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NCERT Solutions for Class 10 Science Source of energy

1) Name four sources of energy you use from morning when you wake up till you reach the school?  
Answer: The different forms of energy are as below:
               (i) Heat energy to cook food
              (ii) Electrical energy to lighten the room.
              (iii) Muscular energy to carry bag and to run bicycle
             (iv) Sound energy to call friends.

2) Name the four forms of energy and the sources.

Forms of energy
Source of Energy
(i) Heat energy by burning LPG.
(ii) Electricity.

(iii) Muscular energy.
(iv) Sound from electric bell.
(i) Petroleum.
(ii) From hydro energy from thermal power station.
(iii) Food material.
(iv) Hydroelectricity or from thermal power plant.

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

4) What are the criteria you would consider when trying to categorize something as a good fuel?
Answer: Criteria: The fuel should be
                                      High energy – yield.
                                      Easily available.
                                      Environment friendly.
                                      Easily transportable.
                                      Low cost.

5) What is a good source of energy?
Answer: A good source of energy has following characteristics:
                       (i) It could do a large amount of work per unit volume or mass.
                       (ii) It would be easily accessible.
                       (iii) it would be easy to store and transport.
                       (iv) It would be economical.

6) What is good fuel?
Answer:   A good fuel has followed characteristics:
                         (i) It should have high calorific value.
                        (ii) It should have normal ignition temperature.
                        (iii) It should not leave more ash and should not produce harmful gases on       
                        (iv) It should be economical and easy to handle.

7) 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.
8) What are the disadvantages of fossil fuels?   
Answer: Following are the disadvantages of fossil fuels:
(i) Fossil fuels are formed under the earth’s surface by a long process which takes million years to form fossil fuel and there are only limited reserves.
(ii) Fossil fuels are non – renewable sources of energy.
(iii) 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.

9) 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 f task 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 are looking at alternate sources of energy.

10) How has the traditional use of wood and water energy been modify for our convenience?

Answer:  Wood and water are traditional sources 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 eco-friendly as it does not produce smoke on burning. It has higher calorific value and leaves lass 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.

11) 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 reactions coming from sun properly at a point and causes more heating.

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

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

14) 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.

15) Debate the merits and demerits of each sources and select the best source of energy on this basis.
Answer:    The fossil Fuels: Coal and Oil
   (i) Very large amount of energy can be generated cheaply.
   (ii) Transporting it from place to place is easy.
   (iii) Engines using fossil fuel are very efficient. This means, only a very small amount of      
          energy is lost.
   (iv) Fossil fuels are easily obtainable and can be store almost anywhere.
   (i)  Fossil fuels are non – renewable. Once used, they are ended permanently.
   (ii) The major disadvantage of fossil fuels is that the cause pollution.
   (iii) Fossil fuels need to be extracted from the mines. They may cause land – slide, destroying   large areas.
Thermal Power
(i) Very large amount of electricity is generated at a place cheaply.
(ii) Transporting coal, oil and gas to the power station easy.
(iii) Power stations run by petroleum gas are very efficient.
(iv) These can b built almost anywhere.
(i) The major drawback of these plants is the pollution caused by burning of fossil fuels.
(ii) Coal based power stations need huge amount of fuel in order to fulfill the demand. This keeps a large are near the power stations, occupied with piles of coal.
(iii) It is a non – renewable source.
Hydroelectric Power
(i) The water required for running these stations involves no cost.
(ii) It is a renewable source.
(iii) There is no pollution by these stations.
(iv) It helps in irrigation for agriculture.
(v) It helps in control of flood.
(i) These power stations can be built only where there is huge supply of water.
(ii) Constructing a dam is very costly.
(iii) It disturbs the neighboring eco– system. It may even destroy inhabited localities near the dam.
Solar Energy
(i) The cost of cooking is nothing since sun’s energy comes free of cost.
(ii) It does not cause pollution.
(i) It will not work during night or in a cloudy day since sufficient sun ways will not be available.
(ii) It is not possible for all, living in a city to have enough open space where to use solar devices.
Solar Cells
(i) These provide a clean, pollution free and environment friendly source of energy.

(ii) These can be used anywhere as a self – generating source of electricity.
(iii) One constructed, there is no further cost of production or maintenance in such devices.
(iv) It is a renewable source.

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

(i) Very large amount of energy is obtained in economical way.
(ii) The by – products after fission may be further utilize. [However, scientists are still to find an effective work to do so.

