Exercise : Solution of Questions on page Number : 121
Answer 1:
Nuclear fuel, such as uranium, is a non-renewable source of energy other than fossil fuels.
Answer 2:
The amount of heat produced when a unit mass of a fuel is completely burnt is known as the calorific value of the fuel.
Answer 3:
When you say that the caloric value of cooking gas (liquefied petroleum gas, or LPG) is 50 kJ/g, it means that when 1 gram of LPG is burnt completely, it produces 50 kilojoules of energy in the form of heat.
Answer 4:
Liquefied petroleum gas, or LPG, produces more heat than coal, because LPG (50 kJ/g) has higher a calorific value than coal (25-30 kJ/g).
Exercise : Solution of Questions on page Number : 122
Answer 5:
The ignition temperature of a fuel is the minimum temperature to which a fuel must be heated so that it catches fire and starts burning.
Answer 6:
If the ignition temperature of a fuel is 80°C, it means that the fuel must be heated up to 80°C to burn. No fuel can burn unless it is heated to its ignition temperature.
Answer 7:
The amount of heat produced by burning a unit mass of a fuel completely is known as its ‘calorific’ value.
Answer 8:
A source of energy is that which provides a required amount of energy in a convenient form for a long time.
There are basically two types of sources of energy:
a) Renewable source of energy
b) Non-renewable source of energy
Answer 9:
Four characteristics of a good source of energy are as follows.
a) It should have a high calorific value.
b) It should be cheap and easily available.
c) It should be safe to handle and use.
d) It should be environment friendly, i.e, it should not cause pollution.
Answer 10:
The sources of energy that have accumulated in nature over a very long time and cannot be quickly replaced when exhausted are called non-renewable sources of energy.
The two examples of non-renewable sources of energy are:
a) Fossil fuels (such as coal, petroleum and natural gas)
b) Nuclear fuels (such as uranium).
Answer 11:
The sources of energy that are being produced continuously in nature and are inexhaustible are called renewable sources of energy. Examples: Wind, wood, sunlight and biomass.
Answer 12:
| Renewable Sources of Energy | Non-Renewable Sources of Energy |
| 1. Renewable sources of energy are those sources of energy that are being produced continuously in nature.
2.These sources of energy can be used again and again. 3. Examples of renewable sources of energy are wind, sunlight and tides. |
Non-renewable sources of energy are those sources of energy that have accumulated in nature over a very long time. These sources of energy cannot be used again and again. Examples of non-renewable sources of energy are coal, petroleum and natural gas. |
Answer 13:
Fossil fuels are classified as non-renewable sources of energy because they are present in limited amounts on the earth. Once they are exhausted, they cannot be replenished, and they will no longer be available to us.
Answer 14:
The following are two sources of energy that are renewable.
a) Wind, which is used to generate energy using wind turbines
b) Water, which is used to generate hydroelectricity from flowing water
They are renewable because they are abundant in nature and can be used again and again..
Answer 15:
The following are two non-renewable sources of energy.
a) Coal
b) Crude oil
They are non-renewable because they are present in limited amounts on the earth. Once they are exhausted, they cannot be replenished, and they will no longer be available to us.
Answer 16:
(a) Coal, petroleum and natural gas are non-renewable sources of energy, whereas wind, tides and wood are renewable sources of energy.
(b) The basis of the above classification is the abundance or depletion of the energy sources mentioned and the amount available for the future.
Answer 17:
Coal is classified as a non-renewable source of energy because, when trees die and are buried in the earth, they take many years to decompose and form coal. So, they are not easily available. But wood will continue to be available to us if we use it properly as per a plan.
Answer 18:
a) Materials that burn to produce heat energy are known as fuels.
Examples of fuels are wood, coal, petrol, diesel and liquefied petroleum gas (LPG).
b) The characteristics of a good fuel are the following.
1. It should have a high calorific value, i.e., it should give an adequate amount of heat per unit mass.
2. It should burn without giving out any harmful gases, so that it does not pollute the air.
3. It should have a proper ignition temperature, so that it burns easily. (Its ignition temperature should neither be too low nor too high.)
4. It should be cheap, easily available and easy to handle.
5. After burning, it should not leave much ash.
c) Advantages of fuel ‘A’.
1.It has a moderate ignition temperature.
2. It does not produce any harmful gases.
Disadvantage:
1. It has a low calorific value and produces less heat per unit mass.
Advantage of fuel ‘B’:
1. It has a high calorific value and hence generates more heat.
Disadvantages:
1. It has a very low ignition temperature and hence is unsafe to use.
2. It produces harmful gases, such as CO and SO2.
Answer 19:
(c) Biogas
Biogas is a renewable source of energy .
Answer 20:
(d) natural gas
Natural gas is a non-renewable source of energy as it takes thousands of years to form.
Answer 21:
(c) Fossil fuels
Fossil fuels are non-renewable sources of energy because they are present in limited amounts on the earth.
Answer 22:
(c) high calorific value and moderate ignition temperature
A good fuel should have a high calorific value so that it can give us more heat per unit mass. It should also have a moderate ignition temperature.
Answer 23:
(b) methane gas
Methane has a calorific value of 55 kJ/g.
Answer 24:
(d) 15 years
A plant sapling takes more than 15 years to grow and mature into a tree.
Answer 25:
(c) Hydrogen gas
Hydrogen gas has the highest calorific value (150 kJ/g).
Answer 26:
(b) wood
Wood has the lowest calorific value among the given fuels.
Exercise : Solution of Questions on page Number : 123
Answer 27:
(b) less of carbon but more of hydrogen gas
The fuel with the highest calorific value would have less carbon and more hydrogen, because hydrogen has a higher calorific value than carbon.
Answer 28:
(d) high ignition temperature
A good fuel should not have a high ignition temperature.
Answer 29:
(c) Biogas
Fossils form deep under the earth’s surface from the prehistoric remains of living organisms. Biogas is a mixture of methane, carbon dioxide, hydrogen and hydrogen sulphide. It is produced by the anaerobic degradation of animal wastes such as cow dung in the presence of water.
Answer 30:
Fuel A has a low calorific value compared with the other fuels, and therefore, it cannot be considered the most ideal fuel.
Fuel B is the most ideal fuel of the three for the following reasons.
(a) It has a high calorific value.
(b) It burns completely without leaving any residue.
Fuel C has both the characteristics of fuel B but is unsafe as it explodes.
Hence, B is the ideal fuel.
Answer 31:
Fuel Y is the better fuel for the following reasons.
(a) It has a moderate ignition temperature of 75°C, i.e., neither too high nor too low.
(b) It does not produce harmful gases such as carbon monoxide (CO).
Answer 32:
(i) Cooking gas is D.
(ii) Alcohol is E.
(iii) Wood is B.
(iv) Hydrogen gas is C.
