Class 10 Science Chapter 14 Question Answers - Sources of Energy - 1

Q1. Explain why only a part of the solar energy that strikes the upper regions of the atmosphere, reaches the surface of the earth.

The ultra-violet rays and the gamma rays present in the solar energy is absorbed by the atmosphere as it comes down towards the surface of the earth, hence we say that only a part of solar energy reaches the surface of the earth.

Q2. Exposure to which component of solar radiation could be a health hazard?

Ultra-violet rays and gamma rays are the components of solar radiation which could a health hazard.

Q3. With the help of a diagram, explain the construction and working of a box-type solar cooker.

The heat absorbing property of black surfaces and the green house effect are utilised in making solar cookers. There are various types of solar cooker out of which, the box type solar cooker is one. The box of the solar cooker is made from non-conducting materials like plastics or fiberglass, with its inner walls painted in black. The walls of the box are made thick with a lining of non-conducting material to prevent loss of heat. Usually, a plane mirror is hinged on the top of the box. The mirror is adjusted in such a manner that it reflects sunlight into the box. The main purpose of the mirror is to increase the effective area for the collection of solar energy. The box is covered with a glass sheet that facilitates retention of heat inside due to green house effect. The temperature inside this type of solar cookers can go from 100 ºC to 140 ºC when they are kept in the sun for two to three hours. This type of cookers can be used to prepare food items that require slow heating, for example, for boiling rice, vegetables or dal. The solar cookers are provided with small containers with their outer surfaces painted in black. Two or three items could be prepared simultaneously by using these containers.

Q4. Mention any two differences between a box type and a concentrator type solar heater.

Box-type solar heater:

• In this type of heater, a plane reflector is used which does not concentrate solar energy at a point.
• It produces comparatively low temperature.

Concentrator type solar heater:

• In this type of heater a concave reflector or a parabolic reflector is used which concentrates the solar energy in a small region called focus.
• It produces high temperature.

Q5. Why is it not possible to make use of solar cells to meet all our energy needs? State at least two reasons to support your answer.

• The electricity generated by solar cells during the day is stored by using it to change storage batteries. These storage batteries give us direct current, but to operate the various devices, alternating current is required.
• The usage of solar cells becomes expensive since the direct current given by the storage batteries has to be changed into alternating current using suitable appliances.

Q6. Mention any four areas where solar cells are being used as a source of energy.

Solar cells are used for street-lighting, for traffic signals, for operating water-pumps and for running radio and television sets in remote areas.

Q7. Why can wind energy farms be established only at specific locations? Give reasons to support your answer.

Wind energy farms can be established over a big area of land, where sufficient wind is blowing always, because electricity generated by a single wind turbine is quite small, so in order to generate a large amount of electricity, a large number of wind turbines are erected over a big area of land.

Q8. It is difficult to use hydrogen as a source of energy, although its calorific value is quite high. Explain. 

• When hydrogen gas is lighted, it burns with an explosion which is very unsafe.
• It is very expensive to produce hydrogen gas by the electrolysis of water because it requires a lot of electricity.
• It is very difficult to store and transport hydrogen gas as it is explosively inflammable.

Hence it is difficult to use hydrogen as a source of energy, though it has high calorific value.

Q9. State the forms in which energy stored in the oceans manifests itself. Which one of these is utilized in OTEC systems?

The energy from oceans is obtained in three forms:

• Tidal energy
• Ocean waves energy and
• Ocean thermal energy

Ocean thermal energy is used in OTEC system.

Q10. With the help of a diagram, explain the process of destructive distillation of wood.

Let us take two glass tubes. In one tube we put some small pieces of softwood and in another tube, we put some water. These two tubes are then arranged as shown in the figure. When we heat the tube with a burner, we can find that gas goes out through the open end of the delivery tube. If we alight a match stick near this gas, it starts burning showing that it is a combustible gas. This combustible gas obtained from wood is called wood gas and it is used as a fuel. In another tube, we can find that there are drops of deep black liquid at the bottom of the tube, underwater. This deep black liquid is called tar. The black residue left behind the other tube is called charcoal. When this tube is cooled we can remove the charcoal from it.

Q11. Why is charcoal considered a better fuel than wood? What are the disadvantages of converting wood into charcoal?

