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Page No 59:

Question A:

Solve the following crossword puzzle:

figure

Across
2. Liquids and gases are heated by this process
4. It does not allow heat to pass
5. Heat is measured in this unit

Down
1. It is the effect of heat
3. It is a form of energy

Answer:

 

 

Page No 59:

Question B.1:

Which is correct among the following?
(a) A clinical thermometer consists of long narrow and uniform glass tube.
(b) Outside the bulb, a small shining thread of mercury can be seen.
(c) The glass tube has a bulb at one end containing mercury.
(d) All the above.

Answer:

(d) All the above
A clinical thermometer consists of a long, narrow and uniform glass tube. It has a bulb at one end which contains a small shining thread of mercury.

Page No 59:

Question B.2:

A wooden spoon dipped in the ice cream cup,
(a) becomes cold by conduction
(b) becomes cold by convection
(c) does not become cold
(d) none of these

Answer:

(c) does not become cold
A wooden spoon dipped into an ice cream cup does not become cold as wood is a poor conductor of heat.



Page No 60:

Question B.3:

One litre of water at 30°C is mixed with one litre of water at 50°C. The temperature of the mixture will be
(a) 80°C
(b) more than 50°C but less than  80°C
(c) 20°C
(d) between 30°C and 50°C

Answer:

(d) between 30°C and 50°C
Heat is transmitted from the hotter object to the colder object till the temperatures of both the objects become equal. Therefore, the temperature will be between 30
°C and 50°C.

Page No 60:

Question B.4:

Land breeze blows due to
(a) conduction
(b) convection
(c) radiation
(d) all of these

Answer:

(b) convection
Land breeze occurs because of convection currents.
At night, since the land cools down faster than the sea, cold air from the land moves towards the sea because the hot air above the sea rises up.

Page No 60:

Question B.5:

When you touch a cold object to
(a) heat flows from your body to the object
(b) temperature flows from your body to the object
(c) heat flows from object to your body
(d) temperature flows from object to your body

Answer:

(a) heat flows from your body to the object
 When somebody touches a cold object, heat flows from his/her body to the cold object.

Page No 60:

Question B.6:

Molecules of not move from the hotter body to the colder body in
(a) conduction
(b) convection
(c) radiation
(d) none of these

Answer:

(a) conduction
Conduction is the process of heat transfer from a hot body to a cold body in which molecules do not move.

Page No 60:

Question B.7:

Which colour is the best radiator of heat?
(a) black
(b) green
(c) red
(d) yellow

Answer:

(a) Black
Black colour is the best radiator and the best absorber of heat.

Page No 60:

Question B.8:

Room heater works on the principle of
(a) conduction
(b) convection
(c) radiation
(d) insulation

Answer:

(c) radiation
Room heater works on the principle of radiation.

Page No 60:

Question B.9:

Conduction takes place in
(a) solids only
(b) liquids only
(c) gases only
(d) solids as well as liquids

Answer:

(a) solids only
Conduction is the transfer of heat without the actual motion of molecules and it takes place only in solids.

Page No 60:

Question B.10:

Handles of cooking utensils should be made of a material that
(a) conducts heat well
(b) does not conduct heat well
(c) radiates heat well
(d) none of these

Answer:

(b) does not conduct heat well
Handles of cooking utensils should be made of materials that are poor conductors of heat as they do not absorb heat quickly and readily. This enables us to hold the utensils comfortably while cooking.

Page No 60:

Question C:

Fill in the blanks:
1. .......................... is a form of energy.
2. Heat travels through  .......................... without the actual displacement of particles.
3. Heat is transferred from a hot body to a cold body by ...................., ...................., ................. .
4. Thermometer is used to measure .......................... .
5. .......................... coloured bodies are good absorbers of heat.

