Class 7 Science Heat Exam Notes

Class 7 Science Heat Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Heat is a form of energy that causes the sensation of hotness or coldness. This Class 7 Science exam notes page covers the different scales of temperature (Celsius, Fahrenheit and Kelvin), the role of temperature in the transfer of heat, the unit of heat energy, the effects of heat on temperature, expansion and change of state, the three methods of heat transfer (conduction, convection and radiation), land and sea breezes, and the points to remember from the chapter.

Topics Covered

1. Different scales of temperature
2. Role of temperature in transfer of heat energy
3. Unit of heat energy
4. Effects of heat
5. Transfer of heat (conduction, convection and radiation)

INTRODUCTION

Heat: It is a form of energy which causes the sensation of hotness or coldness.

For example, if we dip our finger in hot water, we have a sensation of hotness. Similarly, if we touch a block of ice, the sensation is that of coldness. In the former case the heat energy has moved into the finger, while in the latter case it has moved out of the finger. Thus, hotness or coldness basically indicates whether heat energy is flowing into our body or out of it.

Temperature: It is the effect of heat energy which determines the thermal state of a given substance. In other words, it determines the degree of hotness or coldness of a substance. If a body is at a higher temperature than its surroundings, heat energy will flow out of the body. Similarly, if a body is at a lower temperature than its surroundings, heat energy will flow into the body.

Measurement of temperature:
The instrument used for the measurement of temperature is called a thermometer.

DIFFERENT SCALES OF TEMPERATURE

(A) Celsius or centigrade scale:
As the name suggests, this scale has 100 divisions between the upper and lower standard points. This scale was introduced by the Swedish astronomer Celsius, and it is known after his name. On this scale, 0°C represents the melting point of ice and 100°C represents the steam point. Each division on this scale is called one degree centigrade or one degree Celsius, and it is written as °C.

(B) Fahrenheit scale:
This scale was introduced by Fahrenheit. On this scale, 32°F represents the melting point of ice and 212°F represents the steam point. Zero is marked 32°F below the ice point. The length between the standard points is divided into 180 equal parts. Each division on this scale is called 1°F. This scale is widely used for meteorological and clinical purposes.

[Figure: Comparison of the Fahrenheit and Celsius scales. Upper fixed point (boiling point of water): 212°F and 100°C. 180 divisions between them on the Fahrenheit scale and 100 divisions on the Celsius scale. Lower fixed point (melting point of ice): 32°F and 0°C]

(C) Kelvin scale:
The scale of measurement of temperature in which the lowest temperature is zero Kelvin (-273°C) is called the Kelvin scale. It is also called the S.I. scale of temperature.

[Figure: Comparison of the Celsius and Kelvin scales. 100 divisions from 0°C to 100°C correspond to 273 K to 373 K, with the same size of division]

Characteristics of the Kelvin scale:
(i) There cannot be any temperature below zero Kelvin.
(ii) A rise in temperature in Kelvin is equal to the rise in temperature in degree Celsius.

ROLE OF TEMPERATURE IN TRANSFER OF HEAT ENERGY

When two bodies at different temperatures are brought in contact with each other, heat energy always flows from a body at higher temperature to a body at lower temperature, till the temperatures become equal. Thus, it is the temperature of a body which determines the direction of flow of heat energy.

UNIT OF HEAT ENERGY

Heat energy is measured in calories.
The quantity of heat energy required to raise the temperature of 1 g of pure water through 1°C (from 14.5°C to 15.5°C) is called one calorie.

EFFECTS OF HEAT

Temperature rises with heat:
When you heat something, or supply it with heat, its temperature rises. You would have noticed this in the activity in which you measured the temperature of boiling water. The converse is also true. When you cool something, its temperature falls. Cooling something is the same as taking heat away from it.

Heat causes expansion:
All substances, whether solid, liquid or gaseous, expand when they are heated and contract when they are cooled. However, the extent to which they expand is different. Expansion is the greatest in gases, less in liquids, and the least in solids.

Example: Fit a balloon to the mouth of a glass bottle. Place the bottle in a pan of water and heat it. The balloon will inflate as the air inside the bottle becomes warm and expands. Take the bottle out of the pan and allow it to cool. The balloon will deflate as the air inside the bottle contracts.

