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Chapter 6

Heat and Energy

Class 9 - Concise Physics Selina Solutions


Exercise 6(A)

Question 1

What is heat? Write it's S.I. unit.

Answer

Heat is the form of energy that is transferred between two bodies at two different temperatures when kept in contact.

The S.I. unit of heat is joule (J).

Question 2

Two bodies at different temperatures are placed in contact. State the direction in which heat will flow.

Answer

The flow of heat is from the body at a higher temperature to a body at a lower temperature. So, when a hot body is kept in contact with a cold body, the hot body becomes less hot and the cold body becomes less cold.

Question 3

Name the S.I. unit of heat. How is it related to the unit calorie?

Answer

The S.I. unit of heat is joule (J).

1 J = 0.24 cal (approximately)

Question 4

Define temperature and write it's S.I. unit.

Answer

Temperature is a quantity which indicates the thermal state of a body (i.e., the degree of hotness or coolness of the body). It determines the direction of flow of heat when two bodies at different temperatures are placed in contact.

The S.I. unit of temperature is Kelvin (K)

Question 5

Why does a piece of ice when touched with hand, appear cool? Explain.

Answer

When we touch a piece of ice with our hand, heat passes from our hand to the ice. This happens because flow of heat is from a hot body to a cold body and as our hand is at a higher temperature than the ice cube therefore heat passes from our hand to the ice and we feel cold.

Question 6

Distinguish between heat and temperature.

Answer

HeatTemperature
Heat is a form of energy obtained due to random motion of molecules in a system.Temperature is a quality which tells the thermal state of a body (i.e., the degree of hotness or coolness of the body) and determines the direction of flow of heat on keeping the two bodies at different temperatures in contact.
S.I. unit of heat is Joule (J)S.I. unit of temperature is Kelvin (K)
The amount of heat contained in a body depends on mass, temperature, material of the bodyThe temperature of a body depends on the average kinetic energy of it's molecules due to their random motion.
Heat is measured by the principle of calorimetry.Temperature is measured by a thermometer.
Two bodies having same quantity of heat may differ in their temperature.Two bodies at same temperature may differ in the quantities of heat contained in them.
When two bodies are placed in contact, the total amount of heat is equal to the sum of heat of the individual bodies.When two bodies at different temperatures are placed in contact, the resultant temperature is a temperature in between the two temperatures .

Question 7

What do you understand by thermal expansion of a substance?

Answer

The expansion of a substance on heating is called the thermal expansion of that substance.

Question 8

Name two substances which expand on heating.

Answer

The two substances that expand on heating are Brass and Iron.

Question 9

Name two substances which contract on heating.

Answer

The two substances that contract on heating are —

  1. Water from 0°C to 4°C
  2. Silver iodide from 80°C to 141°C

Question 10

What do you mean by anomalous expansion of water?

Answer

The expansion of water when it is cooled from 4°C to 0°C, is known as anomalous expansion of water.

If we take water at 0°C and start heating it, we find that it contracts (instead of expanding) in the temperature range from 0°C to 4°C. On heating it further above 4°C, it expands. Similarly, if water initially at a temperature above 4°C is cooled, it contracts till the temperature of water reaches 4°C.

On further cooling it below 4°C to 0°C, it expands. This unusual expansion of water on cooling it in the temperature range 4°C to 0°C, is called anomalous expansion of water.

Question 11

At what temperature the density of water is maximum? State it's value.

Answer

The density of water is maximum at 4°C. It's value is 1000 kg m-3.

Question 12

State the volume changes observed when a given mass of water is heated from 0°C to 10°C. Sketch a temperature-volume graph to show the behavior.

Answer

The volume of water first decreases on heating it from 0°C to 4°C and then increases on further heating it from 4°C to 10°C. The volume of water is thus minimum at 4°C.

Temperature-Volume graph showing the variation in volume of water from 0°C to 10°C is shown below:

State the volume changes observed when a given mass of water is heated from 0°C to 10°C. Sketch a temperature-volume graph to show the behavior. Heat and Energy, Concise Physics Solutions ICSE Class 9.

Question 13

Draw a graph to show the variation in density of water with temperature in the temperature range from 0°C to 10°C.

Answer

The following graph shows the variation in density of water with temperature in the range of 0°C to 10°C.

Draw a graph to show the variation in density of water with temperature in the temperature range from 0°C to 10°C. Heat and Energy, Concise Physics Solutions ICSE Class 9.

Question 14

A given mass of water is cooled from 10°C to 0°C. State the volume changes observed. Represent these changes on a temperature-volume graph.

Answer

On cooling water from 10°C, the density of water first increases up to 4°C and then decreases on cooling further below 4°C to 0°C.

