# Class 9 Physics Chapter 9

Updated: 29 Oct 2023

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Class 9 Physics Chapter 9 is about “Transfer of Heat“. This article includes topics such as Transfer of Heat, Conduction of Heat, Mechanism of Heat Conduction, Good and Bad Thermal Conductors, Practical Applications of Conduction of Heat (Cooking pots and pans, Plastic Foam, Wire Gauze, Pot holders and Table mats, Woolen Clothes, Igloos, Ice Box, Soldering), Thermal Conductivity, Convection of Heat, Mechanism of Heat Convection, Practical Applications of Heat Convection (Heating Water, Sea Breeze, Riding on Thermals, Refrigerator, Ventilation), Radiation of Heat, Mechanism of Radiation of Heat, Good and Bad Emitters and Absorbers, Practical Application of Radiation of Heat (Colouring Materials, Texture of the Surface, Satellite Protective Coating, Thermos Flask), Green House Effect and Global Warming, Mechanism of Green House Effect, Importance of Green House Effect.

## Class 9 Physics Chapter 9 Notes

### Transfer of Heat Notes

Class 9 Physics Chapter 9 Notes

### Class 9 Physics Chapter 9 SLO Notes

Class 9 Physics Chapter 9 SLO Notes

## Class 9 Physics Chapter 9 MCQs

### Transfer of Heat

1 How many primary ways are there for heat transfer?
(a) One
(b) Two
(c) Three
(d) Four

Three

2 Which of the following is NOT a form of heat transfer?
(a) Conduction
(b) Convection
(d) Reflection

Reflection

### Conduction of Heat

3 What is the term for the flow of thermal energy through matter from higher temperature to lower temperature without matter movement?
(b) Convection
(c) Insulation
(d) Conduction

Conduction

4 What is the process by which heat energy is transferred from particle to particle within a material?
(a) Convection
(c) Conduction
(d) Insulation

Conduction

### Mechanism of Heat Conduction

5 How is heat transferred through conduction in a solid material like an iron rod?
(a) By the movement of the material as a whole
(b) Through the rapid movement of free electrons
(c) By emitting electromagnetic waves
(d) By convection currents

Through the rapid movement of free electrons

6 What happens to the atoms in the hotter part of the iron rod in the conduction process?
(a) They remain stationary
(b) They vibrate with less energy
(c) They vibrate more violently
(d) They emit light

They vibrate more violently

7 What are free electrons in a metal responsible for?
(a) Blocking heat transfer
(c) Transferring energy through collisions
(d) Generating convection currents

Transferring energy through collisions

8 How is heat energy transferred from particle to particle in conduction?
(a) Through the emission of light
(b) By the movement of the material as a whole
(c) Through collisions between particles
(d) By electromagnetic waves

Through collisions between particles

9 What is the term for the electron that can move freely throughout the atom?
(a) Bound electron
(b) Proton
(c) Free electron
(d) All of these

Free electron

### Good and Bad Thermal Conductors

10 Which term describes materials that allow heat to pass through them easily?
(a) Heat insulators
(b) Good conductors
(d) Insulating conductors

Good conductors

11 Which term is used for materials that do not allow heat to pass through them easily?
(a) Heat insulators
(b) Good conductors
(c) Thermal transmitters
(d) Conductive insulators

Heat insulators

12 Why are good conductors of heat often used in cooking utensils?
(a) To make food taste better
(b) To reduce the weight of the utensils
(c) To evenly distribute heat and cook food more quickly
(d) To make the utensils more colorful

To evenly distribute heat and cook food more quickly

13 Which of the following materials is a good conductor of heat?
(a) Wood
(b) Rubber
(c) Copper
(d) Plastic

Copper

14 Among solids, liquids, and gases, which state of matter typically conducts heat the best?
(a) Solids
(b) Liquids
(c) Gases
(d) All conduct heat equally well

Solids

15 Why are metals generally good conductors of heat?
(a) They have closely connected atoms
(b) They contain trapped air
(c) They have large inter-molecular distances
(d) They lack free electrons

They have closely connected atoms

16 Why is water a poor conductor of heat?
(a) It contains free electrons
(b) It has closely packed molecules
(c) It has small inter-molecular distances
(d) It has relatively large inter-molecular distances