Charcoal is a better fuel than wood because of the following reasons:

• Charcoal has better calorific value than wood, i.e., charcoal produces more heat on burning than an equal mass of wood.
• Charcoal does not produce smoke while burning whereas wood produces a lot of smoke on burning and pollutes the air.
• Charcoal is a compact fuel which is easy to handle and convenient to use.

Disadvantages of converting wood into charcoal:

Cutting down of trees in the forests has created the shortage of wood to prepare sufficient charcoal. As a result of this, charcoal has now become expensive fuel. Hence the use of charcoal as a fuel is discouraged so as to prevent deforestation and hence to conserve the ecosystem from the ill effects of deforestation.

Q12. Describe the steps involved in obtaining biogas and explain what is meant by anaerobic decomposition.

Two types of biogas plants are being used in our country; they are
1. The fixed-dome type
2. The floating gas-holder type
The main raw material used in these plants is animal dung (cow-dung, dung of horse, elephant, goat, etc). Plant wastes like vegetable skins, fruit pulp and human excreta may be added to the animal dung.
Biogas is produced by the anaerobic decomposition of animal wastes like animal dung in the presence of water. The decomposition which takes place in the absence of oxygen is called anaerobic decomposition. This decomposition is carried out by anaerobic micro-organisms called anaerobic bacteria in the presence of water but in the absence of oxygen. Animal dung and plant wastes contain a lot of carbon compounds like carbohydrates, proteins and fats. The anaerobic bacteria decomposes these carbon compounds to form methane gas, which is the main constituent of bio-gas. Some other gases like carbon dioxide, hydrogen and hydrogen sulphide are also formed.

Q13. State the advantages of obtaining biogas from animal dung and bio-wastes.

• The bio-gas obtained from animal dung and bio-wastes can be used as a smokeless fuel.
• It gives us a clean fuel.
• The spent dung can be used as a manure.

Q14. Why are fossils fuels classified as non-renewable sources of energy? What steps should be taken to conserve these sources?

Fossil fuels are non-renewable source of energy, because if they are exhausted, it cannot be regenerated in a short time. This is because of the fact that, the fossil fuels which we use today, took millions of years to be formed. The fossil fuels are being fast depleted. To overcome this depletion, we have to conserve energy. We can follow the following simple rules to conserve energy:

• Switch off lights and fans when not in use.
• Use solar cookers, heaters, dryers wherever possible.
• Use tube lights which consumes less energy instead of incandescent lamp wherever possible.
• Use efficient home appliances and fuel efficient stove.

Q15. The heat produced on complete combustion of 10 g of a fuel could raise the temperature of 2 kg of water from 20° C to 70° C. Calculate the calorific value of the fuel, if the specific heat capacity of water was 4.2 J/g ° C. Assume that the heat taken by the container is negligible.

Mass of water = 2 kg = 2000 g
Specific heat of water = 4.2 J/g ° C
Rise in temperature, t = 70 ° C – 20 ° C = 50° C
We know that heat produced Q = m × s × t
= 2000 × 4.2 × 50
= 420000 joules
This heat of 420000 joules has been produced by burning 10 g of fuel.
Let us calculate the heat produced by burning 1 g of fuel, hence
10 g of fuel produce heat = 420000 joules
and 1 g of fuel produce heat = 420000 /10 = 42000 joule.
Thus, the calorific value of the fuel is 42000 joules per gram.
To convert it into kilojoules per gram, we have to divide by 1000, hence
Calorific value of fuel = 42000/1000 kJ/g = 42 kJ/g.

Q16. How much energy will 1 kg mass of wood yield on complete combustion if its calorific value were 15 kJ/g?

Calorific value of wood = 15 kJ/g = 15000 J/g
Mass of wood = 1 kg = 1000 g
Therefore heat produced by 1 kg of wood = 15000 × 1000 = 15 × 10⁵ J.

Q17. Suppose the average solar energy incident on the green canopy (leaves) of a tree is 10⁸ J per day. The calorific value of the wood obtained from the tree could convert 1% of incident solar energy as wood, how many days will it take to produce 10 kg of wood?