Answer:

1. Heat is a form of energy.

2. Heat travels through solids without the actual displacement of particles.

3. Heat is transferred from a hot body to a cold body by conduction, convection, radiation.

4. Thermometer is used to measure temperature.

5. Dark coloured bodies are good absorbers of heat.

Page No 60:

Question D:

Match the items in Column A with the items in Column B:

Column A Column B
1. Good conductor (a) Heat convector
2. Convection (b) Aluminium
3. Radiation (c) Ebonite
4. Insulator (d) Room heater
5. Alcohol (e) Thermometer

Answer:

Sl. No. Column A Column B
1. Good conductor (b) Aluminium
2. Convection (a) Heat convector
3. Radiation (d) Room heater
4. Insulator (c) Ebonite
5. Alcohol (e) Thermometer



Page No 61:

Question 1:

Why are freezers placed on top of refrigerators?

Answer:

Freezers are placed on the top of refrigerators because they work on the principle of convection. Since cold air is heavier than hot air, it travels downwards and warm air, being light, travels upwards. Due to this, the cold air that is generated in the freezer travels to the bottom of the refrigerator and creates a cooling effect. And, the warm air in the refrigerator travels upwards and gets cooled in the freezer once again.

Page No 61:

Question 2:

Why are cloudy nights warmer than clear nights?

Answer:

Cloudy nights are warm because clouds prevent the loss of heat from the Earth. This happens because clouds are made up of water vapours that are poor conductors of heat. However, in a clear night, the Earth radiates the heat that was absorbed from the Sun during the daytime back into the space.

Page No 61:

Question 3:

How do the feathers of a bird protect it from cold?

Answer:

A bird fluffs its feathers to form tiny air spaces (pockets) to trap air. As air is a poor conductor of heat, the trapped air prevents the flow of heat from the body of the bird to the surroundings.

Page No 61:

Question 4:

Which will cool faster-water kept in a black pot or kept in a silver pot?

Answer:

Water kept in a black pot will cool down faster than that in a silver pot. This is because black coloured objects absorb and radiate heat energy faster than light coloured objects.

Page No 61:

Question 5:

Why does an ordinary glass tumbler crack if very hot water is poured into it?

Answer:

When hot water is poured into a glass tumbler, it gets cracked due to uneven expansion. The inner surface of the glass tumbler becomes hot and it expands, but the outer surface does not become equally hot.

Page No 61:

Question 6:

The desert sand is very hot in a day and very cold at night. Explain the reason.

Answer:

Desert sand has a very low specific heat capacity because it absorbs and radiates heat readily and quickly. During the day, the sand absorbs heat from the Sun rapidly and holds on to it, making the sand very hot. During the night, the sand quickly radiates all the heat gained during the day into the space as there are no clouds to trap the heat. This makes the desert sand very cold at night.

Page No 61:

Question 7:

Why are gaps left between lengths of railway tracks?

Answer:

Gaps are left between lengths of railway tracks so that the rails can expand whenever the temperature rises. In the absence of gaps, the rails would buckle on expansion, which can lead to train accidents.

Page No 61:

Question 8:

Why are gunny bags used to prevent melting of ice?

Answer:

A gunny bag is used to prevent the melting of ice because air, which is trapped in the pores of the gunny bag, is a poor conductor of heat. It does not allow heat from the surroundings to reach the ice.

Page No 61:

Question 9:

Why two thin blankets are usually warmer than one thick blanket?

Answer:

Two thin blankets are usually warmer than one thick blanket because the two blankets trap air in between them. This trapped air prevents the flow of heat from the body to the surroundings.

Page No 61:

Question 10:

In the adjacent figure, it is observed that heating the tube from above does not affect the state of ice cubes in metal gauge. Justify your answer.
Figure

Answer:

The melting of ice takes time in the given activity. Since water is a poor conductor of heat, heat does not get transported easily by convection to the bottom of the tube to melt the ice.

Page No 61:

Question 11:

In the figure given below, balloon gets inflated as it is kept in a trough of boiling water. Explain, why did this happen?

Answer:

The balloon kept in a trough of boiling water gets inflated because air turns into air vapour and expands on heating. The air in the bottle absorbs the heat from the boiling water.