Hence, the density of water is maximum at 4°C which is equal to 1 g cm-3 (or 1000 kg m-3 )

Temperature-Volume graph showing these changes is given below:

A given mass of water is cooled from 10°C to 0°C. State the volume changes observed. Represent these changes on a temperature-volume graph. Heat and Energy, Concise Physics Solutions ICSE Class 9.

Question 15

Describe an experiment to show that water has maximum density at 4°C. What important consequences follow from this peculiar property of water? Discuss the importance of this phenomenon in nature.

Answer

Hope's apparatus can be used for demonstrating that water has maximum density at 4°C. The apparatus consists of a tall metallic cylinder with two side openings P, near the top and Q near the bottom, fitted with thermometers T1 and T2 respectively.

Describe an experiment to show that water has maximum density at 4°C. What important consequences follow from this peculiar property of water? Discuss the importance of this phenomenon in nature. Heat and Energy, Concise Physics Solutions ICSE Class 9.

The central part of the cylinder is surrounded by a cylindrical trough containing a freezing mixture of ice and salt. The cylinder is filled with pure water at room temperature. The temperature recorded by both the thermometers is observed at regular intervals of time.

Observations —

  1. Initially both thermometers T1 and T2 show same temperature (i.e., room temperature).
  2. First the temperature recorded by lower thermometer T2, starts decreasing and finally it becomes steady at 4°C, while the temperature recorded in upper thermometer T1 remains almost unchanged during this time.
  3. While the temperature recorded by lower thermometer T2 remains constant at 4°C, the upper thermometer T1 shows a continuous fall in temperature up to 0°C and then it also becomes steady.
  4. At this stage, the lower thermometer T2 shows the temperature 4°C at which water has the maximum density while the upper thermometer T1 shows the temperature of water and ice at 0°C.

Important consequences that follow due to this peculiar property of water (i.e., anomalous expansion of water) are:

  1. It is responsible for bursting of water pipelines and destruction of crops during very cold nights.
  2. It helps in preserving aquatic life during very cold weather.

In nature, during winter when the atmospheric temperature starts falling below 0°C, water at the surface of a pond initially at temperature above 4°C, begins to radiate heat to the atmosphere, so the temperature of water near the surface starts falling.

Describe an experiment to show that water has maximum density at 4°C. What important consequences follow from this peculiar property of water? Discuss the importance of this phenomenon in nature. Heat and Energy, Concise Physics Solutions ICSE Class 9.

When temperature of water at the surface falls below 4°C, water contracts and it's density increases and therefore, it sinks to the bottom.

This continues till temperature of entire water reaches to 4°C. Now, further cooling of top layers below 4°C results in expansion of water and so it's density decreases.

As a result, water does not sink further, but it remains on the surface. When the temperature of atmosphere falls below 0°C, water on the surface looses further heat to the atmosphere and gradually freezes into ice, but water below the ice layer remains at 4°C.

The water layer just below the ice in contact with it will be at 0°C, as shown in figure. Since, ice is a poor conductor of heat, so ice now prevents the flow of heat from water of the pond to the atmosphere.

Thus, temperature of water in contact with ice is at 0°C and that of layers below the ice gradually increases to 4°C. As a result, fish and other aquatic creatures remain alive in water of the pond (or lake), though water on the surface has frozen into ice. Nature thus protects the aquatic life during the winter season.

Question 16

Deep pond of water has it's top layer frozen during water. State the expected temperature of water layer (i) just in contact with ice, (ii) at the bottom of pond.

Answer

(i) The temperature of water layer just in contact with ice is 0°C.

(ii) The temperature of water layer at the bottom of pond is 4°C.

Question 17

Draw a diagram showing the temperature of various layers of water in an ice covered pond.

Answer

Below diagram shows the temperature of various layers of water in an ice covered pond:

Draw a diagram showing the temperature of various layers of water in an ice covered pond. Heat and Energy, Concise Physics Solutions ICSE Class 9.

Question 18(a)

Explain the following:

Water pipes in colder countries often burst in winter.

Answer

In colder countries, during winter nights, as the temperature starts falling below 4°C, water in pipe lines expand and it exerts large pressure on the pipes, causing them to burst.

Question 18(b)

Explain the following:

In winter, water tank (or ocean) starts freezing from the surface and not from the bottom.

Answer

During winter when the atmospheric temperature starts falling below 0°C, water at the surface of water tank (or ocean) initially at temperature above 4°C, begins to radiate heat to the atmosphere, so the temperature of water near the surface starts falling.

When temperature of water at the surface falls below 4°C, water contracts and it's density increases and therefore, it sinks to the bottom.

This continues till temperature of entire water reaches to 4°C. Now, further cooling of top layers below 4°C results in expansion of water and so it's density decreases.

As a result, water does not sink further, but it remains on the surface. When the temperature of atmosphere falls below 0°C, water on the surface looses further heat to the atmosphere and gradually freezes into ice, but water below the ice layer remains at 4°C.