Heat causes change of state:
When you heat a solid, it melts. When you heat a liquid, it ultimately boils and changes into vapour. Conversely, when you cool a vapour, or take heat away from it, it changes into a liquid. And when you cool a liquid, or take heat away from it, it changes into a solid.

[Figure: Heat causes change of state. Supplying heat changes Ice to Water and then to Vapour. Taking heat away changes Vapour to Water and then to Ice. Heat X and heat Y are the amounts of heat supplied or taken away]

TRANSFER OF HEAT

Heat can be transferred by three methods: (a) conduction, (b) convection and (c) radiation.

(a) Conduction:
Heat is transferred by the process of conduction when bodies at different temperatures are in direct or indirect physical contact. Conduction is a process of transfer of heat from the hotter end to the colder end, from particle to particle of the medium. It is the process of transmission of heat in solids. The molecules of the solid do not move from their positions; they only oscillate back and forth about their fixed positions, but they transfer the heat energy in the form of kinetic energy from one molecule to the next.

Thus, a medium is required for the transfer of heat by conduction, and conduction is not possible in vacuum. In solids, heat is transferred mainly by the process of conduction.

Types of conductors:
(i) Good conductors: The substances through which heat energy can easily flow by conduction are called good conductors. Metals in general are good conductors. Amongst the metals, silver is considered the best conductor. Amongst non-metals, graphite is a good conductor. Metals are good conductors of heat.
(ii) Bad conductors: The substances which do not allow heat energy to flow through them easily are called poor conductors or bad conductors. Amongst solids, glass, wood, clay, asbestos, rubber, plastics and wax are poor conductors. All liquids except mercury are poor conductors. All gases, without any exception, are poor conductors.

Practical application of good conductors:
- Copper tubing is used in automobile radiators, as it readily takes up heat from the hot water coming from the engine.
- Cooking vessels are made of metals, so that they can readily absorb heat energy and transfer it to the food.
- Mercury is used as a thermometric liquid, as it is a good conductor of heat.
- Cooling coils of an air conditioner and a refrigerator are made of copper, as they readily conduct heat.

Practical application of bad conductors:
- We wear woollen clothes in winter, because woollen clothes contain a large amount of trapped air. Since air is a bad conductor of heat, it does not allow body heat to flow outward. As our body stops losing heat, we feel warm.

(b) Convection:
Convection is a process of transfer of heat by the actual movement of the medium particles. Liquids and gases are bad conductors of heat, and they are heated mainly by the process of convection. In a solid, the atoms cannot move from their positions, so solids are not heated by convection. A medium is required for the transfer of heat by convection, so heat cannot be transferred by convection in vacuum.

Consequence of convection: land and sea breeze
In coastal regions during summer, a breeze generally blows from the land towards the sea during the night (or early morning). This is called the land breeze.

Land is a better absorber of heat than water. During the day, the land gets hotter, the air above it rises, and cooler air from over the sea flows in to take its place. This gives rise to a sea breeze, which cools the land.

During the night, the land radiates the heat it had absorbed during the day and cools down faster than the sea. Above the sea, the air is warmer. It rises, and cooler air from the land moves towards the sea to take its place. This gives rise to a land breeze. Thus, we have a sea breeze during the day and a land breeze at night.

[Figure: Daytime diagram (sun shown), labels: Hot air rises, Cold air from sea, sea, Land. Hot air rises over the land and cold air from the sea moves towards the land. This is a sea breeze. Night diagram (moon and stars shown), labels: Hot air rises, cold air from land, sea, Land. Hot air rises over the sea and cold air from the land moves towards the sea. This is a land breeze.]

(c) Radiation:
Radiation is the process of heat transfer in which heat passes directly from one body to another without affecting the medium. Thus, no medium is required for heat transfer by radiation. In vacuum, heat transfer takes place only by radiation.

The heat energy transferred by the process of radiation is called radiant heat or thermal radiation.

Nature of radiant heat: Heat energy is transferred by radiation in the form of electromagnetic waves. These waves can travel even in vacuum. They travel in all directions in straight lines with a speed equal to the speed of light (3 × 10⁸ m/s). They do not heat the medium through which they pass. They are reflected by a polished and white surface. When radiant heat falls on an object, it is partly absorbed and partly reflected. Dull, black or coloured surfaces are good absorbers and good radiators of heat.