(v) Kerosene is A.
Answer 33:
The fuels in the decreasing order of calorific values are as follows.
Hydrogen gas (150 kJ/g) > Methane (55 kJ/g) > Petrol (50 kJ/g) > Kerosene (48 kJ/g) > Biogas (40 kJ/g) > Wood (17 kJ/g).
Answer 34:
The fuels in the increasing order of calorific values are as follows.
Dung cakes < Coal < Alcohol < Diesel < LPG.
Answer 35:
Hydrogen gas has a high calorific value (150 kJ/g) but does not contain carbon.
Exercise : Solution of Questions on page Number : 130
Answer 1:
Diesel is used to drive heavy vehicles.
Answer 2:
Liquefied petroleum gas (LPG) is used as a domestic fuel.
Answer 3:
Kerosene is obtained during the fractional distillation of petroleum.
Answer 4:
Fossil fuels, such as coal, oil and gas, are burnt to generate electricity in a thermal power plant.
Answer 5:
Four fractions obtained from petroleum that are used as fuels are petrol (gasoline), diesel, kerosene and petroleum gas.
Exercise : Solution of Questions on page Number : 131
Answer 6:
Liquefied petroleum gas (LPG) consists mainly of butane along with smaller amounts of propane and ethane.
Answer 7:
Compressed natural gas (CNG) is being increasingly used in transport vehicles.
Answer 8:
The full forms of the abbreviations LPG and CNG are as follows.
(i) LPG is liquefied petroleum gas.
(ii) CNG is compressed natural gas.
Answer 9:
The main constituents of these gases are as follows.
(i) Petroleum gas: Butane
(ii) Natural gas: Methane
Answer 10:
Methane is found in natural gas as well as in biogas.
Answer 11:
Two uses of natural gas are
a) as a fuel in thermal power plants
b) as a fuel in transport vehicles
Answer 12:
Compressed natural gas (CNG ) is used as a fuel for transport vehicles.
Answer 13:
Domestic gas cylinders, such as those of Indane, mainly contain ‘butane’.
Answer 14:
Natural gas is a good fuel because it has a high calorific value of up to 50 kJ/g. It burns with a smokeless flame and causes no air pollution. There is also no need for additional storage and transport because it can be directly supplied from gas wells to homes.
Answer 15:
The traditional sources of energy that are familiar to most people are called conventional sources of energy.
Coal and petroleum are two conventional sources of energy.
Answer 16:
A diagram showing the working of a thermal power plant:
Explanation:
In a thermal power plant, electricity is generated using coal as the fuel. Coal is used to generate steam, which helps rotate turbines connected to a generator. The generator converts mechanical energy into electrical energy.
Answer 17:
The disadvantages of burning fossil fuels are as follows.
(a) The burning of fossil fuels produces gases such as sulphur dioxide and nitrogen oxide, which cause acid rain, damaging trees and land and affecting aquatic life.
(b) It produces carbon dioxide, which affects all kinds of organisms.
(c) It produces smoke and ashes, which cause air pollution.
Answer 18:
The burning of fossil fuels produces gases such as sulphur dioxide and nitrogen oxides that cause acid rain. Acid rain damages trees, land and buildings and affects aquatic life. The burning of fossil fuels produces carbon dioxide, which is responsible for the greenhouse effect on the earth. It also produces smoke and ashes, which pollute the air.
Answer 19:
Various steps can be taken to reduce the pollution caused by burning fossil fuels. These are as follows.
(a) The air pollution caused by the burning of petroleum fuels in vehicles can be reduced by using catalytic converters in vehicles. Catalytic converters convert harmful gases into harmless gases.
(b) The air pollution caused by the burning of coal in thermal power plants can be reduced by washing smoke and acidic gases with water in scrubbers.
(c) The air pollution caused by thermal power plants can also be reduced by installing electrostatic precipitators in their chimneys. The unburnt carbon particles and fly ash get deposited in the chimneys before the exhaust gases from the power plants are let into the air.
Answer 20:
Natural gas can be used for heating and cooking food as it is a clean source of energy. It has a high calorific value and does not produce a large amount of smoke on burning. It is easy to use and transport.
Answer 21:
Liquefied petroleum gas (LPG) is considered a good fuel for the following reasons.
a) It has a high calorific value (50 kJ/g).
b) It burns with a smokeless flame and hence does not pollute the air.
c) It is easy to handle and convenient to store.
d) It is a clean domestic fuel and does not produce any harmful gas.
Answer 22:
Liquefied petroleum gas (LPG) is considered a better fuel than coal for the following reasons.
a) It has a higher calorific value (50 kJ/g) than coal (30 kJ/g).
b) It burns with a smokeless flame and hence does not pollute air, whereas coal produces smoke on burning.
c) It is easier to handle and more convenient to store than coal.
d) It is a clean domestic fuel and does not produce any harmful gas, whereas coal, on burning, produces carbon dioxide, which causes air pollution.
Answer 23:
Liquefied petroleum gas (LPG) is odourless, but to detect leakage of the gas, a small quantity of strong-smelling ethyl mercaptan (C2H5SH) is added to it. Ethyl mercaptan has a foul smell resembling that of hydrogen sulphide gas, which can be easily detected.
The following steps are to be taken when leakage of LPG is detected in the kitchen.
a) Lighted matchsticks and other burning substances should be kept away.
b) The knob of the gas stove must be closed.
c) The valve of the gas cylinder should be checked and changed immediately if any damage is found.
Answer 24:
(a) Natural fuels that are found deep inside the earth and are formed from the prehistoric remains of organisms are called fossil fuels. Three examples of fossil fuels are coal, petroleum and natural gas.
(b) Plants and animals that died millions of years ago got buried deep inside the earth and covered by mud and sand. In the absence of oxygen and because of the chemical effects of pressure, heat and bacteria, the remains of the plants and animals were, over the years, converted into fossil fuels, such as coal, petroleum and natural gas. The buried remains of large plants got converted into coal and those of small plants and animals into petroleum and natural gas.
(c) We know that green plants use sunlight for photosynthesis, which is the process by which they produce food for their growth. When animals eat plants for energy, they consume food made with the help of sunlight. When plants and animals die, their remains, which consist of material directly or indirectly made with the help of sunlight, are buried in the earth and converted into fossil fuels, such as coal and petroleum. Hence, the sun is the ultimate source of fossil fuels.
(d) Petroleum and natural gas are formed from the buried remains of small plants and animals.
(e) Coal is formed from the buried remains of large plants.
Answer 25:
(c) butane
Butane is the main constituent of petroleum gas.
Answer 26:
(a) methane
Natural gas consists mainly of methane.
Answer 27:
(c) sodium oxides
The burning of fossil fuels produces sulphur dioxide, nitrogen dioxide and carbon dioxide.