Due to this reason, water tank starts freezing from the top and not from the bottom.

Question 18(c)

Explain the following:

Fishes survive in ponds even when the atmospheric temperature is below 0°C.

Answer

In nature, during winter when the atmospheric temperature starts falling below 0°C, water at the surface of pond initially at temperature above 4°C, begins to radiate heat to the atmosphere, so the temperature of water near the surface starts falling.

When temperature of water at the surface falls below 4°C, water contracts and it's density increases and therefore, it sinks to the bottom.

This continues till temperature of entire water reaches to 4°C. Now, further cooling of top layers below 4°C results in expansion of water and so it's density decreases.

As a result, water does not sink further, but it remains on the surface. When the temperature of atmosphere falls below 0°C, water on the surface loses further heat to the atmosphere and gradually freezes into ice, but water below the ice layer remains at 4°C.

The water layer just below the ice in contact with it will be at 0°C, as shown in figure. Since, ice is a poor conductor of heat, so ice now prevents the flow of heat from water of the pond to the atmosphere.

Thus, temperature of water in contact with ice is at 0°C and that of layers below the ice gradually increases to 4°C. As a result fish remain alive in water of the pond, though water on the surface has frozen into ice.

Question 18(d)

Explain the following:

A hollow glass sphere which floats with it's entire volume submerged in water at 4°C, sinks when water is heated above 4°C.

Answer

As we know that, the density of water decreases on heating water above 4°C. Hence, the upthrust acting on the glass sphere decreases and becomes less than the weight of the glass sphere, hence the glass sphere sinks.

Question 18(e)

Explain the following:

A glass bottle completely filled with water and tightly closed at room temperature, is likely to burst when kept in the freezer of a refrigerator.

Answer

Due to the anomalous nature of water it expands when the temperature goes below 4°C inside the freezer. As the glass bottle is completely filled with water and tightly closed, so there is no space for the water to expand and hence the bottle bursts.

Multiple Choice Type

Question 1

Calorie is the unit of:

  1. heat ✓
  2. work
  3. temperature
  4. food

Answer

Calorie is the unit of heat. 1 cal = 4.186 J

Question 2

1 J equals to:

  1. 0.24 cal ✓
  2. 4.18 cal
  3. 1 cal
  4. 1 kcal

Answer

1 cal = 4.186 J
∴ 1 J = 0.24 cal

Question 3

S.I. unit of temperature is:

  1. cal
  2. joule
  3. celsius
  4. kelvin ✓

Answer

S.I. unit of temperature is Kelvin (K)

Question 4

Water is cooled from 4°C to 0°C. It:

  1. contracts
  2. expands ✓
  3. first contracts, then expands
  4. first expands, then contracts

Answer

Due to anomalous expansion of water, it expands when cooled from 4°C to 0°C.

Question 5

Density of water is maximum at:

  1. 0°C
  2. 100°C
  3. 4°C ✓
  4. 15°C

Answer

The density of water is maximum at 4°C. It's value is 1000 kg m-3.

Exercise 6(B)

Question 1

What is an ecosystem? Name it's two components.

Answer

A unit composed of biotic components (i.e., producers, consumers and decomposers) and abiotic components (heat, rain, humidity, inorganic and organic substances) is called an ecosystem.

The two components of ecosystem are —

  1. Biotic component,
  2. Abiotic component.

Question 2

What is the source of energy for all ecosystems?

Answer

The most significant source of energy for all ecosystems is the sun.

Question 3

State the importance of green plants in an ecosystem.

Answer

Green plants absorb most of the energy that is incident on them and by the process of photosynthesis they produce food for the consumers. Thus, they serve as the primary producers in the ecosystem. They also help in maintaining the balance of Oxygen and Carbon dioxide on earth.

Question 4

Differentiate between the producers and consumers.

Answer

ProducersConsumers
Producers produce their own food using energy from the Sun.Consumers do not produce their own food and obtain their food from producers.
Example — Green plantsExample — Herbivores

Question 5

State the functions of decomposers in an ecosystem.

Answer

The role of a decomposer in an ecosystem is to break down dead organisms and then feed on them. The nutrients created by the dead organisms are returned to the soil to be later used by the producers. Once these deceased organisms are returned to the soil, they are used as food by bacteria and fungi by transforming the complex organic materials into simpler nutrients. The simpler products can then be used by producers to restart the cycle.

Question 6

What is a food chain?

Answer

In an aquatic ecosystem, photosynthetic plants and bacteria act as producers. The food synthesized by producers is utilized by primary consumers (such as krill). The primary consumers are eaten by the secondary consumers (such as small fish) and in turn are consumed by the tertiary consumers (such as large fish).

