1. What is a good source of energy?
Answer
A good source of energy fulfils the following criteria:
→ It produces a lot of heat per unit mass.
→ It does a huge amount of work per unit mass.
→ It is easily accessible.
→ It is easy to store and transport.
→ It is economical.
→ It produces less amount of smoke.
2. What is a good fuel?
Answer
A good fuel produces a huge amount of heat on burning, does not produce a lot of smoke, and is easily available.
3. If you could use any source of energy for heating your food, which one would you use and why?
Answer
Natural gas can be used for heating and cooking food because it is a clean source of energy. It does not produce huge amount of smoke on burning. Although it is highly inflammable, it is easy to use, transport, and it produces a huge amount of heat on burning.
Page No: 248
1. What are the disadvantages of fossil fuels?
Answer
The disadvantages of fossil fuels are:
→ Burning of coal and petroleum produces a lot of pollutants causing air pollution.
→ Fossil fuels release oxides of carbon, nitrogen, sulphur, etc. that cause acid rain, which affects the soil fertility and potable water.
→ Burning of fossil fuels produce gases such as carbon dioxide that causes global warming.
2. Why are we looking at alternate sources of energy?
Answer
Fossil fuels which are traditionally used by human beings everywhere as an energy sources are non-renewable sources of energy. These sources of energy are limited and will disappear after some time. They are being consumed at a large rate. Therefore, we should conserve the energy sources. Hence, we should look for alternate sources of energy.
3. How has the traditional use of wind and water energy been modified for our convenience?
Answer
Earlier, the windmills were used to harness wind energy to do mechanical work such as lifting/drawing water from a well. Today, windmills are used to generate electricity.In windmills, the kinetic energy of wind is harnessed and converted into electricity.
Water energy which was used for transportation before is now a good source to generate electricity. Dams has been constructed on river for generating electricity. Waterfalls were used as a source of potential energy which was converted to electricity with the help of turbines.
Page No: 253
1. What kind of mirror - concave, convex or plain - would be best suited for use in a solar cooker? Why?
Answer
A concave mirror is used in a solar cooker as it uses heat of the sunlight to cook food. The mirror focuses all the incident sunlight at a point. The temperature at that point increases, thereby cooking and heating the food placed at that point.
2. What are the limitations of the energy that can be obtained from the oceans?
Answer
The forms of energy that can be obtained from the ocean are tidal energy, wave energy, and ocean thermal energy. There are several limitations in order to harness these energies.
→ Tidal energy depends on the relative positioning of the Earth, moon, and the Sun.
→ High dams are required to be built to convert tidal energy into electricity.
→ Very strong waves are required to obtain electricity from wave energy.
→ To harness ocean thermal energy efficiently, the difference in the temperature of surface water (hot) and the water at depth (cold) must be 20ºC or more.
3. What is geothermal energy?
Answer
Geothermal power plants use heat of the Earth to generate electricity. This heat energy of the Earth is known as geothermal energy.
4. What are the advantages of nuclear energy?
Answer
The advantages of nuclear energy are:
→ Large amount of energy is produced per unit mass.
→ It does not produce smoke. It is a clean energy.
→ Fission of one atom of uranium produces 10 million times the energy released by burning of one atom of carbon.
→ Fusion of four hydrogen atoms produces huge amount of energy approximately equal to 27 MeV.
1. Can any source of energy be pollution-free? Why or why not?
Answer
No source of energy can be pollution-free. Every source of energy has some type of pollution. For example, the wastes of nuclear reaction are very dangerous to the environment.
2. Hydrogen has been used as a rocket fuel. Would you consider it a cleaner fuel than CNG? Why or why not?
Answer
Hydrogen gas is cleaner than CNG. CNG contains hydrocarbons. Therefore, it has carbon contents. Carbon is a form of pollutant present in CNG. On the other hand, hydrogen is waste-free. The fusion of hydrogen does not produce any waste. Hence, hydrogen is cleaner than CNG.
Page No: 254
1. Name two energy sources that you would consider to be renewable. Give reasons for your choices.
Answer
Two renewable sources of energy are :
→ Sun: The energy derived from the Sun is known as solar energy. Solar energy is produced by the fusion of hydrogen into helium, fusion of helium into other heavy elements, and so on. A large amount of hydrogen and helium is present in the Sun. The Sun has billions years more to burn. Therefore solar energy is a renewable source of energy.
→ Wind: Wind energy is derived from fast blowing air. Wind energy is harnessed by windmills in order to generate electricity. Air blows because of uneven heating of the Earth. Since the heating of the Earth will continue forever therefore wind energy will also be available forever.
2. Give the names of two energy sources that you would consider to be exhaustible. Give reasons for your choices.
Answer
Two exhaustible energy sources are as follows:
→ Coal: It is produced from dead remains of plants and animals that remain buried under the earth's crust for millions of years. It takes millions of years to produce coal. Industrialization has increased the demand of coal. However, coal cannot replenish within a short period of time. Hence, it is a non-renewable or exhaustible source of energy.
→ Wood: It is obtained from forests. Deforestation at a faster rate has caused a reduction in the number of forests on the Earth. It takes hundreds of years to grow a forest. If deforestation is continued at this rate, then there would be no wood left on the Earth. Hence, wood is an exhaustible source of energy.
Excercise
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
(c) a hot day
(d) a windy day
► (b) a cloudy dayPage No: 255
2. Which of the following is not an example of a bio-mass energy source?
(a) wood
(b) gobar gas
(c) nuclear energy
(d) coal
► (c) nuclear energy
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) Nuclear energy
(d) Bio-mass
► (c) Nuclear energy
4. Compare and contrast fossil fuels and the Sun as direct sources of energy.
Answer
Fossil fuels are energy sources, such as coal and petroleum, obtained from underneath the Earth's crust. They are directly available to human beings for use. Hence, fossil fuels are the direct source of energy. These are limited in amount. These are non-renewable sources of energy because these cannot be replenished in nature. Fossil fuels take millions of years for their formation. If the present fossil fuel of the Earth gets exhausted, its formation will take several years. Fossil fuels are also very costly.
On the other hand, solar energy is a renewable and direct source of energy. The Sun has been shining for several years and will do so for the next five billion years. Solar energy is available free of cost to all in unlimited amount. It replenishes in the Sun itself.
5. Compare and contrast bio-mass and hydro electricity as sources of energy.
Answer
Bio-mass and hydro-electricity both are renewable sources of energy. Bio-mass is derived from dead plants and animal wastes. Hence, it is naturally replenished. It is the result of natural processes. Wood, gobargas, etc. are some of the examples of bio-mass.
Hydro-electricity, on the other hand, is obtained from the potential energy stored in water at a height. Energy from it can be produced again and again. It is harnessed from water and obtained from mechanical processes.
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