It has relatively large inter-molecular distances

17 What is the thermal conductive ability of air compared to water?
(a) Air conducts heat better than water
(b) Air and water have similar thermal conductive abilities
(c) Water conducts heat better than air
(d) Air does not conduct heat at all

Water conducts heat better than air

### Practical Applications of Conduction of Heat

#### Cooking pots and pans

18 Why are cooking pots and pans typically made of metals, which are good thermal conductors?
(a) To make them heavy
(b) To conduct heat to the food and spread it evenly
(c) To make them more durable
(d) To insulate the food from heat

To conduct heat to the food and spread it evenly

#### Plastic Foam

19 Why are plastic foam and fiberglass used in the walls and ceilings of homes?
(a) To make the walls and ceilings heavier
(b) To conduct heat effectively
(c) To keep homes cool in summer and warm in winter
(d) To reduce the insulation properties of the home

To keep homes cool in summer and warm in winter

#### Wire Gauze

20 What is the purpose of placing wire gauze over a flame when heating a glass beaker?
(a) To conduct heat into the glass beaker
(b) To protect the glass beaker from concentrated heat
(c) To insulate the flame
(d) To increase the temperature of the flame

To protect the glass beaker from concentrated heat

#### Pot holders and Table mats

21 Why are pot holders and table mats for hot pans made of poor conductors like cloth and wood?
(a) To make them heavier
(b) To conduct heat to the hands
(c) To avoid burning of hands
(d) To make them more durable

To avoid burning of hands

#### Woolen Clothes

22 How do woolen clothes help in keeping the body warm in winter?
(a) By conducting heat away from the body
(b) By trapping heat and preventing its flow out
(c) By making the body sweat more
(d) By increasing body heat production

By trapping heat and preventing its flow out

#### Igloos

23 What property of ice and snow in igloos helps minimize the loss of heat from the inside due to conduction?
(a) They conduct heat efficiently
(b) They trap heat
(c) They act as thermal insulation
(d) They melt easily

They act as thermal insulation

#### Ice Box

24 How does an ice box keep ice from melting?
(a) By conducting heat into the box
(b) By preventing the flow of outside heat into the box
(c) By increasing the temperature inside the box
(d) By using metal walls

By preventing the flow of outside heat into the box

#### Soldering

25 In which process, heat transferred by conduction when objects are in direct contact?
(a) Cooking
(c) Convection
(d) Soldering

Soldering

### Thermal Conductivity

26 What does the thermal conductivity of a substance measure?
(a) Heat conduction
(b) Temperature
(c) Material properties
(d) Length

Heat conduction

27 What factors affect the rate of heat flow through a medium?
(a) Length, area, material
(b) Color, texture, size
(c) Pressure, volume, density
(d) Temperature, color, size

Length, area, material

28 Which substance has the highest thermal conductivity?
(a) Silver
(b) Copper
(c) Gold
(d) Aluminum

Silver

29 Which of the following materials has the lowest thermal conductivity?
(a) Silver
(b) Copper
(c) Steel

30 How the length of a rod affects the rate of heat flow:
(a) Longer length results in faster heat flow
(b) Longer length results in slower heat flow
(c) Length has no effect on heat flow
(d) None of these

Longer length results in slower heat flow

31 How does the cross-sectional area of a rod affect the rate of heat flow?
(a) Larger cross-sectional area allows for faster heat flow
(b) Smaller cross-sectional area allows for faster heat flow
(c) Cross-sectional area has no effect on heat flow
(d) None of these

Larger cross-sectional area allows for faster heat flow

32 How does the difference of temperature between two faces of a slab affect the rate of heat flow?
(a) Increases the rate of heat flow
(b) Decreases the rate of heat flow
(c) Has no effect on the rate of heat flow
(d) None of these

Increases the rate of heat flow

33 What is the symbol used to represent thermal conductivity?
(a) T
(b) H
(c) k
(d) C

k

34 What is the SI unit for thermal conductivity?
(a) Kelvin (K)
(b) Watts per meter per kelvin (W/m.K)
(c) Joules per second (J/s)
(d) Celsius per meter ({}^oC/m)

Watts per meter per kelvin (W/m.K)

### Convection of Heat

35 What is the term for the transfer of heat from one place to another by the bulk motion of fluids?
(a) Conduction
(c) Convection
(d) Insulation