Properties of heat radiation:
(A) Heat radiations travel with the speed of light.
(B) Heat radiations can travel through vacuum.
(C) Heat radiations travel in straight lines.

POINTS TO REMEMBER

- Heat is a form of energy. We use this energy to generate electricity, among other things.
- The hotness or coldness of a body is relative.
- The temperature of a body is a measure of the degree of hotness of the body. We measure it by comparing it with a universal standard.
- The Celsius and Fahrenheit scales are commonly used to measure temperature. Ice melts at 0°C or 32°F, and water boils at 100°C or 212°F.
- Heat causes a rise in temperature. It also causes expansion and change in the state of matter.
- Heat flows or travels in three ways: conduction, convection and radiation.
- Conduction is the process by which heat flows through a substance without the movement of the substance itself.
- Convection is the process of transfer of heat in a liquid or gas by the movement of the liquid or gas.
- Radiation is the process by which heat travels without the help of a material medium. Radiant heat travels in straight lines, in all directions from a hot body.
- Every body emits (gives out) radiant heat. The emission of radiant heat from a body depends upon its temperature and colour.
- When radiant heat falls on a body, a part of it is absorbed and the rest is reflected. How much radiant heat a body absorbs and how much it reflects depends on its colour and the nature of its surface. Shiny white bodies are the best reflectors, while rough black bodies are the best absorbers of radiant heat. Black bodies absorb and emit radiant heat the best.

EXERCISE

Single Correct Answer Type Questions

1. Radiation
(A) does not require a material medium
(B) is the process of the transfer of heat in liquids
(C) is the process of the transfer of heat in which heat travels in one direction
(D) occurs in solids

Answer: (A) does not require a material medium

1. Radiation transfers heat as electromagnetic waves, which need no medium.
2. Heat transfer in liquids is convection, and heat transfer in solids is mainly conduction.
3. Hence, the correct option is (A).

Teacher's Note:
a) Radiation is the only method of heat transfer that works in vacuum.
b) Heat from the Sun reaches the Earth by radiation.

2. Which of the following statements is correct?
(A) Metals are bad conductors.
(B) Some metals conduct heat better than others.
(C) Heat can be conducted from one metal to another even if they are not in contact with each other.
(D) When two metal rods are placed in contact with each other, heat can flow from one to the other even if they are at the same temperature.

Answer: (B) Some metals conduct heat better than others.

1. Metals are good conductors of heat, so option (A) is wrong.
2. Silver is the best conductor, and other metals conduct less well, so option (B) is correct.
3. Conduction needs contact and a temperature difference, so options (C) and (D) are wrong.

Teacher's Note:
a) Heat flows only from a hotter body to a colder body.
b) Silver is the best conductor among metals.

3. The capillary tube of a clinical thermometer has a kink
(A) to increase the expansion of mercury
(B) so that the level of mercury does not fall as soon as the thermometer is taken out of the mouth
(C) to use less mercury
(D) to help us see it better

Answer: (B) so that the level of mercury does not fall as soon as the thermometer is taken out of the mouth

1. The kink is a narrow constriction in the capillary tube.
2. When the thermometer is taken out, the mercury thread breaks at the kink and stays at the reading.
3. Hence, the correct option is (B).

Teacher's Note:
a) This allows the reading to be taken after the thermometer is removed from the mouth.
b) The kink does not change the expansion of mercury.

4. A polished. silvery surface is a
(A) good absorber and good reflector of heat
(B) good absorber and bad radiator of heat
(C) poor absorber and good reflector of heat
(D) poor reflector and good radiator of heat

Answer: (C) poor absorber and good reflector of heat

1. A polished, silvery surface reflects most of the radiant heat falling on it.
2. Because it reflects most of the heat, it absorbs very little and radiates very little.
3. Hence, the correct option is (C).

Teacher's Note:
a) Shiny surfaces are good reflectors, so they are used for thermos flasks.
b) Dull black surfaces are the opposite: good absorbers and good radiators.

5. A black body with a rough surface is a good
(A) reflector and poor absorber of heat
(B) good absorber and good radiator of heat
(C) absorber and poor radiator of heat
(D) reflector and poor radiator of heat

Answer: (B) good absorber and good radiator of heat

1. A black, rough surface absorbs most of the radiant heat that falls on it.
2. A body that absorbs well also radiates well, so it is a good radiator.
3. Hence, the correct option is (B).

