable and can be stored almost anywhere.

Disadvantages :

• Fossil fuels are non-renewable. Once used, they are ended permanently.
• The major disadvantage of fossil fuels is that they cause pollution.
• Fossil fuels need to be extracted from the mines. This may cause land-slide, destroying large areas.

Thermal Power –

Advantages :

• Very large amount of electricity is generated at one place cheaply.
• Transporting coal, oil and gas to the power stations is easy.
• Power stations run by petroleum gas are very efficient.
• These can be built almost anywhere.

Disadvantages :

• The major drawback of these plants is the pollution caused by burning of fossil fuels.
• Coal based power stations need huge amount of fuel in order to fulfill the demand. This keeps a large area near the power stations, occupied with piles of coal.
• It is a non-renewable source.

Hydroelectric Power –

Advantages :

• The water required for running these stations involves no cost.
• It is a renewable source.
• There is no pollution by these stations.
• It helps in irrigation for agriculture.
• It helps in control of flood.

Disadvantages :

• These power stations can be built only where there is huge supply of water.
• Constructing a dam is very costly. .
• It disturbs the neighbouring eco-system. It may even destroy inhabited localities near the dam.

Solar Energy –

Advantages:

• The cost of cooking is nothing since sun’s energy comes free of cost.
•  It does not cause pollution.

Disadvantages :

• It will not work during night or in a cloudy day since sufficient sun rays will not be available.
• It is not possible for all, living in a city to have enough open space where to use solar devices.

Solar Cells –

Advantages :

• These provide a clean, pollution free and environment friendly source of energy.
• These can be used anywhere as a self- generating source of electricity.
• Once constructed, there is no further cost of production or maintenance in such devices.
• It is a renewable source.

Disadvantages :

• These cannot work during night or in cloudy weathers.
• Manufacturing a solar cell is very costly.
• Efficient system to store electricity generated by solar cell is not available.
• These produce only direct current (DC). To run the devices which use alternating current (AC), converters are required which cost lot of energy.
• These cannot produce large amount of electricity unless the area is big.

Nuclear Reactor –

Advantages :

• Very large amount of energy is obtained in economical way
• The by-products after fission may be further utilized, [However, scientists are still to find an effective way to do so. |

Disadvantages :

• The radioactive materials required for fission emit harmful radiations like gamma rays which cause cancer, leukaemia and other genetic disorders.
• In case of an accident, it will explode like a very big nuclear bomb destroying all lives around.
• Radioactive materials are non-bio- degradable. This means waste product will continue to emit harmful radiation for a long time which will affect the living beings. That is why nuclear power plants should be kept under close observation for possible leakages.

Bio-mass –

Advantages:

• The biogas produced’from bio-mass is a clean fuel i.e., it does not cause any pollution.
• The remains of bio-mass is a very good natural fertilizer.
• It is obtained free of cost.
• It gives a safe method to dispose off wastes.
• It has no storage problem.

Disadvantages :

• The process of biogas generation is very slow, which reduces its utility for daily uses.
• The amount of biogas generated from the biomass produced in a locality is not enough to serve the fuel requirements of that locality.

Wind Energy –

Advantages :

• It is a renewable source of energy.
• It does not cause air pollution.
• It is very useful in agricultural farms for running the water pumps or grinding machines.
• It can be very useful for small scale industries which do not require high amounts of electricity.
• Once constructed, it supplies energy free of cost.
• It is the best source of energy for coastal areas.

Disadvantages :

• The amount of power generated is low.
• It causes noise pollution.
• It disturbs the rainfall pattern of the locality.
• Since wind is a natural phenomena and not accurately predictable, it is not possible to have controlled production of electricity.
• Construction of wind farms is very costly.
• It is not feasible in all localities. [Infact, only coastal areas are suited for wind energy.]
• Wind farms distort TV signals.
• Wind farms occupy a large area.

Geothermal Energy –

Advantages :

• It is dean and non-polluting.
• It can be harnessed 24 hours a day throughout the year.
• The cost of production also is very small.

Disadvantages :

• These are possible sources of energy only, in a limited number of locations.
• Some energy is consumed in pumping water down to the reservoir.

