Lakhmir Singh Physics Class 10 Solutions Page No:141
Question 16: Compare the sun and the fossil fuels as the sources of energy.
Solution : Sun is a renewable source of energy, where as fossil fuels are non-renewable sources of energy.
Question 17: What kind of mirror, concave, convex or plane, would be best suited for use in a solar cooker ? Why ?
Solution : Concave and plane mirrors are used for making solar cooker because concave mirror converges a large amount of sun’s rays at a point that is required for high heating and plane mirror reflects the rays of light in the form of a strong beam of sunlight on the top of the box that is required for moderate heating.
Question 18: (a) Name that part of a box-type solar cooker which allows the sun’s heat rays to enter the box but does not
allow inside heat to go out.
(b) Explain why, a plane mirror reflector is used in a box-type solar cooker.
Solution : (a) Glass sheet cover
(b) A plane mirror reflector is used in a box type solar cooker so as to get a strong beam of sunlight after reflection from the mirror.
Question 19: What are the advantages and disadvantages of using a solar cooker ?
Solution : (a) Advantages of using solar cooker
(i) It saves precious fules like coal, kerosene etc.
(ii) It does not produce any smoke or ash.
(iii) The food cooked in a solar cooker has all its nutrients intact.
(b) Disadvantages of using solar cooker
(i) Solar cooker cannot be used during night time.
(ii) If the day-sky is covered with clouds, it will not be possible to cook the food using solar cooker.
(iii) Direction of reflector has to be changed from time to time to keep it facing the sun.
Question 20: (a) What is a solar cell ? Draw the labelled diagram of a solar cell.
(b) Name the semi-conductor material which is usually used for making solar cells.
(c) Write the uses of solar cells.
Solution : (a) Solar cell is a device which converts solar energy directly into electrical energy.
(c) Use of solar cell:
i. To provide electricity in artificial satellites and space probes.
ii. To provide electricity to remote areas where normal electricity transmission lines do not exist.
iii. To provide electricity to light houses
iv. To operate traffic signals, watches, calculators and toys.
Question 21: State the advantages and disadvantages of using solar cells.
Solution : (a) Advantage of solar cell
(i) They have no moving parts and require no maintenance.
(ii) They can be set up in remote and inaccessible areas
(b) Disadvantage of solar cell
(i) They are very expensive.
(ii) They are less efficient. They can convert only about 25 % of light falling on them into electricity.
(iii) They cannot work during night time.
Question 22: What is a solar cell panel ? For what purpose is it used ? State its two main advantages.
Solution : Solar cell panel consists of a large number of solar cells joined together in a definite pattern. It is used to convert solar energy into electricity.
Advantages of solar cell panel are:-
1. It provides much more electric power than a single solar cell.
2. It is used to provide electricity in remote and inaccessible rural areas.
Question 23: (a) What is solar constant ? What is the value of solar constant ?
(b) If the energy received by 5 m2 area in 10 minutes is 4200 kj, calculate the value of solar constant.
Solution : (a) The amount of solar energy received per second by one metre square area of the near earth space (exposed perpendicularly to the sun rays) at an average distance between the sun and the earth, is known as solar constant. Its value is 1.4 kJ/s/m2.
(b) Area, A=5m2; time,t=10 min=600sec; E=4200 kJ.
Solar constant =E/(Axt)
Question 24: How has the traditional use of energy of flowing water been modified for our convenience ?
Solution : Traditionally, the energy of flowing water has been used for rotating the water-wheels and for driving water-mills to grind wheat to make flour. The traditional use of energy of flowing water has been modified by improvements in technology and it is now used to generate electricity.
Question 25: How has the traditional use of wind energy been modified for our convenience ?
Solution : Traditionally, wind energy was used through windmills to pump water from a well and to grind wheat into flour. But the traditional use has been modified and now it is used for the generation of electricity.
Question 26: (a) What is hydroelectricity ? Explain the basic principle of generation of hydroelectricity with the help of a
(b) State two advantages of producing hydroelectricity.
(c) State two disadvantages of producing hydroelectricity.
Solution : (a) The electricity generated from hydro power plant is known as hydro electricity.
Water is collected in a reservoir at a height, so the water has potential energy stored in it. When the water flows down through this large height, its potential energy gets converted into kinetic energy. The fast flowing water rotates the turbine which is connected to generator through its shaft. The generator produces electricity.
(b) Advantage of producing hydroelectricity:
(i) It does not cause any environmental pollution.
(ii) It uses the energy of flowing water, which is a renewable source of energy
(c) Disadvantage of producing hydroelectricity
(i) Large areas of land are required.
(ii) Large eco-systems get destroyed.
Question 27: (a) With the help of a labelled diagram, explain the construction and working of a solar cooker.
(b) Why is the solar cooker box painted black from inside ?
(c) Why is the solar cooker box covered with a glass sheet ?
Solution : (a) Construction and working of solar cooker
A solar cooker consists of an insulated metal box or wooden box which is painted all black from inside. There is a thick glass sheet cover over the box and a plane mirror reflector is also attached to the box.
The food to be cooked is put in metal containers which are painted black from outside. These metal containers are then placed inside the solar cooker box covered with the glass
sheet. The sun’s rays fall on the reflector, get reflected into the box through the upper glass and get trapped within it.
(b) Solar cooker inner body is painted black because black color is a good absorber of heat, so it traps the maximum rays from the sun.
(c) Solar cooker is covered with a glass sheet so as to trap the heat rays in the interior of the box of the cooker.
Question 28: (a) What is wind ? What type of energy is possessed by wind ?
(b) Explain how, wind energy can be used to generate electricity. Illustrate your answer with the help of a labelled diagram.
(c) State two advantages of using wind energy for generating electricity.
(d) Mention two limitations of wind energy for generating electricity
Solution : (a) Moving air is known as wind. Wind posses kinetic energy.
(b) The fast moving wind strikes the wind turbine which starts rotating. The shaft of the wind turbine also rotates and drives the generator which produces electricity.
(c) Advantages of using wind energy for generating electricity:
(i) It does not cause any pollution.
(ii) It is available free of cost.
(d) Limitations of wind energy:
(i) Wind energy farms cannot be established everywhere. They can be established only at those places where wind blows for most part of the year.
(ii) Wind energy farms require large area of land for their establishment.