NCERT Solutions For Class 10 Science Chapter 14 Sources Of Energy

NCERT Solutions For Class 10 Science Chapter 14 Sources Of Energy Long Question And Answer

Question 1. What is a good source of energy?
Answer: A good source of energy would depend on the requirement for which energy is needed.

For example, we use one source say gas for cooking food and another source say electricity for heating a room. In general, a good source of energy would be one

1. Which would do a large amount of work per unit mass/volume.
2. Which would be easily accessible.
3. Which can be easily stored and transported.
4. Which is also reasonably economical
5. It could cause the least environmental damage.

Question 2. What is a good fuel?
Answer: The following should be the characteristics of a good fuel:

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1. It should be fairly cheap.
2. It should be easily available.
3. Its ignition temperature should be well above normal temperature.
4. It should be conveniently handled and transported.
5. It should not produce, during burning, any poisonous materials or irritating fumes etc.
6. Its combustion rate should be steady and controllable.
7. It should not leave any residue or ash after burning.
8. It should bum in a way that can be controlled.
9. It should produce a large amount of heat per unit vol. or mass.

Question 3. What is nuclear energy? What is nuclear fission? How is nuclear energy used for peaceful purposes?
Answer: When the nucleus of an unstable atom breaks up or a radioactive substance emits radiations (a, p, yrays), the energy so released together with radiations is called nuclear energy.

This process is called nuclear fission. This nuclear energy when used in a controlled manner finds use in the following fields:

1. In curing diseases, for example, tumours and cancer.
2. Radio isotopes are nowadays used in research in various fields like industry, agriculture, science etc.
3. Preservation of food, e.g., potatoes can be preserved for long periods if irradiated with y-rays.
4. In determining the age of archaeological remains of wood by a process known as
   carbon dating.
5. As a fuel—a small amount of fuel produces a large amount of energy.

Question 4. Name the three forms in which energy from oceans is made available for use. What are OTEC power plants? How do they operate?
Answer: The three forms of energy from the oceans are:

1. Tidal energy
2. Ocean wave energy and
3. Ocean thermal energy.
4. The devices used to harness ocean thermal energy are called Ocean Thermal Energy
   Conversion (OTEC) power plants.

In one of the OTEC power plants, the warm surface water is used to boil ammonia. The vapours of this liquid are then used to drive turbines and hence generators. The cold seawater is pumped up again to convert ammonia into vapour. This system can be operated 24 hours a day.

Question 5. What is geothermal energy? How can it be utilized?
Answer: Due to high temperatures in the interior of the earth the rocks present there melt which is known as magma. Due to some geological changes inside the earth, this magma is pushed upward and is trapped at some depth below the surface of the earth.

These are called hot spots and serve as the source of geothermal energy. Water is trapped in the earth at several places.

At places where trapped water comes in contact with hot spots, it gets converted into steam. Since this steam remains trapped between rocks, it gets compressed to a very high pressure.

On drilling and introducing pipes upto the compressed steam, the trapped steam can be driven out at high speed. This can be utilized to turn the turbines and generate electricity.

In some places, steam and hot water find their way through large cracks between rocks and serve as natural geysers.

Question 6. What are semiconductors? Explain the principle of working of solar cells made of semiconductors. Why are solar panel cells used in artificial satellites?
Answer: Semiconductors are certain types of metals which allow only partial current to pass through them in one direction only. The conductivity of semiconductors increases when light falls on them. Their conductivity also increases if certain impurities are added to them. Materials like silicon, selenium, cadmium sulphide or copper sulphide act as semiconductors.

In a solar cell, the pieces (waters) of semiconducting materials containing impurities are so arranged that a potential difference develops between two regions of the semiconductors when light falls on them.

Thus, when solar radiations fall in solar cell, it converts solar energy into electrical energy. A lead storage battery is connected to the circuit which gets charged and can be used as and when desired. The power generated by a single solar cell is very low.

A single solar cell with an area of 4 cm2 has an output voltage of about 0.4-0.5 volts at 60 milliamperes of current. Therefore, a large number of solar cells are mounted together to generate appreciable electrical power.

The solar cells are arranged in series to form solar panels. Solar panel cells are mostly used in space satellites which remain in outer space for many years.

The space satellite contains solar panels which convert sunlight energy into electrical energy and thus electrical energy is used by the satellite to operate various functions.