Tidal Energy –

Advantages :

• It is a renewable source of energy.
• It does not cause any pollution.
• Once constructed, there is no further cost involved.

Disadvantages :

• It cannot produce large amounts of electricity.
• It can be constructed only in particular areas of a bay.
• The construction of dam is very costly.

Considering all the advantages and disadvantages of all energy sources, we conclude that solar energy is the mos; preferable energy.

Activity 14.9 (Page 254)

Question 1.
Debate these two issues in class:
(а) The estimated coal reserves are said to be enough to last us for another two hundred years. Do you think we need to worry about coal getting depleted in this case ? Why or why not ?
(b) It is estimated that the Sun will last for another five billion years. Do we have to worry about solar energy getting exhausted? Why or why not?
Answer:
(a) It would be concluded tnat coal is exhaustible source of energy and a prime source of energy. So, we should use coal as source of energy judiciously and should use alternate source of energy. We need not to worry because upto that time we have another source of energy.
(b) No. I hope upto that time, we can discover another solar system.

Question 2.
Give one example of each kind of sources of energy –

1. exhaustible
2. inexhaustible
3. renewable
4. non-renewable

Answer:
Source of energy :

• Energy from coal, petroleum and natural gas
• Energy from wind
• Hydro and wind energy
• Coal, petroleum and natural gas.

Kind, of source :

• Exhaustible
• Inexhaustible
• Renewable source of energy
• Non-renewable source of energy.

Bihar Board Class 10 Science Chapter 14 Sources of Energy NCERT Exemplar Problems

Short Answer Type Questions

Question 1.
Why is there a need to harness non-conventional sources of energy? Give two main reasons.
Answer:
(i) Day-by-day our demand for energy is increasing to improve the quality of life and growth of population,
(ii) Fossil fuels are limited.

Question 2.
Write two different ways 6f harnessing energy from ocean.
Answer:
Tidal Wave and OTEC.

Question 3.
What steps would you suggest to minimise environmental pollution caused by burning of fossil fuels? .
Answer:

• Smokeless appliances should be used.
• Afforestation should be promoted.

Question 4.
What is the role of a plane mirror and a glass sheet in a solar cooker?
Answer:
Plane mirror acts as a reflector. It reflects sun rays into the box. Glass sheet gives green house effect i.e. it traps heat.

Question 5.
Mention three advantages of a solar cell.
Answer:

• Solar cells have no moving parts.
• Requires little maintenance.
• They can be set tip in remote areas.

Question 6.
What are the limitations in obtaining energy from wind?
Answer:

• Because of it being erratic, energy cannot be harnessed continuously.
• Large areas are required for wind farms.

Question 7.
Suggest the steps to minimise the pollution caused by various sources of energy including non- conventional sources of energy.
Answer:
Steps to minimise the pollution :

• The pollution caused by burning of fossil fuel can be reduced by increasing efficiency of combustion and using various techniques to reduce the emission of harmful gases into the surroundings.
• By improving the technology for using conventional sources of energy we can improve the efficiency of these fuels.
• By destructive distillation of wood, we can get charcoal which is better fuel than biomass. Theuse of charcoal gives less smoke, and it has a higher heat generation efficiency.
• The efficiency of agricultural waste and waste of animal arid plant can be increased by making their use to produce biogas.
• Nuclear wastes should be disposed off safely.

Long Answer Type Questions

Question 8.
Which is the process used to harness nuclear energy these days? Explain it briefly.
Answer:
Nuclear energy is generated by the process of nuclear fission. In this process called nuclear fission, the nucleus of a heavy atom, (such as’uranium, plutonium or thorium), when bombarded with low-energy neutrons, can be split apart into lighter nuclei. When this is done, a tremendous amount of energy is released. The fission of an atom of uranium, for example, produces 10 million times the energy produced by the combustion of an atom of carbon from coal. In a nuclear reactor designed for electric power generation, such nuclear ‘fuel’ can be part of a self-sustaining fission chain reaction that releases energy at a controlled rate. The released energy can be used to produce steam and further generate electricity. –

Question 9.
How can solar energy be harnessed? Mention any two limitations in using solar energy. How are these limitations overcome?
Answer:
Solar energy can be trapped by devices like solar cooker, solar heater arid solar cells. Solar cooker is a device where solar energy is utilised to cook food. All solar devices are covered with glass sheet. Glass has unique property to propagate short wavelength radiations but it does not allow to pass longer wavelength radiations. Thus, the shorter wavelength radiations are trapped. So solar devices are covered with glass sheet.