Convection

36 What is convection of heat primarily associated with?
(a) Solids
(b) Liquids
(c) Gases
(d) Fluids

Fluids

37 In which state of matter does convection occur most prominently?
(a) Solids
(b) Liquids
(c) Gases
(d) Plasma

Gases

### Mechanism of Heat Convection

38 How is heat transferred in convection?
(b) By direct contact between particles
(c) Through the bulk movement of fluids
(d) Via conduction through solid particles

Through the bulk movement of fluids

39 What happens to the particles in a fluid when it is heated, causing convection?
(a) They remain stationary
(b) They vibrate in place
(c) They become more tightly packed
(d) They become less dense and rise

They become less dense and rise

40 What is the term for the circulation of fluid due to temperature differences, causing warmer fluid to rise and cooler fluid to sink?
(a) Conduction
(c) Convection current
(d) Insulation

Convection current

### Practical Applications of Heat Convection

#### Heating Water

41 What establishes a continuous circulatory flow in convection within the water?
(a) Expansion of cold wate
(b) Expansion of warm water
(c) Conduction

Expansion of warm water

42 What causes the heated portion of water at the bottom to move upward?
(a) It becomes more dense
(b) It contracts
(c) It expands and becomes less dense
(d) It remains stationary

It expands and becomes less dense

43 Why is convection important for boiling water in an electric kettle?
(a) It lowers the temperature of water
(b) It causes the water to freeze
(c) It circulates heat, warming the lower layers of water first
(d) It prevents the water from heating up

It circulates heat, warming the lower layers of water first

#### Sea Breeze

44 What causes coastal breezes during the day near the sea?
(a) Hot air from the land rises, drawing in cold sea air
(b) Cold air from the land rises, drawing in hot sea air
(c) The sea freezes, creating a breeze
(d) There is no breeze near the sea during the day

Hot air from the land rises, drawing in cold sea air

#### Riding on Thermals

45 How do thermals benefit birds and glider airplanes?
(b) They allow them to fly without flapping their wings
(c) They cause turbulence in the air
(d) They cool the surrounding air

They allow them to fly without flapping their wings

#### Refrigerator

46 In a refrigerator, how is convection used to keep food cold?
(a) By freezing the air inside the refrigerator
(b) By circulating warm air around the food
(c) By circulating cold air around the food
(d) By blocking all air circulation

By circulating cold air around the food

#### Ventilation

47 How is convection used in ventilation systems?
(a) By heating the air inside the room
(b) By blocking airflow near the ceiling
(c) By allowing warm air to escape while drawing in fresh air
(d) By creating a vacuum inside the room

By allowing warm air to escape while drawing in fresh air

48 How is the transfer of heat from a hotter place to a colder place with or without a material medium defined?
(a) Convection of heat
(b) Conduction of heat

49 How does heat from an open campfire primarily reach us when sitting beside it?
(a) By conduction
(b) By convection
(d) By direct contact with flames

50 What is the primary method of heat transfer responsible for warming the Earth from the Sun?
(a) Conduction
(b) Convection
(d) Insulation

51 Why doesn’t conduction or convection play a significant role in the transfer of heat from the Sun to the Earth?
(a) The Sun is not hot enough to emit heat
(b) There’s no material medium for conduction or convection in the vacuum of space
(c) Conduction and convection are more efficient on Earth
(d) None of these

There’s no material medium for conduction or convection in the vacuum of space

52 What is unique about radiation of heat compared to conduction and convection?
(a) It requires a material medium for transfer
(b) It always occurs at a slower rate
(c) It doesn’t require a material medium for transfer
(d) It only takes place in solids

It doesn’t require a material medium for transfer

### Mechanism of Radiation of Heat

53 How heat energy is primarily transferred through radiation?
(a) By conduction of molecules
(b) By convection of particles
(c) By electromagnetic waves
(d) By direct contact between objects

By electromagnetic waves

54 What causes electromagnetic radiation, which is responsible for heat transfer through radiation?
(a) The movement of molecules
(b) The acceleration of electric charges
(c) The rotation of objects
(d) The expansion of materials

The acceleration of electric charges

55 At what temperature does an object stop radiating heat energy through radiation?
(a) 0 degrees Celsius
(b) 0 degrees Fahrenheit
(c) Absolute zero (0 K)
(d) Room temperature

Absolute zero (0 K)

56 What part of the electromagnetic spectrum is typically associated with the radiation of heat, although it is not visible light?
(a) Ultraviolet
(b) X-rays
(c) Infrared
(d) Microwave

