Chapter Review:

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• Properties of good fuels (i) they doesn't  cause  air pollution (ii) Which would do a large amount of work per unit volume or mass. (iii) Be easily accessible.

• Coal, petroleum and natural gas are called fossil fuels. As Such fuels were formed by the slow decomposition of fossils (remains of plants and animals) which got buried under the surface of earth and take millions years to form. 

• Energy can only be converted from one form to another. It can neither be created nor destroyed. The total energy before and after the transformation remains the same. This law is valid in all situation and transformation. That is called the law of conservation of energy. 

• The turbine works by mechanical energy which it gets from various sources like water energy, Wind energy and nuclear energy etc.

• Some gases like CO₂ and methane traps heat from sun rays. Our atmosphere does not allow the radiations to go out. This is called green house effect.

• Oxide of sulpher and nitrogen react with water vapour in atmosphere and produce acidic substance, its rain is called acid rain.

• The source of energy obtained from plants and animals said to be Bio-mass. Ex-: Wood, cow-dung etc. 

• Mixture of cow dung and water is said to be slurry. 

• The black colour absorbs heat more frequently than others.

• Solar cell is a device which converts sunlight into electricity.

• Silicon metal is used for making solar cell. 

• Silver metal is used for interconnection of solar cells.

• The dynamo convert the mechanical energy into electrical energy.

• Geological changes, molten rocks formed in the deeper hot regions
  of earth’s crust are pushed upward and trapped in certain regions called ‘hot spots’.

Text-book Questions

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Page no. 243 

Q1. What is a good source of energy?

Ans: The best source of energy is the one which

1. Do more work per unit volume or mass per unit.

2. Which is easily available.

3. Be easy to store and transport.

4. It should be cheap.

5. Do not spread pollution on burning.

Q2. What is a good fuel?

Ans: A good fuel should have the following properties -

1. More heat is released per unit mass after combustion.

2. It should be easily available at affordable rates.

3. Do not generate excessive smoke on burning.

4. Its ignition temperature is suitable and the calorific value is high.

Q3. If you could use any source of energy for heating your food, which one would you use and why?

Ans: We will use LPG gas or electrical equipment because -

1. It generates more heat.

2. Its combustion does not emit smoke.

3. It is easily available and can be used easily at any time.

4. It is cheap and can be stored and transported easily.

5. From this the desired energy can be obtained as per the requirement.

Page no. 248

Q1. What are the disadvantages of fossil fuels?

Ans: Following are the disadvantages of fossil fuels –

1. Fossil fuels cost hundreds of years to make and their reserves are limited.

2. Fossil fuels are non-renewable sources of energy.

3. Burning of fossil fuels causes air pollution.

4. Green house effect occurs due to the increase in the amount of carbon in the air.

Q2. Why are we looking at alternate sources of energy?

Ans: We know that fossil fuels are non-renewable sources of energy. So there is a need to save them. Coal, petroleum, natural gas etc. are present in limited quantities inside the earth. If we keep using them like this, they will soon be exhausted. Therefore, we should pay attention to alternative sources of energy.

Page no. 253

Q1. What kind of mirror – concave, convex or plain – would be best suited for use in a solar cooker? Why?

Ans: The most suitable mirror for a solar cooker is a concave mirror, as it is a converging mirror. Which focuses the rays of the sun on a point, due to which its temperature soon increases further.

Q2. What are the limitations of the energy that can be obtained from the oceans?

Ans: The energy received from the oceans has the following limits -

1. For the commercial use of wave energy, it is necessary to have very strong waves.

2. Tidal energy is obtained by rising and falling of the water level at the time of tide. The turbines located at the entrance of the dam convert tidal energy into electrical energy. But this dam is successful only in some areas.

3. The cost of construction of power plants is very high and the production of energy is less.

Q3. What is geothermal energy?

Ans: When underground water comes in contact with hot spots, steam is produced. When this steam gets trapped between the rocks, its pressure increases. This steam at high pressure is removed by pipes, this steam spins the turbine of the electric generator and electricity is generated. The energy obtained from these hot spots is called geothermal energy.

Q4. What are the advantages of nuclear energy? 

Ans: he energy produced by nuclear energy is called nuclear energy. A large amount of energy is released by this process. This energy is used to generate electricity by making steam.

Page no. 253 

Q1. Can any source of energy be pollution-free? Why or why not?

Ans: No, there is no such source of energy which is pollution free. Although the solar cell is free from pollution, it may cause environmental damage in mobilizing that device.