The tertiary consumers may be eaten by humans. Humans may be last consumer in this chain of energy transfer when they eat the fish. This simple food chain is shown in the figure below:

What is a food chain? Heat and Energy, Concise Physics Solutions ICSE Class 9.

Question 7

Draw a simple diagram showing a food chain.

Answer

Below diagram shows a food chain:

Draw a simple diagram showing a food chain. Heat and Energy, Concise Physics Solutions ICSE Class 9.

Question 8

Describe the energy flow in an ecosystem.

Answer

The diagram below shows the energy flow in the ecosystem.

Describe the energy flow in an ecosystem. Heat and Energy, Concise Physics Solutions ICSE Class 9.

The producers (photo-synthetic plants) synthesize organic substances by the process of photosynthesis. The chemical energy so stored in plants is called gross primary production. The producers themselves first use the synthesized organic substances in the process of respiration in which some energy is used in oxidation of organic substances. The rest of the energy, called the net primary production, is stored for the growth, development and important metabolic processes.

Next the primary consumers (herbivores) obtain their food from the producers, so they obtain only a small part of energy from the producers and the rest is wasted in decay of producers. A small part of the energy obtained by the primary consumers is utilized in respiration and the remaining part is stored in them as food.

Afterwards, a small part of the energy stored as food in the primary consumers is obtained by the secondary consumers (carnivores). They again make use of a part of the energy in respiration and rest is stored in them as food.

By repeating the sequence, the tertiary consumers (carnivores) obtain energy as food from the secondary consumers and utilise a small part of it in respiration and remaining energy is wasted in their decay and decomposition.

The energy flow in ecosystems is thus linear i.e., it moves in a fixed direction. At the end, the energy reaches to the degraded (or unuseful) state. It does not return to the sun to make the process cyclic.

Question 9

State the law which governs the energy flow in an ecosystem.

Answer

The flow of energy in the process of entrance, transformation and diffusion in ecosystem is governed by the laws of thermodynamics that are stated below:

  1. First law of thermodynamics (Law of conservation of energy) — Energy can be transformed from one form to another form, but it can neither be created nor destroyed.
  2. Second law of thermodynamics — When energy is put to work, a part of it is always converted in unuseful form mainly as heat, due to friction and radiation.

Question 10

Show that the energy flow in an ecosystem is linear.

Answer

The diagram below shows the energy flow in the ecosystem.

Show that the energy flow in an ecosystem is linear. Heat and Energy, Concise Physics Solutions ICSE Class 9.

Plants being primary producers, absorb solar energy to prepare food. This food is then used up by primary consumers, so they obtain only a small part of energy from the producers and rest is wasted in decay of producers.

Afterwards, a small part of energy stored as food in primary consumers is obtained by secondary consumers (carnivores). They again use a part of energy in respiration and rest is stored in them as food.

Now, the tertiary consumers (carnivores) obtain energy as food from the secondary consumers and utilise a small part in respiration and rest is wasted in their decay and decomposition.

The energy flow in ecosystems is thus linear i.e., it moves in a fixed direction. At the end, the energy reaches to the degraded (or unuseful) state. It does not return to the sun to make the process cyclic.

Question 11

Draw a simple diagram showing the energy flow in a food chain.

Answer

Below diagram shows the energy flow in a food chain:

Draw a simple diagram showing the energy flow in a food chain. Heat and Energy, Concise Physics Solutions ICSE Class 9.

Question 12

Draw a diagram to show that the energy flow in an ecosystem is governed by the law of conservation of energy.

Answer

Below diagram shows that the energy flow in an ecosystem is governed by the law of conservation of energy:

Draw a diagram to show that the energy flow in an ecosystem is governed by the law of conservation of energy. Heat and Energy, Concise Physics Solutions ICSE Class 9.

Multiple Choice Type

Question 1

Food chain begins with:

  1. respiration
  2. photosynthesis ✓
  3. decomposition
  4. decay

Answer

In a food chain, the first level are the producers (photosynthetic plants). They synthesize organic substances by the process of photosynthesis (i.e., they combine simple compounds with the help of solar energy into complex organic substances).

Question 2

The source of energy in an ecosystem is:

  1. sun ✓
  2. decayed bodies
  3. green plants
  4. sugar

Answer

The most significant source of energy for all ecosystems is the sun..

Question 3

Energy enters in a food chain through:

  1. primary consumers
  2. secondary consumers
  3. tertiary consumers
  4. producers ✓

Answer

In a food chain, the first level are the producers (photosynthetic plants). They utilize the energy received from the sun to synthesize organic substances by the process of photosynthesis (i.e., they combine simple compounds with the help of solar energy into complex organic substances).

Question 4

The place of human beings in food chain in an ecosystem is as:

  1. producer
  2. consumer ✓
  3. decomposer
  4. both (a) & (b)

Answer

Human beings are consumers in the food chain.