Infrared

### Good and Bad Emitters and Absorbers

57 Which color is often associated with being a good absorber of radiation?
(a) White
(b) Black
(c) Silver
(d) Red

Black

58 Why does a black car warm up more quickly in the sun than a car of any other color?
(a) Black is a poor absorber of heat
(b) Black is a good reflector of heat
(c) Black is a good absorber of heat
(d) Black is a poor radiator of heat

Black is a good absorber of heat

59 What affects the rate of energy transfer by radiation?
(a) The shape of the object
(b) The size of the object
(c) The color and texture of the surface
(d) The weight of the object

The color and texture of the surface

60 Why are white clothes worn in hot climates?
(a) Good reflector, poor absorber
(b) Good heat absorption
(c) Reflective and high absorptive
(d) Cooler choice

Good reflector, poor absorber

### Practical Application of Radiation of Heat

#### Colouring Materials

61 Why are the cooling fins on the back of a refrigerator often painted black?
(a) To absorb heat more effectively
(b) To reflect heat away
(c) To prevent heat loss
(d) To improve the appearance

To absorb heat more effectively

#### Texture of the Surface

62 What type of material is often used as a radiant barrier to reduce heat transfer through radiation?
(a) Polished metal
(b) ARMA foil
(c) Black paint
(d) Wood

ARMA foil

#### Satellite Protective Coating

63 What is the purpose of the highly reflective metal foil covering satellites like the Hubble Space Telescope?
(a) To increase temperature changes
(b) To minimize temperature changes
(c) To absorb more heat
(d) To improve communication

To minimize temperature changes

64 How does a thermos flask prevent heat transfer?
(a) By conducting heat away
(b) By allowing convection currents
(c) By minimizing conduction, convection, and radiation
(d) By absorbing heat from the surroundings

By minimizing conduction, convection, and radiation

### Green House

65 Warming of the Earth’s surface due to the atmosphere is said to be:
(a) Global cooling
(b) The greenhouse effect
(c) Atmospheric cooling
(d) Solar heating

The greenhouse effect

66 How does the greenhouse effect impact the Earth’s surface temperature?
(a) It decreases the surface temperature
(b) It has no effect on the surface temperature
(c) It warms the surface temperature
(d) None of these

It warms the surface temperature

67 What factors determine the strength of the greenhouse effect?
(a) The color of the atmosphere
(b) The composition of the atmosphere
(c) The distance from the sun
(d) The amount of water vapor in the atmosphere

The composition of the atmosphere

68 Which of the following is a greenhouse gas?
(a) Oxygen
(b) Nitrogen
(c) Carbon dioxide
(d) Hydrogen

Carbon dioxide

#### Mechanism of Green House Effect

69 How does Earth receive energy from the Sun?
(a) In the form of ultraviolet radiation only
(b) In the form of visible, ultraviolet and near-infrared radiation
(c) In the form of X-rays
(d) In the form of gamma rays

In the form of visible, ultraviolet and near-infrared radiation

70 What percentage of solar energy is reflected to space by the atmosphere and clouds?
(a) 26%
(b) 19%
(c) 45%
(d) 60%

26%

71 What percentage of solar energy is absorbed by the atmosphere and clouds?
(a) 10%
(b) 19%
(c) 26%
(d) 40%

19%

72 How does the atmosphere gain heat in addition to the absorption of solar and thermal radiation?
(a) By conduction
(b) By convection
(c) By latent heat fluxes from the surface
(d) By reflection

By latent heat fluxes from the surface

#### Importance of Green House Effect

73 Why is the greenhouse effect important for the survival of life on Earth?
(a) It cools the Earth’s surface
(b) It prevents the formation of clouds
(c) It warms the Earth’s surface, making it habitable
(d) It causes extreme temperature fluctuations

It warms the Earth’s surface, making it habitable

74 How much warmer is the Earth’s surface due to the presence of greenhouse gases in the atmosphere?
(a) 10 \ {}^o C warmer
(b) 20 \ {}^o C warmer
(c) 33 \ {}^o C warmer
(d) 50 \ {}^o C warmer

33 \ {}^o C warmer

75 Which greenhouse gas is primarily responsible for the increase in Earth’s temperature due to human activities?
(a) Methane (CH_4)
(b) Carbon dioxide (CO_2)
(c) Ozone (O_3)
(d) Water vapor (H_2O)