Answer 28:
(c) Diesel
Diesel is used to drive heavy vehicles.
Answer 29:
(b) kerosene
A special grade of kerosene oil is used as aviation fuel in jet aeroplanes.
Answer 30:
(c) sun
The sun is the ultimate source of energy stored in all fossil fuels.
Answer 31:
(b) Cow-dung cakes
Fossil fuels take thousands of years to form deep under the earth. Cow dung does not come under this category of fuels.
Answer 32:
(b) uranium
Thermal power plants use only coal, oil or gas as fuel.
Exercise : Solution of Questions on page Number : 132
Answer 33:
(a) butane
Liquefied petroleum gas (LPG) consists mainly of butane, along with smaller amounts of propane and ethane.
Answer 34:
(c) as a reducing agent
Coke is used as a reducing agent in the extraction of metals from their ores.
Answer 35:
(d) charcoal
Charcoal cannot be obtained from coal.
Answer 36:
(c) carbon monoxide
Only nitrogen oxides, sulphur dioxide and carbon dioxide are responsible for acid rain.
Answer 37:
(a) Element X is carbon.
(b) Another element found in combination with carbon in fossil fuels is hydrogen.
Answer 38:
We know that green plants use the energy from sunlight for photosynthesis and growth. They store this energy in the form of carbon compounds. Thus, every leaf and every bit of wood in plants and trees is made using the energy from sunlight. Animals eat plants for energy, which is originally received from sunlight. These plants and animals die and their remains get converted into coal and petrol, respectively, which are energy-rich compounds of carbon that were made by plants using solar energy.
Answer 39:
(a) Substance X is ethyl mercaptan (C2H5SH).
(b) Ethyl mercaptan is a pungent substance and hence easy to detect.
Answer 40:
(a) The device is a ‘catalytic converter’.
(b) A catalytic converter converts the harmful gases from the exhaust of vehicles into harmless gases. For example, it converts carbon monoxide into carbon dioxide, and nitrogen oxide into nitrogen gas.
Exercise : Solution of Questions on page Number : 140
Answer 1:
A hydropower plant converts potential energy (of stored water) into electrical energy by rotating turbines in flowing water.
Answer 2:
Flowing water possesses kinetic energy.
Answer 3:
The kinetic energy of flowing water is used to rotate a turbine.
Answer 4:
The sun is the original source of wind energy.
Answer 5:
The minimum wind speed for the satisfactory working of a wind-powered electric generator is 15 km/h.
Answer 6:
(a) In the past, wind energy was used in windmills to pump water from wells and to grind wheat into flour.
(b) Nowadays, wind energy is used to generate electricity through wind-powered generators.
40
Answer 7:
The copper tube of a solar water heater is painted black from outside so as to collect as much the sun’s heat as possible, because a black surface absorbs more heat than a white surface.
Answer 8:
The nuclear fusion reaction that occurs inside the sun produces solar energy.
Answer 9:
Solar cookers, solar water heaters and solar cells are solar energy devices.
Answer 10:
A plane mirror is used as the reflector in a box-type solar cooker.
Answer 11:
The range of temperature achieved in a box-type solar cooker in two to three hours is 100°C to 140°C.
Answer 12:
Solar cells convert sunlight into electricity.
Answer 13:
Solar constant = 1.4 kW/m2 = 1.4 kJ/s/m2
1 hour in seconds = 60 × 60 = 3600
Energy received in one hour = 3600 × 1.4 kJ/m2 = 5040 kJ/m2
Answer 14:
A solar cell converts solar energy into electrical energy.
Answer 15:
(a)
| Hydropower plant | Thermal power plant | |
| 1. | A hydropower plant uses flowing water to rotate turbines. | A thermal power plant uses coal, oil or gas to drive turbines. |
| 2. | A hydropower plant does not cause air pollution. | A thermal power plant causes air pollution. |
(b) A thermal power plant causes serious air pollution. This is because, in a thermal power plant, large amounts of fossil fuels are burnt every day to generate electricity.
Exercise : Solution of Questions on page Number : 141
Answer 16:
| Fossils fuels | The sun |
| 1. Fossils fuels are formed from the remains of dead plants and animals. 2. The burning of fossil fuels causes air pollution. 3. Fossils fuel are non-renewable sources of energy. |
Inside the sun, nuclear fission occurs, which produces enormous amounts of heat. The use of solar energy does not cause any air pollution. Sunlight is a renewable source of energy. |
Answer 17:
A concave mirror is best suited for use in a solar cooker because it converges a large amount of the sun’s heat rays at its focus, because of which a high temperature is produced in the focus area.
Answer 18:
(a) A glass-sheet cover in a box-type solar cooker allows the sun’s heat to enter the box but does not allow the heat inside to go out.
(b) A plane mirror reflects the sun’s rays in the form of a strong beam of light. This property of a plane mirror is used in a box-type solar cooker.
Answer 19:
The advantages of using a solar cooker are as follows.
a) It does not require fuels such as coal, kerosene and liquefied petroleum gas (LPG).
b) It does not produce harmful smoke that pollutes the air.
c) It does not spoil the nutrients in food.
d) It allows for the preparation of more than one item at a time.
The disadvantages of using a solar cooker are as follows.
a) It cannot be used to cook food at night.
b) A cloudy sky affects its working.
c) The direction of the reflector has to be changed from time to time.
d) It cannot be used for baking.
Answer 20:
(a) A solar cell is an electrical device that converts light energy into electrical energy.
(b) A semiconductor material, such as silicon, is used in making solar cells.
(c) The uses of solar cells are given below.
(1) To provide electricity to electrical appliances such as bulbs and geysers
(2) To generate power in a space station
Answer 21:
The advantages of solar cells are that they do not have moving parts, they require no maintenance and they work with any kind of light-focussing devices. Moreover, they can be used even in remote areas.
The disadvantages of solar cells are that they require an expensive special grade of silicon or silver wire, they have low efficiency and they convert only 25 per cent of light falling on them into electricity.
Answer 22:
A solar cell panel consists of a large number solar cells joined together in a series in a definite pattern.
Solar cell panels are used to provide electricity in remote and inaccessible rural areas.
The advantages of solar cell panels are given below.
(a) They provide a larger amount of electricity than a single cell and hence can be used to operate TVs and water pumps.
(b) The electricity produced by solar cell panels can be stored in batteries and used at night.
Answer 23:
(a) The amount of solar energy received per second on one square metre area of the near-earth space at an average distance between the sun and the earth is called the solar constant. Its value is 1.4 kJ/s/m2 or 1.4 kW/m2.