Page No 61:

Question E:

Write True (T) or False (F) against the following statements in the given brackets:
1. Temperature is the causes of heat. ( )
2. Hotter body transfers heat to a cooler body.
3. Places near the sea receive sea breeze during daytime. ( )
4. Heat cannot travel without a medium. ( )
5. Air is a good conductor of heat. ( )

Answer:

1. False
Heat is the cause of temperature.

2.True

3.True

4. False
Radiation is a mode of heat transmission in which heat travels without any medium.

5. False
Air is a poor conductor of heat.



Page No 63:

Question A.1:

What is the cause behind increase of decrease in temperature?

Answer:

Heat is the cause behind the increase or the decrease in temperature.

Page No 63:

Question A.2:

What are the different modes of transfer of heat?

Answer:

There are three modes of transfer of heat:
1. Conduction
2. Convection
3. Radiation

Page No 63:

Question A.3:

What are the scales commonly used to measure temperature?

Answer:

Celsius and Fahrenheit are the two scales commonly used to measure temperature.

Page No 63:

Question A.4:

What happens to radiant heat when it falls on a body?

Answer:

When radiant heat falls on a body, a major part of it is reflected and a small part is absorbed by the body. The power of absorption depends on the nature and the colour of the body.

Page No 63:

Question A.5:

Give two examples from everyday life where heat is transmitted by radiation.

Answer:

 Two examples of radiation in everyday life are as follows:

1. Heat from the Sun is transmitted by radiation.

2. Room heaters transmit heat by radiation.

Page No 63:

Question A.6:

Name the liquid used as thermometric liquid in extremely cold regions.

Answer:

Alcohol is used as thermometric liquid in extremely cold regions.



Page No 64:

Question B.1:

What is convection? Does it need any medium for transfer of heat?

Answer:

Convection is a process of transmission of heat from a hot object to a cold object by the actual movement of the particles of the medium. 
Yes, the transfer of heat takes place through the movement of the particles of the fluid (liquid and gas) medium.

Page No 64:

Question B.2:

In what way is conduction different from convection?

Answer:

Sl. No. Conduction Convection
1. Conduction is the process of transfer of heat from a hot object to a cold object without actual movement of the particles of the medium. Convection is the process of transfer of heat from a hot object to a cold object with actual movement of the particles of the medium.
2. Conduction takes place when two objects of different temperatures are in contact with each other. It occurs in solids, liquids and gases. Convection requires the movement of the particles of the medium for transmission of heat. Hence, it does not take place in solids. It occurs in fluids (liquids and gases).

Page No 64:

Question B.3:

State two applications of conduction.

Answer:

The two applications of conduction are as follows:
1. Protecting animals from cold: The hair present on the body of an animal protects it from cold as it is a poor conductor of heat.

2. Making handles of cooking utensils: The handles of cooking utensils are made of wood or ebonite. Since these materials poor conductors of heat, the handles can be held comfortably while cooking.

Page No 64:

Question B.4:

How does heat form the Sun reach us?

Answer:

The heat from the Sun reaches us by radiation. This process does not require a medium and can occur in vacuum as well. So, we can feel the heat from the Sun despite the intervening 150 million kilometres of vacuum.

Page No 64:

Question B.5:

Why are houses made of bricks and mud in rural areas?

Answer:

The houses in rural areas are built with bricks and mud so as to keep the houses warm in winters and cool in summers.
Since these materials are poor conductors of heat, they do not allow heat to pass through them easily.

Page No 64:

Question B.6:

State one use of good and bad conductors of heat.

Answer:

(a) Application of good conductors of heat:
Metals are used in making cooking utensils because they are good conductors of heat.

(b) Application of bad conductors of heat:
The hair and the fur on the bodies of animals are poor conductors of heat. They do not allow the heat to escape to the surroundings, thereby
protecting the animals from cold weather​.

Page No 64:

Question B.7:

Give the difference between: (a) Sea breeze and land breeze (b) Laboratory thermometer and clinical thermometer.