Exercise 6(C)

Question 1

State two characteristics which a source of energy must have.

Answer

The characteristics that a source of energy must have are —

  1. A source of energy should be such that it can provide an adequate amount of useful energy at a steady rate over a long period of time.
  2. It should be safe and convenient to use.

Question 2

Name the two groups in which various sources of energy are classified. State on what basis are they classified.

Answer

From the point of view of availability, the various sources of energy are classified into the following two groups —

  1. Renewable or non-conventional sources of energy
  2. Non-renewable or conventional sources of energy

Question 3

What is meant by renewable and non-renewable sources of energy? State two differences between them, giving two examples of each.

Answer

Renewable sources of energy or non-conventional source are sources from which energy can be obtained continuously over a long period of time.

Non-renewable source of energy or conventional source are sources from which energy cannot be continuously obtained over a long period of time.

Difference between renewable and non-renewable sources of energy are as follows —

Renewable sources of energy or non-conventional sourceNon-renewable sources of energy or conventional source
These sources can be regenerated.These sources cannot be regenerated.
These are natural sources which will not get exhausted.These are natural sources which will get exhausted with time.
Example — Sun, wind, water etc.Example — Coal, petroleum, natural gas.

Question 4

Select the renewable and non-renewable sources of energy from the following —

(a) Coal
(b) Wood
(c) Water
(d) Diesel
(e) Wind
(f) Oil

Answer

(a) Coal — non-renewable source
(b) Wood — renewable source
(c) Water — renewable source
(d) Diesel — non-renewable source
(e) Wind — renewable source
(f) Oil — non-renewable source

Question 5

Why is the use of wood as a fuel not advisable although wood is a renewable source of energy?

Answer

Although wood, obtained from trees, is also considered a renewable source of energy, trees usually takes more than 15 years to grow fully, therefore, renewable of wood as source of energy takes a long time.

Further, cutting of trees on a large scale causes depletion of forests which results in global warming and environmental imbalance. Hence, use of wood as a source of energy must be avoided

Question 6

Name five renewable and three non-renewable sources of energy.

Answer

The five renewable sources of energy are:

  1. Sun
  2. Wind
  3. Flowing water
  4. Bio mass and bio fuels from waste
  5. Tides

The three non-renewable sources of energy are:

  1. Coal
  2. Petroleum
  3. Natural gas

Question 7

What is (i) tidal, (ii) ocean and (iii) geo thermal energy? Explain in brief.

Answer

(i) Tidal energy — the energy possessed by rising and falling water in tides is known as tidal energy.

But this is not a major source of energy because of the following two reasons —

  1. The rise and fall of sea water during tides is not enough to generate electricity on a large scale.
  2. There are very few sites which are suitable for building tidal dams.

(ii) Ocean — Water in oceans possesses energy in two forms:

  1. Ocean thermal energy — The energy available due to the difference in temperature of water at the surface and at deeper levels of ocean.
  2. Oceanic (or sea) waves energy — the kinetic energy possessed by fast moving oceanic (or sea) waves.

(iii) Geo thermal energy — The heat energy possessed by rocks inside the earth is called geothermal energy.
It is harnessed to produce electricity. The rocks present at hot spots, heat the underground water and turn it into steam, which gets compressed at high pressure between the rocks.
By drilling holes into the earth up to the hot spots, steam is extracted through pipes which is utilized to rotate the turbine connected to the armature of an electric generator to produce electricity.

Question 8

What is the main source of energy for the earth?

Answer

The main source of energy for the earth is the Sun.

Question 9

What is solar energy? How is solar energy used to generate electricity in a solar power plant?

Answer

The energy obtained from the sun is called solar energy.

To obtain electricity from solar energy two devices are used — a solar cell and a solar power plant.

The device which converts solar energy directly into electricity is called a solar cell.

On the other hand, a solar heating device used to generate electricity from solar energy, is called a solar power plant.

The sun's rays after reflection from a large concave reflector gets concentrated at it's focus. The rays have sufficient heat energy which can boil water, if it is placed at the focus of the reflector. This principle is used in a solar power plant.

A solar power plant consists of a number of big concave reflectors, at the focus of which, there are black painted water pipes. The reflectors concentrate the heat energy of the sun rays on the pipes due to which water inside the pipes starts boiling and produces steam.

The steam thus produced is used to rotate a steam turbine which drives a generator producing electricity.

Question 10

What is a solar cell? State two uses of solar cells. State whether a solar cell produces a.c. or d.c. Give one disadvantage of using a solar cell.

Answer

Solar cell — The device which converts solar energy directly into electricity is called solar cell.