Q2. Hydrogen has been used as a rocket fuel. Would you consider it a cleaner fuel than CNG? Why or why not?  

Ans: Hydrogen is a clean fuel because it does not generate CO₂, but it generates water when it is combusted, so pollution does not spread.

Page no. 254 

Q1. Name two energy sources that you would consider to be renewable. Give reasons for your choices.

Ans: (a) Wind Energy :

(i) Energy from the winds is found only in those places where the wind blows at high speed during the rainy season.

(ii) For the speed of turbines, the minimum speed of the wind should be more than 15 km/h.

(iii) Auxiliary cells should also be available for its use.

(b) Hydroelectric Energy :

(i) It is also a renewable source of energy.

(ii) Hydroelectricity is generated from water.

Q2. Give the names of two energy sources that you would consider to be exhaustible. Give reasons for your choices.

Ans: Those sources of coal and petroleum are exhaustible. Their reserves are limited in nature and they will definitely get exhausted one day. It took millions of years for these fuels to be manufactured. Therefore it is impossible to recreate them.

NCERT Solutions Execise 

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Q1. 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.

Ans: (b) a cloudy day.

Q2. Which of the following is not an example of a bio-mass energy source?

(a) wood

(b) gobar-gas

(c) nuclear energy

(d) coal

Ans: (c) nuclear energy

Q3. 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) nuclear energy

(d) bio-mass.

Ans: (c) nuclear energy

Q4. Compare and contrast fossil fuels and the Sun as direct sources of energy.

Ans:

fossil fuels:

(i) It is terminating.

(ii) It is an expensive source of energy.

(iii) They also affect the greenhouse.

  Sun :

(i) It is not tenable.

(ii) It is a cheap source of energy.

(iii) This equipment is a source of clean energy.

Q5. Compare and contrast bio-mass and hydro electricity as sources of energy.

Ans: 

Biomass :

(i) It is a renewable source of energy.

(ii) Its cost is not high.

(iii) is due to the generation of bio gas.

Hydroelectricity:

(i) It is also a renewable source of energy.

(ii) Hydroelectricity is generated from water.

(iii) Its cost is expensive to some extent.

Q6. What are the limitations of extracting energy from—

(a)the wind?

(b) waves?

(c) tides?

Ans: (a) Winds :

(i) The energy from the winds is found in those places where the wind blows at a high speed during the rainy season.

(ii) For the speed of turbines, the minimum speed of the wind should be more than 15 km/h.

(iii) Auxiliary cells should also be available for its use.

(b) Waves :

(i) The waves must be strong to get the energy generated by the waves

(ii) The same electric power cannot be obtained while using it.

(iii) The equipment required for this is expensive.

(c) Tidal :

(i) It is necessary to build a dam to get this kind of energy.

(ii) The use of energy generated by tides is limited.

(iii) The cost of land for its construction is high.

Q7. On what basis would you classify energy sources as

(a) renewable and non-renewable?

(b) exhaustible and inexhaustible?

Are the options given in (a) and (b) the same?

Ans: 

(a) Renewable energy is the energy which we can use indefinitely for a long time. Its reserves remain renewable. Eg: Solar power and wind power.

 Non-renewable energy is the energy which if we use it once, it cannot be recovered and its reserves remain exhausted. Such as: fossil fuels (coal, petrol, natural gas).

(b) The options for (a) and (b) are the same because renewable energy is renewable but non-renewable energy is non-renewable.

Q8. What are the qualities of an ideal source of energy?

Ans: Characteristics of an ideal source of energy:-

(i) can be easily obtained.

(ii) should also be cheap.

(iii) do more work per unit volume and mass.

Q9. What are the advantages and disadvantages of using a solar cooker? Are there places where solar cookers would have limited utility?

Ans: Benefits of using solar cooker:

(i) This equipment is cheap.

(ii) There is no pollution by using it.

(iii) They do not have any moving parts.

Disadvantages of Solar Cooker:

(i) This device is used only in sunlight.

(ii) It takes more time to cook food.

   Yes, there are many such areas. Solar-cells are expensive. Where solar cookers have limited utility eg: areas with high rainfall, hilly areas etc.

Q10. What are the environmental consequences of the increasing demand for energy? What steps would you suggest to reduce energy consumption?

Ans: Today's era has become a mechanical age, today's population uses fossil fuels to pollute the environment. Environmentally friendly using solar cells. Many types of diseases are increasing.

  Following are the measures to reduce energy consumption:

(i) Least use of coal, fossil fuel.

(ii) Relying on oneself instead of relying on machines.

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