Answer:

(a) Differences between sea breeze and land breeze:
 

Sl. No. Sea Breeze Land Breeze
1. Breeze blowing from the sea towards the land due to the relative heating up of the land during the daytime is called sea breeze. Breeze blowing from the land towards the sea due to the relative cooling down of the land during the night is called land breeze.
2. Sea breeze is an example of convection current that occurs during the daytime. Land breeze is an example of convection current that occurs at night.


(b) Differences between laboratory thermometer and clinical thermometer:
 
Sl. No. Laboratory Thermometer   Clinical Thermometer
1. Laboratory thermometers are used in laboratories to measure temperature and change in temperature with a high degree of precision. Clinical thermometer, also called doctor's thermometer, is used at home to measure the temperature of the human body.
2. Laboratory thermometers do not have a kink and cannot be used to measure the temperature of the body. Clinical thermometer has a kink to prevent the mercury from sliding back into the bulb.
3. The temperature range of a laboratory thermometer is between -10C to 110C. The temperature range of a clinical thermometer is between 35C to 43C.

Page No 64:

Question B.8:

How does the constriction near the bulb in a clinical thermometer help in measuring body temperature accurately?

Answer:

When the thermometer bulb is placed under the tongue or in the armpit to check the body temperature, the mercury expands until it reaches the maximum temperature of the body. The constriction does not allow the mercury from sliding back into the bulb after expansion.

Page No 64:

Question C.1:

Draw a labelled diagram of a mercury thermometer. How does it function? What is its use?

figure

Answer:

(a) Diagram of a mercury thermometer:



(b) Function of a mercury thermometer: A mercury thermometer consists of a bulb containing mercury, which is attached to a narrow glass capillary tube. The upper end of the tube is sealed to prevent the evaporation of mercury. The space above the liquid (mercury) is evacuated to allow the expansion of mercury. The capillary tube has a constriction above the bulb to prevent the sliding back of mercury into the bulb after expansion. When the thermometer bulb is placed under the tongue or in the armpit to check the body temperature, mercury expands until it reaches the maximum temperature of the body. The temperature can then be read distinctly from the scale.

(c) Use 
of a mercury thermometer: Mercury has a low specific heat capacity and expands even on the slightest increase in temperature. Hence, it is used in thermometers to measure the temperature of the human body accurately.

Page No 64:

Question C.2:

Describe an experiment to show the formation of convection currents in a liquid.

figure

Answer:

Experiment to show the formation of convection currents in a liquid:

Aim of the experiment: To show the formation of convection currents in a liquid

Apparatus: Beaker, Bunsen burner, potassium permanganate crystals, wire gauge and water

Procedure: Take a beaker and fill half of it with water. Drop a few crystals of potassium permanganate into the beaker. This is done because potassium permanganate crystals add colour to the water and help in observing the convection currents easily. Heat the beaker and observe the movement of the solution.
The arrangement of the apparatus is shown below:


Observation: Due to the presence of potassium permanganate, the following movements of the coloured water are clearly visible:
(i) T
he coloured water at the bottom gets heated first and starts moving in the upward direction.
(ii) The cooler liquid on the top moves downwards, which in turn gets heated up and rises.

Inference: The transmission of heat from the hot water at the bottom of the beaker to the cold water at the top takes place by the actual movement of water.

Conclusion: Convection is a mode of heat transfer that takes place in liquids.

Page No 64:

Question C.3:

Why is it advantageous to have ventilators in houses at the top of the wall?

Answer:

It is advantageous to have ventilators at the top of the wall because they provide outlets for the hot and stale air in the house. The air that we breathe out is hot and light. As the air in the room gets heated, it rises up due to convection and escapes through the ventilators, while fresh air enters through the windows and the doors of the house.

Page No 64:

Question C.4:

Why do we use mercury as a liquid in thermometers?

Answer:

We use mercury as a liquid in thermometers because of the following reasons:

1. Mercury is a metal found in the liquid state. It is a good conductor of heat. It expands quickly even with a slight increase in temperature. Therefore, it does not take much time to show the temperature of the body.