Uses of solar cells are as follows —

  1. A large number of solar cells are arranged over a large area to form a solar panel. They are made to collect a large amount of solar energy to produce sufficient electricity.
  2. Small solar cells are used in watches and calculators.

A solar cell produces d.c. electricity which cannot be used for many household purposes.

One disadvantage of solar cell is that the initial cost of a solar cell is sufficiently high.

Question 11

State two advantages and two limitations of producing electricity from solar energy.

Answer

Advantages of producing electricity from solar energy —

  1. They do not cause any pollution in the environment. They are the cleanest source of energy.
  2. They are most suitable for the remote, inaccessible and isolated places where electric power lines cannot be laid.

Limitations of producing electricity from solar energy —

  1. Initial cost of solar panel is sufficiently high.
  2. The efficiency of conversion of solar energy to electricity is low.

Question 12

What is wind energy? How is wind energy used to produce electricity? How much electric power is generated in India using wind energy?

Answer

The large mass of moving air is called wind. Due to motion, it has kinetic energy. The kinetic energy of wind is called wind energy.

Wind energy is used in a wind generator to produce electricity by making use of a wind mill (or wind turbine) to drive a wind generator.

What is wind energy? How is wind energy used to produce electricity? How much electric power is generated in India using wind energy? Heat and Energy, Concise Physics Solutions ICSE Class 9.

The figure above shows the arrangement of a wind generator in which a small electric generator (or dynamo) is placed at the top of a pillar. The armature of the dynamo is connected to the shaft attached with the blades of a wind mill. When the blowing wind strikes the blades of the wind mill, the kinetic energy of wind changes into rotational kinetic energy of the blades.

The rotation of blades of the turbine rotates the armature of the dynamo in the magnetic field between the pole pieces N and S of a strong magnet, thus an alternating e.m.f is produced between the terminals T1 and T2 and hence electric power is generated.

The electric power generated by a single wind mill generator is small. So to generate a sufficient amount of electric power, a large number of such wind generators are arranged over a big area called a wind farm, and then the electric power generated by each generator is combined together for supply to the consumers.

At present in India, we are generating more than 10,000 MW electric power by this technique in coastal areas of Gujarat and Tamil Nadu.

Question 13

State two advantages and two limitations of using wind energy for generating electricity.

Answer

Advantages of using wind energy for generating electricity —

  1. Does not cause any kind of pollution.
  2. It is an everlasting (renewable) source of energy.

Limitations of using wind energy for generating electricity —

  1. The wind farms can be established only at places near the coastal areas where wind blows around the year steadily with a speed not less than 15 km h-1.
  2. A large area of land is needed for the establishment of a wind farm.

Question 14

What is hydro energy? Explain the principle of generating electricity from hydro energy. How much hydro electric power is generated in India?

Answer

The kinetic energy possessed by flowing water is called hydro energy.

The most important use of hydro energy is to convert it into electrical energy by using a turbine. The electricity so obtained is called hydroelectric power.

What is hydro energy? Explain the principle of generating electricity from hydro energy. How much hydro electric power is generated in India? Heat and Energy, Concise Physics Solutions ICSE Class 9.

The figure shown above shows the principle of hydroelectric power plant. The flowing water of river is collected in a dam at high altitude. The water stored in the dam has potential energy. When water from dam falls on turbine, the potential energy of the water stored in dam changes into kinetic energy and this kinetic energy of water is transferred to the blade of the turbine as rotational kinetic energy.

As the turbine rotates, it rotates the armature of the generator (or dynamo) in the magnetic field between the pole pieces N and S of a strong magnet, due to which an alternating e.m.f. is produced between the terminals T1 and T2

Such a mini hydroelectric power plant can be constructed on the rivers in hilly areas across a small dam of height nearly 10 m.

At present in India we are generating hydroelectricity which is only 14% of the total electricity generated by us.

Question 15

State two advantages and two disadvantages of producing hydro-electricity.

Answer

Advantages of producing hydro-electricity are:

  1. It does not produce any environmental pollution.
  2. It is a renewable source of energy.

Disadvantages of producing hydro-electricity are:

  1. The flowing water is not available everywhere.
  2. Due to the construction of dams over rivers, plants and animals of that place gets destroyed or killed.

Question 16

What is nuclear energy? Name the process used for producing electricity using the nuclear energy.

Answer

Nuclear energy is the energy released in nuclear fission and nuclear fusion reactions. In both these processes, the origin of energy is the loss in mass i.e., the sum of masses of the products of reaction is less than the sum of masses of reactants and this loss in mass is converted into energy E according to Einstein's mass-energy equivalence relation E = mc2 , where c (= 3 x 108 m s-1) is the speed of light and m is the loss in mass.

Nuclear fission with controlled chain reaction in a nuclear reactor is used for producing electricity using the nuclear energy.