(b) Area = 5 m2
Time = 10 min = 10 × 60 sec = 600 sec
Energy = 4200 kJ
Therefore,
solar constant = Energy/(Time × Area)
= 4200/(5 × 600) kJ/s/m2
= 4200/(3000) kJ/s/m2
=1.4 kJ/s/m2
=1.4 kW/m2
Answer 24:
Before the beginning of the Industrial Revolution, water energy served many purposes, but the methods of using it were not efficient. For example, the flow of water was used for transporting wooden logs and grinding wheat to make flour. Nowadays, water energy is harnessed to produce electricity, which is a better and more efficient use of this form of energy.
Answer 25:
Before the beginning of the Industrial Revolution, wind energy served many purposes, but the methods of using it were not efficient. For example, windmills were used to power smaller mills, such as flour mills and saw mills. Nowadays, wind energy is harnessed to produce electricity, which is a better and more efficient use of this form of energy.
Answer 26:
(a) Hydroelectricity is the product of a process by which mechanical energy from the flow of water is used to generate electricity.
It works on the principle of conversion of mechanical energy into electrical energy. Mechanical energy from the flow of water is used to rotate turbines, which are connected to a dynamo that generates electricity.
(b) The advantages of hydroelectricity are given below.
(1) It does not cause any pollution.
(2) It is a renewable form of energy.
(c) The disadvantages of hydroelectricity are given below.
(1) Hydroelectricity plants require the construction of large dams, which affects the ecology of the surroundings.
(2) Hydropower plants require water in reservoirs and are dependent on rain for their functioning.
Answer 27:
(a) A solar cooker is illustrated below.
Construction:
A mirror, black box and glass lid are required to construct a solar cooker..
The mirror is mounted on the box to concentrate light rays onto the cooker box. The box is painted back from the inside to absorb the maximum light. It is covered with a glass lid to produce a greenhouse effect inside it. See the diagram given above.
Explanation:
A solar cooker is a device in which heat energy from sunlight is trapped inside the cooker using the phenomenon of the greenhouse effect.
A solar heating device such as a solar cooker uses a reflector to reflect sunlight to a small area where the cooking container is placed. The light from the sun is focussed on the container, which gets heated, cooking the food.
(b) A solar cooker box is painted back from the inside so that light is not reflected and the maximum absorption takes place.
(c) A solar cooker is covered with a glass sheet to produce a greenhouse effect inside the cooker box. The greenhouse effect is a phenomenon by which heat is trapped.
Answer 28:
(a) Wind is the flow of air and is a source of energy. It possesses mechanical energy.
(b) Wind is the flow of air and possesses mechanical energy. This mechanical energy is used to rotate wind turbines, which are connected to electricity generators. See the diagram given below.
(c) The advantages of wind energy are as follows.
(1) Wind is a clean source of energy. It does not cause any pollution.
(2) It is a renewable source of energy.
(d) The limitations of wind energy are as follows.
(1) The production of energy depends on the flow of air. It varies during different times of the day and is seasonal.
(2) The initial capital cost is very high.
Answer 29:
(b) a cloudy day
A solar water heater receives energy from the sun, but the sun does not appear on a cloudy day. Therefore, a solar water heater does not receive energy on a cloudy day and cannot heat up water.
Answer 30:
(d) potential energy possessed by stored water is converted into electricity
A hydropower plant uses running water to rotate turbines.
Answer 31:
(c) glass-sheet cover
The part of a box-type solar cooker that is responsible for producing a greenhouse effect is its glass-sheet cover.
Answer 32:
(c) semi-conductors
Solar cells are made of semiconductors.
Answer 33:
(c) 1.4 kW/m2
The value of the solar constant is 1.4kW/m2.
Answer 34:
(d) infrared rays
Infrared rays make sunlight warm. The warmth of sunlight is used in a solar cooker.
Answer 35:
(d) transparent glass sheet
A transparent glass sheet does not allow the incident sun’s heat to be reflected back.
Exercise : Solution of Questions on page Number : 142
Answer 36:
(a) 15 km/h
The minimum speed of wind should be 15km/h for the satisfactory working of a wind generator.
Answer 37:
(c) 5040 kJ
Solar constant = 1.4 kW/m2
1 hour = 60 × 60 sec = 3600 sec
Therefore, energy = 1.4 kJ/s/m2 × 3600 s = 5040 kJ.
Answer 38:
(b) the glass-sheet cover of the solar cooker is not closed
The glass sheet prevents the sunlight incident on it from being reflected back.
Answer 39:
(a) Electrical energy from the power station = 2000 MW = 2000000 kW
Energy produced by the wind turbine = 300 kW
Therefore, number of turbines needed = 2000000/300 = 6666.67
Hence, 6667 turbines will be needed.
(b) In the thermal power plant mentioned, coal is burnt to produce electricity. Therefore, the quantum of electrical energy produced is fixed. A windmill produces electrical energy depending on the speed of wind, which may vary from time to time. Therefore, wind turbines cannot replace the coal-fired thermal power plant.
Answer 40:
In a solar water heater, the storage tank is placed at a higher level than the solar panel containing coils because hot water is lighter and rises up.
Answer 41:
Solar cells are connected to rechargeable batteries to store the electricity that we obtain with the help of these cells. The stored electricity can be used later; for example, it can be used at night, when there is no sunlight.
Answer 42:
(a) The solar cells convert the energy from sunlight into electrical energy. This electrical energy is converted into chemical energy when the car batteries are charged. When these charged batteries are used, the chemical energy is converted into electrical energy. In the last stage, this electrical energy is converted into kinetic energy for the car by the electric motor that drives the car.
(b) (i) In bright sunlight, the car batteries are charged quickly.
(ii) On a cloudy day, the car batteries are charged very slowly.
(iii) At night, the car batteries cannot be charged as there is no sunlight.
Exercise : Solution of Questions on page Number : 148
Answer 1:
When wood is burnt in a limited supply of air, a black substance called charcoal is obtained as a residue.
Answer 2:
Geothermal energy is not derived from solar energy directly or indirectly. It is the heat energy obtained from the hot rocks present inside the earth.
Answer 3:
Geothermal energy is the heat energy obtained from the fission of radioactive materials present in the hot rocks inside the earth.
Answer 4:
A micro-organism called ‘anaerobic bacteria’ decomposes animal dung into biogas. Oxygen is not required for this decomposition.
Answer 5:
Methane is the major constituent of biogas that is used as a fuel.
Answer 6:
The constituents of biogas are the following.
1. Methane
2. Carbon dioxide
3. Hydrogen
4. Hydrogen sulphide
Answer 7:
Water is needed and oxygen is not.
Biogas is formed by the anaerobic degradation of cow dung in the presence of water but in the absence of oxygen.
Answer 8:
On the decomposition of the organic matter in cow dung, a clean fuel, ‘biogas’, is obtained.
Answer 9:
Apart from cattle dung, plant wastes can be used as a raw material in a biogas plant.