Question 7. What is hydroelectricity? Explain the basic principle involved in the generation ofthis energy.
Answer: The electricity generated by using the energy of flowing water is called hydroelectricity. The basic principle of the generation of hydroelectricity is as follows:

The water flowing in a river is collected by constructing a high-rise dam D. This water, which has been stored at a high altitude, has a lot of potential energy in it.

The water stored in a high-rise dam is allowed to fall on a water-turbine G located near the base of the dam.

When the fast-moving water falls on the blades of a turbine, then the kinetic energy of fast-moving water rotates the turbine rapidly.

The shaft of the turbine is connected to the coil of an electricity generator or dynamo. So, when the water turbine rotates rapidly, the generator coil fixed on its shaft also rotates rapidly and produces electricity.

Question 8. With the help of a labelled diagram, explain the construction and working of a solar water heater.
Answer: The solar water heater consists of an insulated metallic box which is painted black from inside and outside.

In this box, copper tubes painted black from the outside, are fitted in a zigzag shape. The box and copper tubes are painted black so that they may absorb the maximum radiant heat energy of the sun efficiently.

The box is covered with a glass sheet lid to trap sun rays by producing a greenhouse effect.

The two ends of copper tubes are joined to a water storage tank as shown in the figure. The solar water heater and its water storage tank are fitted on the roof of a building, so that they may absorb the maximum radiant heat energy of the sun.

Working of Solar Water Heater: When the sun shines the radiant heat energy enters the metal box through a glass sheetlid and is absorbed by water present in copper tubes.

This water gets heated and it comes out through the other end of copper tube A and goes into the storage tank.

As long as the sunshine is there, the water keeps circulating between the solar heater box and the insulated storage tank. The water keeps on circulating on its own because water on getting hot expands and pushes itself into the copper tube

Now to replace this hot water, fresh cold water moves into the copper tubes. In this way, all the water in the storage tank gets heated which can be taken out for use from the tap attached to the pipe.

Question 9. Draw a schematic labelled diagram of the type of solar cooker. Name two components of the solar cooker which are responsible for increasing the temperature inside the solar cooker. Explain their function.
Answer: A schematic diagram is as shown:

Two components of the solar cooker are a plane mirror and a Glass sheet.

Functions: The plane mirror maximizes the amount of light/heat entering the solar cooker. The glass sheet traps the infrared radiation or produces a greenhouse effect.

Question 10. Name the major fuel component of biogas. What are its other combustible components? Draw a simple labelled diagram of a fixed dome-type biogas plant. What is the use of residual slurry and why?
Answer: The major fuel components of biogas are:

1. Methane, CO2 and traces of other gases.
2. The spent slurry is used as manure as it is rich in nitrates and phosphates.

Question 11. What is a windmill? With the help of a diagram, describe in brief the construction and working of the windmill. Give uses of wind energy.
Answer: A windmill is a machine which works with the energy of blowing wind. It can be used to run water pumps (to draw water from the ground) and also for grinding grains.

It consists of a circular wheel to which blades are attached radially to catch the wind. The centre of this circular wheel is connected to one end of the crank rod. The U-bend of the crank is connected to the pump rod (piston rod).

Working: When the blowing wind strikes across the blades of a windmill, the KE of wind rotates the blades continuously, as a result crank attached to them rotates.

Now, when the crank rotates, its U-bend lifts the pump rod up and down continuously. This pump rod now operates the pump which lifts the underground water.

Uses of wind energy:

1. Wind energy is used to propel sailboats to transport men and materials from one
   place to another.
2. Wind energy is used to run water pumps to draw out underground water.
3. Wind energy is used to run flour mills to grind the grains.
4. Wind energy is used to generate electricity.

Question 12. What is the basic cause for winds to blow? Name a part of India where wind energy is commercially harnessed. Compare wind power and the power of water flow concerning generating mechanical and electrical energies. What is the hindrance to developing them?
Answer: Sun’s energy is one of the main factors responsible for the movement of air/wind in the atmosphere.

Kanyakumari in Tamil Nadu India is the place where wind energy is commercially harnessed. The power of flowing water is more than that of wind and as such flowing water produces more energy than the wind.

1. The source of wind power i.e., Air and the source of flowing water power i.e., water are inexhaustible and reversible sources.
2. Wind power and flowing water do not cause any pollution.
3. The kinetic energy of wind and flowing water gets transformed into electrical energy.
4. Wind and water are available free of cost.