Question 17

What percentage of total electrical power generated in India is obtained from nuclear power plants? Name two places in India where electricity is generated from nuclear power plants.

Answer

At present, only about 3% of the total electrical power generated in India is obtained from nuclear power plants.

The two places in India from where electricity is generated from nuclear power plants are —

  1. Tarapur in Maharashtra
  2. Narora in Uttar Pradesh

Question 18

State two advantages and two disadvantages of using nuclear energy for producing electricity.

Answer

Advantages of using nuclear energy to produce electricity are as follows —

  1. A very small amount of nuclear fuel (such as uranium-235) can produce a tremendous amount of energy.
  2. Once the nuclear fuel is loaded into the nuclear power plant, it continues to release energy over a long period of time.

Limitations of using nuclear energy to produce electricity are as follows —

  1. It is not a clean source of energy because very harmful nuclear radiations are produced in the process which are highly energetic and penetrating.
  2. The waste obtained from the nuclear power plants causes a high degree of environmental pollution.

Question 19

State the energy transformation in the following —

  1. electricity is obtained from solar energy.
  2. electricity is obtained from wind energy.
  3. electricity is obtained from hydro energy.
  4. electricity is obtained from nuclear energy.

Answer

  1. Light energy is transformed into electrical energy when electricity is obtained from solar energy.
  2. Mechanical energy is transformed into electrical energy when electricity is obtained from wind energy.
  3. Mechanical energy is transformed into electrical energy when electricity is obtained from hydro energy.
  4. Nuclear energy is transformed into electrical energy when electricity is obtained from nuclear energy.

Question 20

State four ways for the judicious use of energy.

Answer

The four ways for the judicious use of energy are —

  1. The fossil fuels such as coal, petroleum, etc. should be used only for a limited purpose when no other alternative source of energy is available.
  2. Wastage of energy should be avoided.
  3. Cutting of trees must be banned and more and more trees should be planted.
  4. Efforts must be made to make judicious use of energy in the community.

Question 21

What do you mean by degradation of energy? Explain it by taking two examples of your daily life.

Answer

The gradual decrease of useful energy due to radiation loss, friction, etc. is called degradation of energy.

Examples of degradation of energy —

  1. In transmission of electricity from a power generating station, a lot of electrical energy is wasted in the form of heat energy in the line wires used for transmission.
  2. When we cook food over a fire, the major part of heat energy obtained from the fuel is radiated out in the atmosphere. This radiated energy is of no use to us. It is thus a degraded form of energy.

Question 22

The conversion of part of energy into an unuseful form of energy is called ______.

Answer

The conversion of part of energy into an unuseful form of energy is called degradation of energy.

Multiple Choice Type

Question 1

The ultimate source of energy is —

  1. wood
  2. wind
  3. water
  4. sun ✓

Answer

The most significant source of energy for all ecosystems is the sun.

Question 2

Renewable source of energy is —

  1. coal
  2. fossil fuels
  3. natural gas
  4. sun ✓

Answer

A natural source providing us energy continuously is called a renewable (or non conventional) source of energy. Sun is the main source of energy for us on earth and is a renewable source.

Exercise 6(D)

Question 1

What do you mean by green house effect?

Answer

Green house effect is the process of warming of the earth's surface and it's lower atmosphere, by absorption of infrared radiations of long wavelength emitted out from the surface of the earth by greenhouse gases such as carbon dioxide, methane, nitrous oxide, ozone, chlorofluorocarbon (CFCs), etc. The below diagram shows the Green House Effect:

What do you mean by green house effect? Heat and Energy, Concise Physics Solutions ICSE Class 9.

Question 2

Name three green house gases.

Answer

Three green house gases are — carbon dioxide, methane, nitrous oxide.

Question 3

Which of the following solar radiations pass through the atmosphere of the earth?

X-rays, ultraviolet rays, visible light rays, infrared radiation.

Answer

The solar radiations that pass through the atmosphere of the earth are visible light rays and infrared radiation.

Question 4

What results in the increase of carbon dioxide contents of earth’s atmosphere?

Answer

The main human activities responsible for increase of the carbon dioxide gas in the earth's atmosphere are —

  1. The burning of fuels, deforestation, transportation and industrial production (particularly cement factories).
  2. Increase of population (human beings emit nearly 32 gigatonnes of carbon dioxide each year).
  3. Imbalance of carbon dioxide cycle (the ocean does not absorb full amount of carbon dioxide and plants are not able to change all the carbon dioxide into oxygen).

The increase in concentration of carbon dioxide due to the above human activities is responsible for 60% increase in green house effect.

Question 5

Name the radiations which are absorbed by the green house gases.

Answer

The radiations which are absorbed by the green house gases are infrared radiation of long wavelength.

Question 6

What would have been the temperature of earth's atmosphere in absence of green house gases in it?