Answer 10:
The following are the three forms of energy that can be harnessed from the sea.
1. Tidal energy
2. Wave energy
3. Ocean thermal energy
Answer 11:
The two forms of energy in which solar energy manifests itself in the sea are sea wave energy and ocean thermal energy.
Answer 12:
The full form of OTE is ‘ocean thermal energy’.
Answer 13:
In a biogas plant, the function of anaerobic micro-organisms such as anaerobic bacteria is the degradation of animal wastes in the presence of water.
Answer 14:
False.
Tidal energy is not one of the forms of energy in which solar energy manifests itself in oceans.
Answer 15:
(a) Biomass is another form in which solar energy manifests itself.
(b) Tidal waves build up and recede twice a day.
Answer 16:
(a) Biomass
Biomass is organic matter that is used as a fuel to produce energy.
Examples: Wood, agricultural wastes and cow dung.
(b) Wood is one of the biomass resources that is still widely used as a source of heat energy in many households in our country.
Answer 17:
The two ways in which cow dung can be used as a fuel are given below.
1. Cow-dung cakes can be used as a fuel for cooking food.
2. Biogas can be made from cow dung for use as a smokeless fuel.
Biogas is a better fuel as it does not cause air pollution. After extracting biogas, the cow dung can still be used as a manure because it continues to retain all the nutrients that plants need.
Exercise : Solution of Questions on page Number : 149
Answer 18:
When wood is burnt in a limited supply of air, water and other volatile materials present in it are removed, and a black substance called charcoal is left behind.
Charcoal is a better fuel than wood for the following reasons.
1. Charcoal produces more heat on burning than an equal mass of wood.
2. Charcoal does not produce smoke while burning, whereas wood produces much smoke and causes air pollution.
3. Charcoal is a compact fuel that is easy to handle.
Answer 19:
| Biomass | Hydroelectricity | |
| 1. | Biomass energy is generated from organic matter including wood, agricultural waste and cow dung. | Hydroelectricity is produced from the potential energy of water stored at a height. |
| 2. | Biomass energy can only be used for heating and cooking. | Hydroelectricity can be used for running all types of electrical appliances. |
| 3. | No special device is required to produce energy from biomass. | Hydroelectricity can be produced by establishing a hydroelectricity plant. |
Answer 20:
Biogas is considered an ideal fuel for domestic use for the following reasons:
1. It does not cause air pollution as it burns without smoke.
2. It produces a large amount of heat per unit mass.
3. It is a clean fuel as it burns completely without leaving behind any residue.
4. It is cheaper than most common fuels.
Answer 21:
(a) The power of the rise and fall of the sea level (high tide and low tide) can be harnessed to generate electricity. During high tide, when the level of water in the sea is high, sea water flows from the sea into a reservoir at a dam and turns turbines. The turbines then turn generators to produce electricity. During low tide, when the level of sea water is low, the sea water stored in the dam reservoir is allowed to flow out into the sea. This also turns the turbines and produces electricity.
(b) Tidal energy is not likely to be a potential source of energy for the following reasons.
1. The rise and fall of sea water during high and low tides are not enough to generate electricity on a large scale.
2. Very few sites are suitable for building tidal dams.
Answer 22:
Two ways in which the energy of sea waves can be harnessed are the following.
1 By setting up floating generators in the sea, which would move up and down with the sea waves. This movement would drive the generators to produce electricity.
2. By letting the sea waves move up and down inside large tubes. As the waves move up, the air in the tubes is compressed. This compressed air can be used to turn the turbines of a generator to produce electricity.
Answer 23:
Ocean thermal energy
The energy that is generated using the difference between the temperatures of water at the surface of the ocean and at deeper levels is called ocean thermal energy.
To generate electricity, we use ocean thermal energy conversion (OTEC) power plants for which a temperature difference of 20 degrees Celsius between the surface water and the water at deeper levels is needed. In this process, the warm surface water of the ocean is used to boil a liquid such as ammonia or chlorofluorocarbon. The high-pressure vapours obtained are used to turn the turbine of a generator to produce electricity.
Answer 24:
The limitations of the energy that can be harnessed from the sea are as follows.
1. Very strong waves are required to obtain electricity from wave energy. Such strong waves are not available in the sea all the time.
2. Tidal energy depends on the relative positions of the earth, the moon and the sun.
3. To harness ocean thermal energy efficiently, the difference between the temperatures of the surface water (hot) and the water at a depth (cold) must be 20ºC or more.
4. Very few sites are suitable for building tidal dams.
Answer 25:
Biowastes and sewage can be disposed of by using them in large gas plants to produce biogas and manure.
This method is advantageous to us for the following reasons.
1. It is a safe method for large-scale utilisation of biowaste.
2. The biogas produced can be used as a fuel.
3. Good quality manure can be obtained by this method.
Answer 26:
The forms of energy that are not derived from the sun are geothermal energy, nuclear energy, hydroelectricity and energy obtained from coal and petroleum.
Geothermal energy is the heat energy obtained from the fission of radioactive materials present in the hot rocks inside the earth. Nuclear energy comes from the nucleus of an atom. Hydroelectricity is obtained from the flow of water.
Answer 27:
(a) Biogas is a mixture of methane, carbon dioxide, hydrogen and hydrogen sulphide.
The major component of biogas is methane. Up to 75 per cent of biogas consists of methane gas.
(b) Biogas is produced by using raw materials such as animal wastes and plant wastes in the presence of water.
(c)
Construction of a biogas plant
A biogas has an underground tank shaped like a well. It is called a digester and is made of bricks and cement. The dome of the digester acts as a storage facility for biogas. On the top of the dome, there is a gas outlet ‘S’ with a valve ‘V’. A sloping inlet chamber ‘I’ is situated on the left side, and on the right side, there is a rectangular outlet chamber ‘O’. The function of the inlet chamber is to introduce fresh dung slurry into the digester tank and that of the outlet chamber is to facilitate the removal of the spent slurry after the extraction of biogas.
Working
A slurry is made in the mixing tank by mixing cow dung and water and then fed into the digester tank through ‘I’. The digester is filled up to the cylindrical level, the dome being left free for the collection of biogas. In 50-60 days, the cow dung undergoes degradation by anaerobic bacteria in the presence of water but in the absence of oxygen, with the gradual evolution of gas. The biogas that is formed gets collected in the dome and exerts pressure on the slurry in the tank for its removal through the outlet chamber.
(d) Two uses of biogas are given below.
1. As a fuel for cooking food
2. As a fuel for lighting
(e) Two advantages of using biogas are the following.
1. Biogas is a clean fuel with a high calorific value, so it is a viable alternative to fossil fuels.
2. Large-scale utilisation of biowastes and sewage for producing biogas is a safe and efficient method of waste disposal and a source of energy.