Calorific value of wood = 15KJ/g
Conversely the same energy (15KJ) is required to form 1g of wood.
Energy required to form 10Kg of wood = 10 × 1000 × 15 = 15 × 10⁴ KJ of energy
The average solar energy received per day = 10⁸J day^-1 
But 1% of incident energy that is useful for the formation of wood = (10⁸) / 100 = 10⁶ J day^-1 
No of days required to form 10kg of wood = (15 x 10⁴ x 10³) / 10⁶ = 15 × 10 = 150 days.

Q18. State the conditions essential for combustion to take place.

The conditions for combustion to take place are:

• Presence of a combustible substance.
• Presence of a supporter of combustion like air or oxygen.
• Presence of a combustible substance to its ignition temperature.

Q19. Explain how knowledge of the conditions of combustion could help in fire fighting.

The combustible substance like furniture, clothes and books, etc., are removed first when fire starts in a room, so that the fire may not spread due to the presence of a large number of combustible substance.

When the burning clothes of a person are covered with a blanket, the supply of air is cut off and burning stops.

A wet cloth does not burn because water present in it keeps the temperature of the cloth below its ignition temperature.

Q20. What constitutes a source of energy?

A resource which can provide adequate amount of energy in a convenient form over a long period of time constitutes a source of energy.

Q21. State the two forms in which energy is mainly utilized at our homes.

Electrical energy is used for lighting bulbs, tubes and to run household appliances. Heat energy obtained by burning wood, coal, kerosene or cooking gas are used for cooking our food.

Q22. Explain why fossils fuels are classified as non-renewable sources of energy.

Fossil fuels are present in limited amounts in the earth and they cannot be replaced quickly when exhausted. Hence they are called non-renewable sources.

Q23. Name any two renewable sources of energy.

• Solar energy
• Wind energy.

Q24. Why is the use of wood as a fuel not advised although forests can be replenished?

Wood produces a lot of smoke on burning and pollutes the air. Hence it is advised not to use it as a fuel.

Q25. How much solar energy will be received by 1m² area in one hour, if the solar constant were 1.4 kilowatt per square metre?

One hour = 60 × 60 = 3600 s
Solar constant = 1.4 kW/m² = 1.4 kJ/s/m²
i.e., in 1s, 1m² area receives energy = 1.4 kJ
So, in 1 hour, i.e., in 3600 s, 1 m² area receive energy = 1.4 × 3600 kJ = 5040 kJ.

Q26. What is the range of the wavelength of electromagnetic waves that constitutes visible radiation?

The wavelength range for the visible radiation is 400 nm in violet to 700 nm in red.

Q27. Name the component of sunlight that mainly carries heat with it.

Infrared rays carries the sensation of heat in sunlight.

Q28. Name any two components of solar radiation that are not visible to us.

Ultraviolet rays and infrared rays.

Q29: Name the type of radiation emitted by a hot electric iron?

Infrared radiation.

Q30. State any two activities from our daily life in which solar energy is utilized.

• Solar energy is used for drying clothes.
• Solar energy is used for the preservation of fruits, vegetables and fish, etc., by the process of sun-drying.

Q31. Wavelength of radiation incident on a surface is 850 nm. Will the surface become visible when exposed to this radiation?

The surface is not visible since the radiation incident is more than the visible range, i.e., the incident radiation is infrared radiation which is invisible.

Q32. Name any two materials that are used for making solar cells.

Solar cells are made from semiconductor materials like silicon and gallium.

Q33. What prevents us in making use of solar cell panels to meet all our domestic needs of electricity?

The electricity generated by solar cell panels during the day is stored by using it to charge storage batteries. These storage batteries give us direct current. But to operate the various devices, alternating current is required. So, the direct current given by storage batteries has to be changed into alternating current by using suitable appliances before it can be used to run various devices. This increases the cost of using solar panels as the source of electricity.

Q34. Explain the principle behind the working of a windmill.

When the blowing wind strikes across the blades of wind-mill, it exerts a force on them due to which the blades of the wind-mill start rotating. As long as the wind is blowing, the blades of wind-mill keep on rotating continuously. The rotational motion of the blades of the wind-mill can be used to drive a large number of machines like water-pumps, flour mills and electric generators.

Q35. Name the substance from which alcohol is manufactured on a large scale. What is this process known as?

Alcohol is made from sugar. It is manufactured in large quantities by fermentation of sugar.