Answer

In the absence of green house gases from the atmosphere, entire heat energy radiated from the earth's surface and objects on it, would escape out into space and then average temperature on earth would be -18°C instead of 15.5°C.

Question 7

State the effect of green house gases on the temperature of earth’s atmosphere.

Answer

The green house gases have an average warming effect of nearly 33°C so that the average temperature on earth's surface is about 15.5°C.

Question 8

What do you mean by global warming?

Answer

Global warming means the increase in average effective temperature near the earth's surface due to an increase in the amount of green house gases in its atmosphere.

Question 9

What causes the rise in atmospheric temperature?

Answer

The reason for rise in atmospheric temperature is the increase in amount of green house gases present in the atmosphere.

As the green house gases increase they trap more and more of heat radiated from the earth's surface. Thereby increasing the temperature of the earth.

Question 10

State the cause of increase of green house effect.

Answer

The reason for the increase in green house effect is the increase in green house gases like carbon dioxide and methane.

Human activities responsible for increase of the carbon dioxide gas in the earth's atmosphere are —

  1. The burning of fuels, deforestation, transportation and industrial production (particularly cement factories).
  2. Increase of population (human beings emit nearly 32 gigatonnes of carbon dioxide each year).
  3. Imbalance of carbon dioxide cycle (the ocean does not absorb full amount of carbon dioxide and plants are not able to change all the carbon dioxide into oxygen).

The increase in concentration of carbon dioxide due to the above human activities is responsible for 60% increase in green house effect.

Apart from this, rice cultivation, animal husbandry, natural gas exploration, burning of bio mass in clearing of forests and leakage in natural gas pipe line have doubled the concentration of methane which is also responsible for the increase in green house effect.

Question 11

What will be the effect of global warming at the poles?

Answer

Due to global warming, melting of ice is more and the area of sea ice around the poles is shrinking. It is expected that in the next 50 years, the ice at both the poles will melt completely and therefore the sea level will rise.

Question 12

State the effect of global warming in coastal regions.

Answer

As more ice is melting due to the effect of global waring, so, buildings and roads in the coastal areas will get flooded and they could suffer damage from hurricanes and tropical storms.

Question 13

How will global warming affect the sea level?

Answer

Due to global warming, melting of ice is more and the area of sea ice around the poles is shrinking. It is expected that in the next 50 years, the ice at both the poles will melt completely and therefore the sea level will rise.

Buildings and roads in the coastal areas will get flooded and they could suffer damage from hurricanes and tropical storms.

Question 14

How will global warming affect agriculture?

Answer

In the near future, warming of nearly 3°C will result in poor yield in farms in low latitude regions and will increase the rise of malnutrition. The crop yield is expected to increase in middle and high latitude regions.

Therefore, the farmers will have to shift their farming region from low latitudes to high latitudes.

Question 15

State two ways to minimize the impact of global warming.

Answer

Ways to minimize the impact of global warming are —

  1. Use of renewable source of energy for generation of electricity instead of electricity from fossil fuel-based power plants.
  2. Use of battery-operated vehicles such as electric cars, bikes and scooters.

Question 16

What is carbon tax? Who will pay it?

Answer

Since, industries emit carbon dioxide to a great extent, to check them, they must be asked to pay carbon tax.

This tax can be calculated on the basis of carbon emission from the industry, number of employee hours and turn over of the industry.

This will encourage the industry to use energy efficient offices and to avoid the travelling of it's employees by having tele-conferencing.

Multiple Choice Type

Question 1

The green house gas is —

  1. oxygen
  2. nitrogen
  3. chlorine
  4. carbon dioxide ✓

Answer

Carbon dioxide is a green house gas. Other green house gases are methane, nitrous oxide, ozone and chlorofluorocarbon.

Question 2

The increase of carbon dioxide gas in atmosphere will cause —

  1. decrease in temperature
  2. increase in temperature ✓
  3. no change in temperature
  4. increase in humidity

Answer

The increase of carbon dioxide gas in atmosphere will cause an increase in green house effect hence increasing global warming and increase in temperature.

Question 3

Without green house effect, the average temperature of earth’s surface would have been —

  1. -18°C ✓
  2. 33°C
  3. 0°C
  4. 15°C

Answer

In the absence of green house gases from the atmosphere, entire heat energy radiated from the earth's surface and objects on it, would escape out into space and then average temperature on earth would be -18°C instead of 15.5°C .

Question 4

The global warming has resulted in —

  1. the increase in yield of crops
  2. the decrease in sea levels
  3. the decrease in human deaths
  4. the increase in sea levels. ✓

Answer

Due to global warming melting of ice is more and the area of sea ice around the poles is shrinking. It is expected that in the next 50 years, the ice at both the poles will melt completely and therefore the sea level will rise.

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