(i) The radioactive materials required for fission error harmful radiations like gamma rays which cause cancer, leukaemia and other genetic disorders.
(ii) In case of an accident, it will explode like a very big nuclear bomb destroying all lives around.
(iii) 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
(i) The biogas produced from bio – mass is a clean fuel i. e. it does not cause any pollution.
(ii) The remains of bio – mass is very good natural fertilizer.
(iii) It is obtained free of cost.
(iv) It gives a safe method to dispose off wastes.
(v) It has no storage problem.

(i) The process of biogas generation is very slow, which reduces its utility for daily uses.
(ii) 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
(i) It is a renewable source of energy.
(ii) It does not cause air pollution.
(iii) It is very useful in agricultural farms for running the water pumps or grinding machines.
(iv) It can be very useful for small scale industries which do not require high amounts of electricity.
(v) Once constructed, it supplies energy free of cost.
(vi) It is the best source of energy for coastal areas.

(i) The amount of power generated is low.
(ii) It causes noise pollution.
(iii) It disturbs the rainfall pattern of the locality.

16) 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 NO2 like harmful gases during burning of wood directly. Even use of solar energy causes excessive trapping of infrared radiation which causes global warming.

17) Hydrogen has been used as rocket fuel. Would you consider it a cleaner fuel than CNG? Why or why not?
Hydrogen is a cleaner fuel than CNG because of the following reasons:
(i) Hydrogen has high calorific value than CNG.
(ii) CNG is conventional source of energy but H2 is not.
(iii) CNG is green – house gas but H2 is not.
(Iv) Burning of CNG gives out CO and CO2 gases but burning of H2 does not give any such harmful gases.

18) Name two energy sources that you would consider to be renewable. give reasons for your choices.
Two renewable sources of energy are:
(i) 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.
(ii)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.

19) Give the name of two energy sources that you would consider to be exhaustible. Give reasons or 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.

20) Compare and contrast fossil fuels and the sun as source of energy.


Fossil fuels
  1. It is conventional source of energy.
  2. Sun is the ultimate source of all fossil fuels.
  3. Fossil fuels cannot be used un-judiciously because it is limited in quantity.
  4. Fossil fuels can be used as source of energy at any time.
  5. Use of fossil fuel causes air pollution because during burning of fossil fuel harmful gases are evolved.
  6. It costs to buy fossil fuel.
  1. It is non – conventional source of energy.
  2. Sun is the ultimate and final source of energy on earth.
  3. Sun is larger source of energy and can be used at large scale as according to our need.
  4. Solar energy can be used only in day time.
  5. Solar energy is a clean fuel and can be harnessed without causing any pollution.
  6. It is cost free source of energy.

21) Compare and contrast biomass and hydroelectricity as a source of energy.

1.Biomass is a renewable and conventional source of energy.
2.Biomass possesses chemical energy.

3. Use of biomass as energy causes pollution of air.
4.Use of Biomass does not cause ecological imbalance.
5. Biomass is relatively more economical source of energy than hydroelectricity.
  1. Hydroelectricity is also a renewable and conventional source of energy.
  2. Hydro energy possesses kinetic energy of running water at high altitudes.
  3. Hydro energy is pollution free source of energy.
  4. Construction of dam for hydroelectricity causes ecological imbalance.
  5. Hydroelectricity is relatively costly source of energy.

22) What are the limitations of extracting energy from?
      (a) the wind    (b) waves   (c) tides?

Source of energy

(i) Wind energy

(ii) Wave energy 

(iii) Tides
(a) Everywhere and every time the blowing air is   not available.
(b) To generate electricity the wind should have speed more than 15 km/hr.
(a) The wave is not available very time for generating electricity.
(b) It is costly to set up device to trap wave energy.
(a) The locations for constructing dams for harnessing tidal energy are limited.
(b) Construction of dams for harnessing tidal energy is costly.

23) On what basis, would you classify energy sources as
      (a) renewable and non – renewable
      (b) exhaustible and inexhaustible?

(a) If the source of energy can 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.

24) What are the qualities of ideal sources of energy?

The qualities of ideal sources of energy are:
    (i) It could do a large amount of work per unit volume of mass.
    (ii) It would be easily accessible.
   (iii) It would be easy to store and transport.
   (iv) The nutrients of food do not get destroyed.
 Disadvantages of use of solar cooker:
    (i) Solar cooker cannot be used at night and in cloudy weather.
   (ii) It takes more time to cook food.
  (iii) The direction of solar cooker is to be changed continuously towards the direction of sun.
  (iv) Sun energy is not available uniformly all the time and at all the places.

  (v) 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.