Teacher's Note:
a) Black bodies absorb and emit radiant heat the best.
b) A rough surface also absorbs and radiates better than a smooth surface.

6. Sweating causes cooling by
(A) conduction
(B) radiation
(C) convection
(D) evaporation

Answer: (D) evaporation

1. Sweat on the skin evaporates, and evaporation needs heat.
2. The heat is taken from the body, so the body cools down.
3. Hence, the correct option is (D).

Teacher's Note:
a) Evaporation takes heat away from the surface it is happening on.
b) This is why a fan makes us feel cooler when we are sweating.

7. Air conditioners are placed high up the walls so that-
(A) they create less noise
(B) cold air from them comes down and cools the room while hot air rises up
(C) They are out of the way
(D) all of the above

Answer: (B) cold air from them comes down and cools the room while hot air rises up

1. Cold air is denser than warm air, so it sinks after it leaves the air conditioner.
2. Warm air rises, and this sets up a convection current that cools the whole room.
3. Hence, the correct option is (B).

Teacher's Note:
a) The cooling of the room is due to convection.
b) Heaters are placed low on the wall, for the same reason in reverse.

8. It is easier to drink hot tea from a porcelain mug than from a steel tumbler as
(A) the porcelain mug has a handle
(B) porcelain is a heat insulator
(C) the tea will cool faster in the steel tumbler
(D) all of the above

Answer: (B) porcelain is a heat insulator

1. Porcelain is a bad conductor of heat, so the heat does not pass quickly to the outside of the mug.
2. The outer surface stays cool, so the mug can be held comfortably.
3. A handle is not the reason, so option (D) is wrong and the correct option is (B).

Teacher's Note:
a) Steel is a good conductor, so the outside of a steel tumbler becomes hot.
b) The handle only helps in holding the mug, and it does not stop the heat.

9. The heat energy emitted by sun reaches the earth through-
(A) conduction
(B) convection
(C) radiation
(D) None of these

Answer: (C) radiation

1. There is no material medium between the Sun and the Earth, so conduction and convection are not possible.
2. Heat from the Sun travels as electromagnetic waves, which is radiation.
3. Hence, the correct option is (C).

Teacher's Note:
a) Radiant heat travels through vacuum, which is why sunlight reaches the Earth.
b) Radiant heat travels at the speed of light.

10. Ice blocks are covered with sawdust because-
(A) sawdust lowers the temperature of the ice
(B) the sawdust, together with the air trapped within it, acts as heat insulator and prevents the heat outside from getting to the ice and melting it
(C) the sawdust absorbs excess water from the ice
(D) None of these

Answer: (B) the sawdust, together with the air trapped within it, acts as heat insulator and prevents the heat outside from getting to the ice and melting it

1. Sawdust contains a lot of air, which is a poor conductor of heat.
2. This slows down the flow of heat from the surroundings to the ice, so the ice melts more slowly.
3. Hence, the correct option is (B).

Teacher's Note:
a) Sawdust does not cool the ice, as option (A) suggests.
b) The same principle keeps the ice cold in a cooler box.

11. Land and sea breezes are based on-
(A) the phenomenon of conduction of heat
(B) the phenomenon of convection of heat
(C) the phenomenon of absorption and radiation of heat
(D) all of the above

Answer: (B) the phenomenon of convection of heat

1. Land and sea breezes are caused by warm air rising and cooler air moving in to take its place.
2. This movement of air is convection.
3. Hence, the correct option is (B). Absorption and radiation only set up the temperature difference that causes the convection.

Teacher's Note:
a) Land absorbs heat faster than water during the day, and the land cools faster at night.
b) Conduction is not the cause, because air is a poor conductor of heat.

12. 1 calorie equals to-
(A) 4.2 J
(B) 0.42 J
(C) 420 J
(D) 4200 J

Answer: (A) 4.2 J

1. One calorie is the heat needed to raise the temperature of 1 g of water by 1°C.
2. This is equal to 4.2 joules.
3. Hence, the correct option is (A).

Teacher's Note:
a) The joule is the S.I. unit of heat energy.
b) A kilocalorie is 1000 calories, that is, 4200 J.