Answer 28:
(a) Geothermal energy
Geothermal energy is the heat energy obtained from the fission of radioactive materials present in the hot rocks inside the earth.
(b) The hot rocks present below the surface of the earth are the sources of the heat provided by geothermal energy.
(c) The hot rocks inside the earth heat the underground water and turn it into steam. As more and more steam is formed between the rocks, it gets compressed to high pressures. A hole is drilled into the earth up to the hot rocks and a pipe is put into it. The steam present around the rocks comes up through the pipe at high pressure, which turns the turbine of a generator and thus produces electricity.
(d) Two advantages of geothermal energy are given below.
1. It is economical to use.
2. It does not cause any pollution.
(e) Two disadvantages of geothermal energy are given below.
1. It is available only in areas where there are hot rocks near the surface of the earth.
2. Deep drilling into the earth to obtain geothermal energy is technically very difficult and expensive.
Answer 29:
(c) atomic energy
Atomic energy is considered to be coming from atoms.
Answer 30:
(b) geothermal energy
Geothermal energy is the heat energy from the hot rocks present inside the earth and does not come directly or indirectly from the sun’s energy.
Answer 31:
(b) methane
Up to 75 per cent of biogas consists of methane gas, which makes it an excellent fuel.
Answer 32:
(d) methane
The major component of biogas is methane gas. Up to 75 per cent of biogas consists of methane.
Answer 33:
(c) burning of charcoal
Charcoal does not produce smoke while burning, so it is more environment friendly than the other fuels mentioned.
Answer 34:
(c) nuclear power
Nuclear power is a non-renewable form of energy, because nuclear materials are limited and hence we will run out of them one day.
Answer 35:
(c) moon
High tide is the rise of sea water due to the gravitational pull of the moon.
Answer 36:
(c) oxygen
Oxygen is not required for the production of biogas.
Answer 37:
(c) coke
Coke cannot be obtained from biomass.
Answer 38:
(c) natural gas
Natural gas is a non-renewable source of energy. The others are renewable sources.
Answer 39:
(a) fission of radioactive materials
Geothermal energy is obtained by the fission of radioactive materials that are naturally present in the hot rocks inside the earth.
Exercise : Solution of Questions on page Number : 150
Answer 40:
(c) hydropower
Hydropower projects have many adverse affects on the surroundings and the environment.
Answer 41:
(c) occurrence of floods
All the other given problems can be the consequences of establishing hydroelectric power plants.
Answer 42:
(a) Ocean thermal energy
(b) Yes. The water at the surface of an ocean gets heated by the heat of the sun and attains a higher temperature than the colder water at deeper levels. So, ocean thermal energy is derived from the sun.
(c) In ocean thermal energy conversion plants, the warm surface water of the ocean is used to boil a volatile liquid such as ammonia or a chlorofluorocarbon. This produces high-pressure vapour, which is used to turn the turbine of a generator and produce electricity.
(d) 20 degrees Celsius
Answer 43:
(a) The name ‘high tide’ is given to the condition of the sea when its water is raised.
(b) The name ‘low tide’ is given to the condition of the sea when its raised water recedes.
(c) The name of the form of energy that can be harnessed by this process is ‘tidal energy’.
(d)
Answer 44:
A: Coal (mined from the earth)
Coal is mined from the earth.
B: Coke. It consists mainly of carbon.
C: Wood. It is obtained from trees.
D: Charcoal. It consists mainly of carbon.
Answer 45:
It is geothermal energy, which is not derived directly or indirectly from the sun and does not cause any pollution. It can be converted into electricity. Also, geothermal energy is not available everywhere and is very difficult and expensive to obtain.
Exercise : Solution of Questions on page Number : 156
Answer 1:
(a) The nuclear fission of the heavy nucleus of a radioactive atom is responsible for the liberation of energy.
(b) The nuclear fusion of hydrogen nuclei into helium nuclei produces energy in the sun.
Answer 2:
The neutrons produced in the nuclear fission cause further fission of the heavy nuclei, leading to a self-sustaining chain reaction.
Answer 3:
The low-energy neutrons bring about the fission of uranium-235.
Answer 4:
The fission of uranium-235 is caused by low-energy neutrons, which are also called slow-moving neutrons.
Answer 5:
(a) Nuclear fusion is involved in the working of a hydrogen bomb.
(b) Nuclear fission is involved in the working of an atom bomb.
Answer 6:
Graphite acts as a moderator by slowing down fast-moving neutrons in a nuclear reactor.
Answer 7:
The control rods of a nuclear reactor are made of chemical elements that are capable of absorbing many neutrons without fissioning themselves, such as boron, silver, indium and cadmium.
Answer 8:
The purpose of the thick, concrete chamber surrounding the reactor of a nuclear power plant is to absorb the dangerous nuclear radiation so as to protect the outside world.
Exercise : Solution of Questions on page Number : 157
Answer 9:
In a nuclear power station, uranium is used as a fuel to produce heat.
Answer 10:
The energy produced during nuclear fission reactions is used for generating electricity .
Answer 11:
The energy released in nuclear reactions is expressed in the unit of ‘electron volt’ (eV) or ‘million electron volt’ (MeV). The electron volt is a small unit, so the energy released in nuclear reactions is usually expressed in terms of million electron volts.
Answer 12:
One atomic mass unit (u) is equivalent to 931 MeV of energy.
Answer 13:
(a) Splitting of a heavy nucleus into two lighter nuclei is called nuclear fission.
(b) Uranium-235 atoms will split when hit by neutrons. This is called nuclear fission.
(c) Nuclear fission is used in nuclear power stations for the production of electricity.
(d) In a nuclear power station, nuclear fission takes place in the reactor.
Answer 14:
Nuclear fission
Nuclear fission is the process by which the heavy nucleus of a radioactive atom splits up into smaller nuclei when bombarded with low-energy neutrons.
Example:
When uranium-235 atoms are bombarded with slow-moving neutrons, the heavy uranium nucleus breaks up to produce two medium-weight atoms, barium-139 and krypton-94, with the emission of three neutrons. A tremendous amount of energy is produced in this process.
Reaction:
U92238 + no1 →fission56Ba139+Kr9436+3n01+ Tremendous amount of energy
Answer 15:
(a) Nuclear fusion
Nuclear fusion is the process by which two nuclei of light elements combine to form a heavy nucleus.
Example:
When deuterium atoms are heated to an extremely high temperature under extremely high pressure, two deuterium nuclei combine to form a heavy nucleus of helium, and a neutron is emitted. A tremendous amount of energy is liberated in this fusion reaction.
Reaction:
H12+H12→H23+n01+Tremendous amount of energy
(b) When two positively charged nuclei are brought together, they repel each other. Therefore, a very high temperature is required to force the lighter nuclei to fuse together to form a bigger nucleus.