Limitations of wind energy:

1. It is not available all the time and at all the places for doing various types of work.
2. Wind energy farms can be established only in those places where the wind blows for the greater part of the year and there is no place in the world where the wind blows all the time.

Limitations of flowing water energy:

1. The construction of dams on rivers results in a variety of ecological changes in the downstream area of the river.
2. The flowing water is not available in plenty everywhere to turn the water wheels
   and run the machines.

Question 13. Describe the steps involved in obtaining biogas and explain what is meant by anaerobic decomposition.
Answer: A slurry of animal dung, for example, cow dung and water is made in the mixing tank from where it is fed into the digester. The digester is usually a closed underground tank made by laying bricks.

1. In the digester, the action of microorganisms breaks down or decomposes the complex compounds of the biomass in the slurry.
2. Anaerobic microorganisms, in the presence of water, do the degradation of biomass in the digester.
3. Since anaerobic microorganisms do not require oxygen, the digesters are designed like a sealed chamber. This process takes a few days to complete during which gases like methane; carbon dioxide, hydrogen and hydrogen sulphide are produced.
4. Biogas, which is a mixture of these gases, is supplied to consumers through pipes. The fermentation is carried out by anaerobic bacteria in the presence of water but in the absence of oxygen.
5. The anaerobic bacteria decompose the carbon compound present in the animal and plant wastes in the form of carbohydrates, proteins and fats into methane gas. This is called anaerobic decomposition.

Question 14. Why are fossil fuels classified as non-renewable sources of energy? What steps should be taken to conserve these sources?
Answer: The term fossil means the remains of prehistoric plants and animals buried under the earth millions of years ago which may be due to some natural calamity.

1. Fossil fuels are formed from dead plants and animals which got buried deep in the earth and covered with sediments like mud and sand in the absence of oxygen.
2. Chemical reactions, high pressure, heat and the action of anaerobic bacteria converted these fossils into fossil fuels like coal, petroleum and natural gas. This happened throughout millions of years due to certain slow changes under special circumstances.
3. Since these conditions are not prevailing now, therefore, no new fossil fuels are being formed now. Hence, they have been classified as non-renewable sources.

The following steps should be taken to conserve these fuels: Use of alternative forms of energy like solar energy; waste biomass energy; sea wave energy: tidal energy; geothermal energy; wind energy and fusion energy. Understand that “Energy saved is energy produced”. Therefore, try to use energy judiciously. Do not waste energy uselessly.

Put off all the lights and fans in your school, home and other places if they are not being used. Use geysers, electric irons and other energy-consuming appliances carefully.

Do not unnecessarily use motor vehicles unless four to five persons are required to travel to reach a destination. Use public transport instead. Such small steps taken by all of us can save the earth from the energy getting exhausted.

Question 15. What is the process used to harness nuclear energy these days? Explain it briefly.
Answer: Nuclear reactoris the process used to harness nuclear energy these days.

Nuclear fission reaction takes place in the nuclear reactor. The reaction in which a heavy nucleus splits into two or smaller nuclei with the evolution large amount ofenergy when it is bombarded with slow-moving neutrons is 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 along with the release of a tremendous amount ofenergy; U-235 nucleus splits up broadly into two groups of nuclei.

1. A heavy group of nuclei, with mass number in the range A = 130 to A = 149.
2. A light group of nuclei, with mass number in the range A = 85 to A = 104.

In the form of a nuclear reaction, we represent the nuclear fission of U-235 as follows:

⇒ \({ }_{92}^{235} \mathrm{U} \xrightarrow[\text { Slow moving }]{\frac{1}{\mathrm{n}}}{ }_{92}^{236} \mathrm{U} \longrightarrow{ }_{56}^{144} \mathrm{Ba}+{ }_{36}^{89} \mathrm{Kr}+3{ }_0^1 \mathrm{n}+\text { Energy (in huge amount) }\)

The major hazards of nuclear power generation are as follows:

1. The improper nuclear waste storage and disposal may result in environmental
   contamination.
2. There is a risk of accidental leakage of nuclear radiation and its leakage causes huge losses to living things.
3. The high cost of installation of nuclear power plants high risk of environmental contamination and limited availability.
4. How can solar energy be harnessed? Mention any two limitations in using solar energy.
5. How are these limitations overcome?
6. Solar energy can be harnessed directly as well as indirectly which is as follows:

Direct utilisation: The direct utilisation of solar energy can be done either by collecting it as heat or by converting it to electricity

Indirect utilisation: It can be done by converting solar energy into chemical energy like biomass of plants etc.