2. Mercury is a shining, silvery white opaque liquid. Therefore, the position of the edge can be read accurately on the scale. Moreover, it does not stick to the walls of the thermometer.

3. Mercury has a very low specific heat capacity and its rate of expansion and contraction is uniform.

4. Mercury has a low freezing point (-39°C) and a high boiling point (357°C). As a result, mercury can be used to measure a considerably wide range of temperatures.

5. Mercury is a metal, which is easily obtained in its pure form.

Page No 64:

Question C.5:

What are the differences between convection and radiation?

Answer:

Sl. No. Convection Radiation
1. The process of transmission of heat from the hotter object to the colder object by the actual movement of the particles of the medium is called convection.  The process of transmission of heat from the hotter object to the colder object in straight lines without any material medium is called radiation.
2. Convection takes place only in liquids and gases. As it involves the actual movement of the particles, convection does not take place in solids. Radiation takes place in solids, liquids and gases. It also takes place in vacuum because radiation does not need a material medium to transfer heat.
3. Example: Land and sea breezes occur due to convection currents. Example: Heat from the Sun is transmitted to the Earth through radiation.

Page No 64:

Question C.6:

Explain with the help of diagrams how land and sea breezes occur.

Answer:

Formation of sea breezes:
During day, land gets heated up faster than sea water. The heated air rises up, thereby creating a temporary low pressure region that draws cold air from the sea towards the land. This happens because the land surface absorbs and radiates heat readily due to its low specific heat capacity. On the other hand, the sea water heats up slowly and radiates the heat in it at a slower rate, as it has a very high specific heat capacity. This breeze from the sea is called sea breeze.



Formation of land breezes:
At night, the land loses heat quickly and cools down faster than sea water, which is still hot as it loses heat slowly. Therefore, the hot air above the sea rises up and the cold air from the land moves towards the sea. This breeze from the land to the sea is called land breeze.

Page No 64:

Question C.7:

State two applications of radiations.

Answer:

Two applications of radiation are as follows:

1. White or light coloured clothes are good reflectors of heat. Therefore, they are preferred in summers as they reflect (radiate) most of the heat from the Sun, thereby keeping our bodies cool.
2. Black colour is a good absorber as well as a good radiator of heat. Therefore, the bases of cooking utensils are usually painted black so that they can absorb more heat and food can be cooked faster.

Page No 64:

Question D.1:

The radiators of cars are painted black.

Answer:

The colour black is a good absorber and a good radiator of heat. Hence, the radiators of cars are painted black to effectively radiate the heat generated by the engine and to produce a cooling effect. 

Page No 64:

Question D.2:

Handles of cooking utensils are made of wood, plastic or bakelite.

Answer:

Utensils made of metals are good conductors of heat and they absorb heat quickly. If the handles of such utensils are also made of metals, one might get hurt while holding them. Therefore, the handles are made up of insulators like wood, plastic or bakelite, which are poor conductors of heat and do not get heated up easily, and so we can hold the utensils comfortably.

Page No 64:

Question D.3:

We prefer wearing dark coloured clothes in winters.

Answer:

Dark coloured clothes are preferred in winters because they are good absorbers of heat and hence they keep our bodies warm.

Page No 64:

Question D.4:

The inner side of the outer wall and outer side of the inner wall of thermos flask is silvered.

Answer:

The inner side of the outer wall and the outer side of the inner wall of thermos flasks are silvered to minimise the loss of heat due to radiation. When the heat from inside the flask strikes the silvered wall, the heat radiation gets reflected back to the flask because silver is a good reflector of light and heat.

Page No 64:

Question D.5:

Ventilators are made close of the ceiling.

Answer:

Ventilators are made close to the ceiling to let out the hot and stale air from a room. When a room gets heated up, hot air, being light, rises upwards and escapes through the ventilators into the surroundings, while fresh air enters through the windows and doors of the room.



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