Answer 16:
Hydrogen atoms can be considered a nuclear fuel in the sun because the sun derives its energy from the fusion of hydrogen nuclei.
The main nuclear fusion reaction taking place in the sun, a reaction that releases a tremendous amount of energy, is the fusion of four hydrogen atom nuclei to form the bigger nucleus of the helium atom. The total energy produced by the fusion of hydrogen into helium is tremendous. All this energy is released in the form of heat and light.
4 H11 →inside the sunnuclear fusion He2 4+ 2e10+Tremendous amount of energy
Answer 17:
(a) Einstein’s mass–energy equation is given as follows:
E=mc2
E is the amount of energy produced if mass, m, is destroyed and c is the speed of light.
(b) If 25 atomic mass units (u) of a radioactive material is destroyed in a nuclear reaction, then the amount of energy released in MeV is given by:
25 × 931 = 23275 MeV (since 1 u = 931 MeV)
Answer 18:
(a) The sun and other stars obtain their energy from the nuclear fusion reactions of hydrogen. So, hydrogen can be considered a source of energy.
(b) We know that a hydrogen bomb consists of deuterium, tritium and lithium. A hydrogen bomb is exploded by using an atom bomb. When an atom bomb is exploded, much heat is produced, which raises the temperature of deuterium and tritium to an extent in a small time. At this temperature, fusion reactions of deuterium and tritium take place, producing a tremendous amount of energy. This process causes the explosion of a hydrogen bomb, releasing an enormous amount of energy in a very short time.
(c) In the sun as well as in a hydrogen bomb, energy is produced by nuclear fusion reactions of hydrogen.
Answer 19:
(a) When the atoms of a heavy element, U-235, are bombarded with slow-moving neutrons, the uranium nucleus breaks up to produce two medium-weight atoms, barium-139 and krypton-94, with the emission of three neutrons. A tremendous amount of energy is produced in this process. The name of this process is nuclear fission.
(b) The nuclear reaction is as follows.
U92235 +n01 →fissionBa56139 +Kr3694 +3n01 +Tremendous amount of energy
(c) The process is used in nuclear power stations.
Answer 20:
(a) The advantages of nuclear energy are given below.
1. In the production of nuclear energy, gases such as carbon dioxide and sulphur dioxide are not produced. So, it does not contribute to the greenhouse effect or acid rain.
2. There is no need for putting in nuclear fuel again and again, because once the fuel is loaded into the reactor, the nuclear power plant can go on producing electricity for two to three years at a stretch.
(b) The disadvantages of nuclear energy are given below.
1. The waste products are radioactive, and they keep on emitting harmful radiations.
2. The availability of fuel is limited.
3. There is the risk of accidents in nuclear reactors, which may lead to the leakage of radioactive materials, causing serious damage to all living things.
Answer 21:
(a) Uranium-235 produces the energy in the fuel rods.
(b) No. But a nuclear power plant can work day and night for two to three years on the same uranium fuel.
(c) Its energy is produced by nuclear fission.
(d) Graphite acts a moderator in the reactor by slowing down fast-moving neutrons in it.
(e) The boron rods act as control rods and absorb excess neutrons, preventing the fission reaction from going out of control.
(f) Liquid sodium is pumped to transfer the heat produced in the reactor by fission to a heat exchanger for converting water into steam.
Answer 22:
(a) Graphite is used as a moderator.
(b) The reactor is enclosed in a concrete chamber to absorb the nuclear radiation.
(c) Uranium-235 is used in the fuel rods.
(d) Boron rods are used as the control rods.
(e) Liquid sodium is used to carry away the heat.
Answer 23:
(a) Control rods control the rate of fission by absorbing excess neutrons.
(b) Liquid sodium is used to remove the heat from the reactor core. It is used as a coolant, and it transfers the heat to the heat exchanger. This heat is used to convert water into steam.
Answer 24:
When the control rods are inserted in the graphite core, they absorb excess neutrons and thus prevent the fission reaction from going out of control.
Answer 25:
Advantages:
1. Nuclear power stations need less fuel than power stations that burn fossil fuels to produce the same amount of electricity.
2. There is no emission of greenhouse gases.
3. Nuclear fuel does not produce sulphur dioxide, a gas that affects health and produces acid rain.
Disadvantages:
1. The initial cost of designing and building a nuclear power station is high.
2. Thermal pollution occurs because of the discharge of hot water from nuclear power stations.
3. There is the risk of accidents in a nuclear reaction.
Answer 26:
(a) A nuclear reactor is a device to harness nuclear energy (fission energy) in a controlled and economical way. The energy produced is used to generate electricity and to power ships.
Uranium is used as a fuel in a nuclear reactor.
(b)
In a nuclear power plant, the fission of U-235 is carried out in a steel vessel (V) of the reactor. Enriched uranium rods (A) are inserted in a graphite core. Here, graphite acts as a moderator that slows down the speed of neutrons. Boron rods (B) are inserted between the uranium rods. These boron rods control the fission reaction. The controlled fission of U-235 produces a large amount of heat energy. Liquid sodium is pumped continuously through the pipes embedded in the reactor, and it absorbs the heat produced in the reactor. The extremely hot sodium is then passed into the coil of a heat exchanger (H) containing water. The water absorbs the heat from the sodium and boils to form steam. The steam thus formed at high pressure is introduced into a turbine chamber (C) with a turbine (T). The turbine rotates because of the pressure of the steam and drives the generator to produce electricity.
(c) In case of an emergency, the control rods are fully inserted in the reactor. They absorb all the neutrons, shutting down the reactor.
(d) Five places in India where nuclear power plants are located are the following.
1. Tarapur (Maharashtra)
2. Kalapakkam (Tamil Nadu)
3. Kakrapar (Gujarat)
4. Narora (Uttar Pradesh)
5. Kaiga ( Karnataka )
Exercise : Solution of Questions on page Number : 158
Answer 27:
(a) Nuclear fission and nuclear fusion can be differentiated as follows.
| Nuclear fission | Nuclear fusion | |
| Definition | Fission is the splitting of the heavy nucleus of a radioactive atom into smaller nuclei. | Fusion is the fusing of two or more lighter nuclei into a heavy nucleus. |
| Natural occurrence of the process | The process of nuclear fission does not normally occur in nature. | Fusion occurs in stars, such as the sun. |
| Energy released | The energy released by fission is a million times greater than that released in chemical reactions, but is lower than the energy released by nuclear fusion. | The energy released by fusion is three to four times greater than the energy released by fission. |
(b)
(i) Nuclear fission can be used for the production of electricity.(ii) The process of nuclear fusion is used for making hydrogen bombs.
(c) Nuclear fusion. The energy released by fusion is three to four times greater than the energy released by fission.