1. The limitations of using solar energy are as follows:
2. Energy reaching the surface is very much diffused, so direct utility is limited.
3. It is not available uniformly all the time and in all the places.

Make a list of conventional and non-conventional sources ofenergy. Give a brief description of harnessing one non-conventional source ofenergy.

Conventional: Fossil fuels, water, wind, biomass etc.

Non-conventional: Nuclear, Solar, Energy from the ocean, Geothermal.

Nuclear energy: Nuclear energy is produced by the release ofheat from unstable elements such as uranium. The energy is harnessed by using the energy to heat water.

Why is there a need for harnessing non-conventional sources ofenergy? How can energy be harnessed from the sea in different ways?

There is a need for harnessing non-conventional sources of energy because of the following reasons:

The energy demand is increasing day by day to meet the basic requirements of our changed lifestyles, growing use of machines and industrialisation to improve our living standards.

Fossil fuels are non-renewable sources of energy and were formed millions of years ago and there are only limited reserves of fossil fuels.

The energy from the sea can be harnessed in different forms which are as follows:

Tidal energy. It is harnessed by constructing a dam across a narrow opening to the sea. A turbine fixed at the opening of the dam converts tidal energy to electricity.

Wave energy. A wide variety of devices have been developed to trap huge waves near the regions for the rotation of turbines and the production of electricity.

Ocean thermal energy: The water at the surface of the sea or ocean is heated by the sun while the water in deeper sections is relatively cold. This temperature difference is exploited to obtain energy in ocean thermal energy conversion plants.

1. These plants can operate if the temperature difference between the surface water and water at depths up to 2 km is 293 K (or 20°C) or more The warm surface water is used to boil a volatile liquid like ammonia. The vapours of the liquid are then used to run the turbine of the generator.
2. The cold water from the depths of the oceans is used to condense vapour again into liquid. The devices used to harness this form of ocean energy are known as Ocean Thermal Energy Conversion [OTEC] power plants.

Question 16. What are the environmental consequences of using fossil fuels? Suggest the steps to minimise the pollution caused by various sources ofenergy including non-conventional sources ofenergy.
Answer: Environmental consequences ofusing fossil fuels: Airpollution, Greenhouse effect, Environment consequences: acid rain, global warming etc.

The following are the steps to minimise the pollution caused by various sources ofenergy including non-conventional sources of energy:

1. The air pollution caused by the burning of coal or petroleum products can be reduced by afforestation.
2. Proper and safe disposal of nuclear wastes.

Question 17. Energy from various sources is considered to have been derived from the sun. Do you agree? Justify your answer.
Answer: Yes, the sun is the ultimate source of energy directly or indirectly, all forms of energy are derived from solar energy for ofthe following reasons:

Non-renewable sources of energy: Fossil fuels like coal, petroleum and natural gas are formed due to the burial, of large plants and ancient creatures whose ultimate source of energy is the sun.

Renewable sources ofenergy: They are indirectly derived from solar energy such as:

Energy from flowing water: Clouds are formed when water in lakes, rivers, seas,
etc., evaporates due to solar energy. They bring rainfall and snowfall. The rain
and melting snow feed rivers, streams, etc. This flowing water can be used for
getting hydroelectricity.

Wind energy: Wind energy arises due to uneven heating ofthe earth’s surface by the sun’s rays at two different adjoining places. Due to this, a pressure difference is created and wind possesses kinetic energy.

Bio-energy: Plants in the process of photosynthesis convert solar energy into food (chemical energy). This food is consumed by animals. Thus, the animal’s wastes and remains of the plants constitute biomass which can be utilised as a source of energy.

Wave energy: The waves are generated by strong winds (due to solar energy)
blov/ing across the sea.

Ocean thermal energy: The Sun is responsible for the temperature difference between
the water at the surface and water at depth in seas and oceans.

Solar heating devices: They derive their energy directly from solar energy and convert it into other usable forms of energy. Thus, the energy from various sources is considered to have been derived from the sun.

Question 18. What is biomass? Explain the principle and workings of a biogas plant using a labelled schematic diagram.
Answer: Biomass refers to the organic fuel obtained from plant and animal wastes like wood, cow dung, residue after harvesting the crop, vegetable waste sewage, etc.

