Text-book Questions

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.