(d) The energy released in joules when 5 g (5/1000 kg ) of a material is completely converted into energy during a nuclear reaction is given by Einstein’s equation as
Answer 28:
(c) graphite
Graphite acts a moderator in the reactor by slowing down fast-moving neutrons in it.
Answer 29:
(d) boron
Boron is used in the control rods as it absorbs neutrons.
Answer 30:
(b) liquid sodium and carbon dioxide
Liquid sodium and carbon dioxide are used in order to transfer the heat produced in the reactor by fission to the heat exchanger, for converting water into steam.
Answer 31:
(b) which transfers the heat from the reactor to the water in the heat exchanger
Liquid sodium or carbon dioxide gas is used as a coolant.
Answer 32:
(b) nuclear energy
Nuclear energy comes from the nucleus of an atom.
Answer 33:
(d) 931 MeV
One atomic mass unit is equal to 931 MeV.
Answer 34:
(b) nuclear fission
In a nuclear reactor, the controlled fission of uranium-235 produces a large amount of heat energy.
Answer 35:
(c) 1.6 × 10−19 J
It is the amount of energy acquired by an electron having a charge of 1.6 × 10−19 C when accelerated through a potential difference of 1 volt.
Answer 36:
(d) neutrons
In both reactions, neutrons can be produced.
Answer 37:
(c) sun
Nuclear fusion reactions are the source of the sun’s energy.
Answer 38:
(d) 9 × 1013 J
The energy produced by converting 1 gram (1/1000 kg) mass of a nuclear fuel is given by Einstein’s equation as
Answer 39:
(a) conversion of hydrogen gas into helium
The sun’s energy is derived from the fusion of hydrogen nuclei into helium nuclei.
Answer 40:
(d) atom bomb
In an atom bomb, the fission reaction of uranium-235 or plutonium-239 is deliberately allowed to go out of control so as to produce an enormous amount of energy in a very short time.
Answer 41:
(a) 1.6 × 10−13 J
One MeV of nuclear energy is equal to 1.6 × 10-13 J.
Answer 42:
(b) nuclear fusion energy
The energy produced in a nuclear fusion reaction is much more than that produced in a nuclear fission reaction and has not been controlled so far.
Exercise : Solution of Questions on page Number : 159
Answer 43:
(d) highly radioactive
The waste materials produced by the fission of uranium-235 in a nuclear power plant are radioactive and hence extremely harmful to all living things.
Answer 44:
(c) conversion of mass into energy
In both the reactions, heat energy is released owing to the conversion of mass into energy.
Answer 45:
(b) deuterium
In nuclear fusion, two nuclei of light elements such as that of hydrogen are combined to form a heavy nucleus. Deuterium atoms are the hydrogen atoms of mass number 2.
Answer 46:
(a) The process represents nuclear fission.
In this process, a heavy nucleus of uranium splits into two smaller nuclei of barium and krypton when bombarded with low-energy neutrons. A tremendous amount of energy is also produced.
(b) Particle c is a neutron and x is equal to 3.
(c) E represents the tremendous amount of energy that is liberated in the reaction.
(d) The given reaction is utilised in nuclear power stations.
(e) An atom bomb is based on a similar type of reaction.
Answer 47:
(a) The process represents nuclear fusion.
In this process, when two smaller nuclei of deuterium are heated to an extremely high temperature under extremely high pressure, a heavy nucleus of helium and a neutron are emitted. A tremendous amount of energy is also liberated.
(b) Particle c is a neutron and x is equal to 1.
(c) E represents the tremendous amount of energy liberated.
(d) The two conditions are given below.
(i) Millions of degrees of temperature
(ii) Millions of pascals of pressure
(e) A hydrogen bomb is based on similar reactions.
Answer 48:
(i) An atom bomb is based on nuclear fission reactions. So, a heavy nucleus D of mass number 235 is the most suitable to make it.
(ii) A hydrogen bomb is based on nuclear fusion reactions. So, a light element A of mass number 2 is the most suitable to make it.
Answer 49:
If the reactor core suddenly gets very hot, then we should insert the control rods of boron a little further into the reactor to reduce the rate of the nuclear fission reaction.
Answer 50:
The control rods should be withdrawn a little more from the inside of the nuclear reactor to increase the rate of the nuclear fission reaction and produce more heat.
Answer 51:
If the control rods are fully inserted into the graphite, they absorb all the neutrons and therefore the nuclear chain reaction stops.
Exercise : Solution of Questions on page Number : 161
Answer 1:
Hydrogen is a cleaner fuel than CNG. This is because the burning of hydrogen produces only water, which is harmless. However, the burning of CNG gives water and carbon dioxide, a gas that can produce the greenhouse effect in the atmosphere, harming the environment.
Answer 2:
A compact fluorescent lamp (CFL) is more energy efficient, because it consumes much less electrical energy than a filament-type electric bulb.
Answer 3:
The amounts of coal, petroleum and natural gas left in the earth are limited. It has been estimated that the world’s coal reserves may last for another 200 years, the known reserves of petroleum for around 40 years and the known reserves of natural gas for around 60 years.
Answer 4:
Two devices that can be utilised for cooking food so as to save fuel are the following.
1. Solar cooker
2. Pressure cooker
Answer 5:
The following are the various factors that we should keep in mind while choosing a source of energy.
1. The ease of extracting energy from the source.
2. The cost of extracting energy from the source.
3. The efficiency of technology used in the extraction.
4. The effect of the energy source on the environment.
Answer 6:
No source of energy can be pollution free.
For example, solar cells, solar cookers and wind generators used for generating energy are pollution free, but the processes involved in making the materials to build these devices damage the environment in some way.
Answer 7:
The following are the environmental consequences of the increasing demand for energy.
1. More nuclear power plants are being set up, increasing the radioactivity in the environment.
2. The ecological balance is being disturbed by the construction of hydropower plants.
3. The amount of the greenhouse gas carbon dioxide in the atmosphere is increasing because of the burning of fossil fuels.
4. The combustion of fossil fuels is producing acid rain and damaging plants, soil and aquatic life.
Answer 8:
The following are a few steps to reduce energy consumption.
1. Switch off electrical devices such as TV, lights and fans when they are not needed.
2. Use energy-efficient electrical appliances.
3. Use pressure cookers and solar cookers for cooking food.
4. Use solar water heaters.
5. Encourage the use of biogas as a fuel in rural areas.
Answer 9:
d) fossil fuels
The burning of fossil fuels produces acidic gases and large amounts of carbon dioxide and smoke, all of which pollute the environment.
Answer 10:
(a) 200 years
The world’s known coal reserves are expected to last for about 200 years.
Answer 11:
(c) petroleum
The known reserves of petroleum are expected to last for about 40 years.
Answer 12:
(b) CFL
A compact fluorescent lamp (CFL) is an energy-efficient device. It consumes much less electrical energy than other devices that provide light.