It has been used as a fuel for a long time. These fuels, however, do not produce much heat on burning and a lot of smoke is given out when they are burnt.

Bio-energy can be produced in a plant, known as a ‘biogas plant or gobar gas plant’ using biomass like cow dung, various plant materials like the residue after harvesting the crops, vegetable waste and sewage are decomposed in the absence of oxygen to give biogas. The labelled diagram of the ‘gobar gas’ plant is given in the figure.

Principle: It is based on the principle that the anaerobic micro-organisms decompose or breakdown complex compounds ofthe cow-dung slurry, in the absence of oxygen, in a few days and generate gases like methane, carbon dioxide, hydrogen and hydrogen sulphide which burn without smoke and leave no residue like ash.

Working: The plant has a dome-like structure built with bricks. A slurry of cow dung, various plant materials like the residue after harvesting the crops, vegetable waste, sewage and water is made in the mixing tank from where it is fed into the digester. The digester is a sealed chamber which is free from oxygen.

Anaerobic micro-organisms decompose or break down complex compounds of the cowdung slurry in a few days 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.

Question 19. Faisal is a student of class X. He knows the principle of conservation ofenergy. One day a declamation contest was held in his school. The topic was “Energy crisis and its possible solution”.

1. Faisal was confused. He. thought when the total energy of the universe is constant then where is the question ofenergy crisis?
2. The next day he posed the same question in his science class. His science teacher was kind enough to explain the paradox.
3. What do you mean by the term ‘energy crisis’?
4. Why do we talk about the energy crisis despite the law of conservation of energy?
5. What characteristic was exhibited by Faisal?

Answer: We generally use fossil fuels like coal, petroleum, CNG gas etc., for obtaining the energy for our daily needs. However, these fossil fuels are exhaustible (or nonrenewable) in nature.

As we use more and more quantities ofthese fossil fuels, their stock on our earth is continuously decreasing and these fuels may be exhausted soon. This situation is called “energy crisis”.

Of course, the total energy of the universe remains constant but a large part is in a form which cannot be utilised by mankind. For example, when we burn petrol/diesel, chemical energy is converted into heat energy and total energy remains constant.

Yet, this heat energy produced goes to the atmosphere and cannot be extracted for our use. That’s why we say that there is a fear of a crisis of usable energy sources.

Faisal has an analytical ability of very high order. He correlated the topic of the declamation contest with the law of science and tried to get the correct answer.

He was satisfied only when he got the correct answer to his doubts from his science teacher.

Question 20. Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fuels is typically millions of years and sometimes exceeds 650 million years.

Fossil fuels contain a high percentage of carbon and include coal, petroleum and natural gas. They range from volatile materials with low carbon hydrogen ratios like methane to liquid petroleum to nonvolatile materials compared to pure carbon like anthracite coal.

Fossil fuels produce around 21.3 billion tonnes of carbon dioxide per year whereas naturalprocesses can only absorb about half of that amount, so there is a net increase of 10.65 billion tonnes of atmospheric carbon dioxide per year.

Carbon dioxide is one ofthe greenhouse gases that enhance radiative forcing and contribute to global warming, causing the average surface temperature rise in response, which the vast majority of climate scientists agree will cause major adverse effects.

Is that the only reason that we should be cautious about using fossil fuels?

What steps would you suggest to be taken?

Answer: On the one hand the amount of available fossil fuels is decreasing and it is non-renewable sources ofenergy. Secondly burning of fossil fuels also produces many other poisonous gases like SO2, NO2, etc.

We should encourage the development of renewable sources ofenergy like solar energy,
geothermal energy, wind energy, hydroelectric energy.

Question 21. Recently when the Government decided to set up a nuclear powerplant in an area NGOs and local people raised their voices against it. They demanded that the Government should assure safety measures before setting up such a plant and the Government assured them of it.

1. Explain the value exhibited.
2. List any two concerns ofthe people for which they were demanding safety measures.

Answer: The NGOs and people did want to set up nuclear power plants for an increased supply of energy and hence growth of the country. At the same time, it was desired that all safety measures be taken to ensure a peaceful and healthy environment around our place.

1. Nuclear radiation is very harmful to the sustenance of life and plants on Earth.
2. To take proper safeguards against the leakage of the radiations.
3. To take steps for the safe arrangement of disposing of nuclear wastes.