13. Fahrenheit scale divides two fixed point into-
(A) 180 parts
(B) 212 parts
(C) 100 parts
(D) 32 parts

Answer: (A) 180 parts

1. The two fixed points on the Fahrenheit scale are 32°F (melting point of ice) and 212°F (steam point).
2. The difference is 212 - 32 = 180, which is divided into 180 equal parts.
3. Hence, the correct option is (A).

Teacher's Note:
a) The Celsius scale divides the same two fixed points into 100 parts.
b) Option (B), 212, is the steam point, not the number of divisions.

14. The normal temperature of human body is-
(A) 37°C
(B) 38°C
(C) 35°C
(D) 98.4°C

Answer: (A) 37°C

1. The normal body temperature of a healthy person is about 37°C.
2. This is equal to 98.4°F, so option (D) is the same value in the wrong unit.
3. Hence, the correct option is (A).

Teacher's Note:
a) The printed value 98.4°C in option (D) should be 98.4°F; 98.4°C is not possible for a human body.
b) Fever is a temperature above about 38°C.

15. Convert 293 K into Celsius scale-
(A) 566°C
(B) 293°C
(C) 20°C
(D) 496°C

Answer: (C) 20°C

1. Temperature in °C = temperature in K - 273.
2. So 293 K - 273 = 20°C.
3. Hence, the correct option is (C).

Teacher's Note:
a) Subtract 273 when converting from Kelvin to Celsius.
b) Add 273 when converting from Celsius to Kelvin.

16. When in thermal contact, the quantity of heat lost by the hotter body is -------- the amount of heat gained by the colder body.
(A) equal to
(B) greater than
(C) less than
(D) cannot say

Answer: (A) equal to

1. In thermal contact with no heat lost to the surroundings, the heat energy is conserved.
2. So the heat lost by the hotter body equals the heat gained by the colder body.
3. Hence, the correct option is (A).

Teacher's Note:
a) Heat flows until both bodies reach the same temperature.
b) This is the principle of heat exchange in a calorimeter.

17. Conduction cannot takes place in
(A) copper
(B) iron
(C) aluminium
(D) vacuum

Answer: (D) vacuum

1. Copper, iron and aluminium are all metals, which are good conductors.
2. Conduction needs a material medium to pass the heat from particle to particle.
3. Vacuum has no particles, so conduction cannot take place in it, and the correct option is (D).

Teacher's Note:
a) Heat can still reach a body through vacuum, but only by radiation.
b) Vacuum flasks use this idea to reduce heat transfer.

18. The snow on the mountains does not melt all the at once when it is heated by the sun because it-
(A) becomes very hard
(B) reflects most of the heat from the sun
(C) has a low specific heat capacity
(D) has a high latent heat of fusion

Answer: (D) has a high latent heat of fusion

1. Melting needs heat energy, which is called the latent heat of fusion.
2. Snow has a high latent heat of fusion, so a large amount of heat is needed to melt it.
3. Hence, the correct option is (D).

Teacher's Note:
a) The temperature of melting snow stays at 0°C until all the snow has melted.
b) Option (B) is not the reason, because snow is heated by the Sun.

19. Conduction is possible-
(A) When the bodies are apart from each other
(B) When the bodies have some temperature and in thermal contact
(C) When they have different temperature maintaining distance between them
(D) bodies should be in contact and should have different temperatures

Answer: (D) bodies should be in contact and should have different temperatures

1. Conduction needs physical contact between the bodies.
2. Heat flows only when there is a temperature difference, so the bodies must be at different temperatures.
3. Hence, the correct option is (D).

Teacher's Note:
a) Option (B) is wrong, because heat does not flow when both bodies are at the same temperature.
b) Conduction does not need the bodies to be apart.

20. It is warmer to have two thin blankets than to have a single thick blanket because-
(A) thick blanket cannot give more warmth
(B) two blankets allow more heat to pass through them
(C) air between the two blankets is a good conductor of heat
(D) air between the thin blankets does not allow heat to pass through it since it is a bad conductor

Answer: (D) air between the thin blankets does not allow heat to pass through it since it is a bad conductor

1. Each thin blanket traps a layer of air, and air is a bad conductor of heat.
2. The air layers between the two blankets slow the loss of body heat.
3. Hence, the correct option is (D).

Teacher's Note:
a) Option (C) is wrong, because air is a bad conductor, not a good one.
b) This is the same reason that woollen clothes keep us warm.

Characteristics of Kelvin scale

Revise the chapter using the notes and the exercise answers given above.