Solar cell is. a device which convert solar energy into electrical energy. Metals like silicon, germanium, selenium etc, are used to fabricate the solar cell. A large number of solar cells are combined in an arrangement to make solar panel.

Limitations :

• It is expensive
• Special grade silican for making solar cells is limited.
• Its efficiency in low.
• It will not work during night or in a cloudy day since sufficient sun rays will not be available.
• It is not possible for all, living in a city to have enough open space where to use solar devices.

Question 10.
Make a list of conventional and non- conventional sources of energy. Give a brief description of harnessing one non-con ventional source of energy.
Answer:
Conventional sources of energy : Fossil fuels, water, wind, Biomass etc.
Non-con ventional sources of energy : Nuclear energy, solar energy, tidal energy, wave energy, ocean thermal energy, geothermal energy.

Tidal energy :
Tidal energy can be harnessed for producing electricity. A dam is constructed across a narrow opening to the ) sea. The water moves in and out of the openings in the dam during high and low tides. When the water flows through the openings, it is made to flow over turbine blades which rotates the armature of the generator and produces electricity.

Question 11.
Why is there a need for harnessing non- conventional sources of energy? How can energy be harnessed from the sea in different ways?
Answer:
With the progress of technology., the demand of energy is increasing continuously. Our changing life style and use of more machines to do more and more of tasks easily and rapidly has increased the demand of energy. This demand of energy cannot be fulfilled by using traditional sources of energy. This is why we are looking at alternate sources of energy.
Futher, burning of fossil fuels cause air pollution. The oxides of carbon, nitrogen and sulphur that are released on burning fossil fuels cause acid rain and other respiratory problems for human beings.

• Tidal energy
• Energy possessed by waves
• Ocean thermal energy
• Energy from sea vegetation, i.e., sea weeds
• Energy from salinity gradient.

Question 12.
Energy from various sources is considered to have been derived from the Sun. Do you agree? Justify your answer. .
Answer:
Sun is the ultimate source of energy. , Energy from various sources is, in fact, derived from the Sun. Fuels like wood, agriculture wastes and waste of animals and plants are possible due to Sun. Plant trap sunlight for photosynthesis and produce food, wood etc. on whiph animals depend. Similaraly fossil fuels are formed from plants and animals. The plants and animals also derived energy from the Sun. Biogas is produced from cow-dung, variouls plants residues etc.

which is also ultimatily depended upon Sun, Further, wind energy is possible due to Sun. Sun creates difference of temperature on earth surface due to which wind flows and then wind energy is possible. Similarly the non-conventional sources of energy like solar energy, tidal energy are possible due to Sun only. It is Sun due to which movement of earth and moon is possible. Moon is responsible for the tides. Wave energy, ocean thermal energy and geothermal energy are also because of the Sun. Then we can say that various sources of energy is derived from the Sun.

Question 13.
What is biomass? Explain the principle and working of a biogas plant using a labelled schematic diagram.
Answer:
Biomass is any part of a living system plants or animals which has been separated from it or used after death. Bio-gas is produced in a plant as shown in the figure given on the next page. The plant has a dome-like structure built with bricks. A slurry of cow-dung and water is made in the mixing tank from where it is fed into the digester. The digester is a sealed chamber in which there is no oxygen.

Anaerobic micro-organisms that do not require oxygen decompose or break down complex compounds of the cow-dung slurry. It takes a few days for the decomposition process to be completed and generate gases like methane, carbon dioxide, hydrogen and hydrogen sulphide. The biogas is stored in the gas tank above the digester from which they are drawn through pipes for use;