25) What are the environmental consequences of the increasing demand for energy? What steps would you suggest to reduce energy consumption?
Modernisation and industrialization 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.
Following steps can be suggested to reduce the energy consumption:
    (i) The judicious use of energy and avoiding misuse of energy can reduce the energy consumption.
   (ii) the alternative sources of energy like solar energy, wind energy, hydro energy and energy of sea should be used to reduce the load of energy or non – renewable source of energy.

26) State examples of any two activities from our daily life in which we use solar energy.
       (i) In drying clothes
       (ii) Drying grains.

27) From where does the earth receive its energy?
Answer: The earth receives its energy from sun.

28) Define hydroelectricity.
Answer: The electricity generated by turning the shaft of generator with the help of flowing water is called hydroelectricity.

29) Give different characteristics of a source of energy.
(i) it should be capable of providing an adequate amount of useful or net energy.
(ii) It should be convenient to use.
(iii) Its transportation should be easy.
(iv) Its storage should be easy.
(v) Its should be capable of delivering desired quantity of energy at a steady rate over a long period of time.

30) Write the working principle of a windmill.
Answer:  A windmill is a machine which works with the help of energy of wind. The blades of windmill are designed in such a manner so as to create a pressure difference between its different regions when wind strikes them. This pressure difference produces a torque (a turning effect) to rotate the blades.

31) Why is charcoal a better fuel than wood? What are the disadvantages of converting wood into charcoal?
Answer: Charcoal has higher calorific value than wood. Wood produces a lot of smoke while charcoal does not produce any smoke on burning.
 (i) One kilogram of wood can change into 0.25 kg of charcoal.
 (ii) There is loss of energy in the destructive distillation process.

32) How is bio-gas obtained?
Bio-gas is obtained by:
(i) Making a slurry of animal dung and water in slurry tank and feeding it into digester.
(ii) Anaerobic microorganisms degrade the biomass in the digester in the presence of water.
(iii) This process takes nearly a month to complete during which gases like methane, carbon dioxide, hydrogen and hydrogen sulphide are produced.

33) Write the differences between box type and a concentrator type solar cooker.
(i) In box type solar cooker, the temperature of box rises slowly. In concentrator type solar cooker, the temperature rises quickly.
(ii) In box type solar cooker, the sunlight is collected over a small area. In concentrator type solar cooker, the sunlight is collected over a large area.
(iii) In box type solar cooker, temperature rises from 100*C to 140* C while in concentrator type solar cooker, the temperature ranges from 180C to 200* C.

34) What are solar cells? How much solar energy a solar cell can convert into electricity?
Answer: A device which converts solar energy into electric energy is called solar cell. A solar cell can convert only 0.7% of the solar energy into electricity.

35) Write two uses and two limitations of wind energy.
Answer: Blowing wind possesses kinetic energy. is termed as wind energy.
  (i) To move the sail boats.
  (ii) To generate electricity.

 (iii) To pump water from the earth.
   (i) Wind energy is not available in plenty in many regions.
  (ii) The speed of wind is not sufficient to run windmills all the times.

36) What is geothermal energy? Write two uses of geothermal energy.   
Answer: The energy which is directly obtained from the heat inside the earth is called geothermal energy.

(i) It is used to produce electricity.
(ii) It is used to provide direct heating for a range of applications in the residential, commercial and industrial sectors.
(iii) it is also used in green – house heating.

37) In which form is energy stored in oceans? Which one of these is utilized in OTEC system?
Answer: Oceans have a good amount of energy as tidal energy. The temperature difference between top and lower layers in oceans is also the source of energy called ocean thermal energy.

The devices used to harness ocean thermal energy are called. Ocean Thermal Energy Conversion (OTEC) power plant

38) Name three forms in which every from ocean is made available for use.

 (i) Tidal energy
(ii) Energy possessed by waves
(iii) Ocean thermal energy
(iv) Energy from sea vegetation, i.e., sea weeds
(v) Energy from salinity gradient.

39) How can tidal energy be harnessed?
Answer: 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 opening 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.

40) List the disadvantages of using geothermal energy.

(i) It is expensive to drill a deep well and take out steam.
(ii) Geothermal energy is available at limited places.

41) Write the efficiency of silicon cells. Which solar cells are sensitive for visible and infrared radiations? Write an advantage of using solar cells. Explain the use of solar panel.
The efficiency of silicon cells is 25%. Solar cells have been designed such that they consist of a number of thin layers of silicon. The multilayered silicon cells have been found sensitive for both visible and infrared radiations. solar cells give us a clean and pollution free environment.
A large number of solar cells combined together can produce more electricity. This arrangement is known as solar panel. The electricity so produced by solar cells is stored in form of charges, i. e., DC. To operate alternating current devices, DC is converted into AC.

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