HEAT FLOW AND TEMPERATURE
Heat: The energy transferred from one body to another body due to temperature difference between them is called heat. It is an invisible energy which causes the sensation of hot and cold. The direction of flow of heat energy is from hot object to cold object.
Temperature: The degree of hotness or coldness of a body is called temperature. Or a reliable measure of the hotness of an object is called its temperature. It gives the measurement of heat energy.
Thermometer: A device used for measuring the temperature of different objects are called thermometer.
Clinical thermometer: A device which measures the temperature of human body is called clinical thermometer. It is marked from 350 C to 420 C.
Q.1 What do you mean by the terms heat and temperature?
Ans. The energy transferred from one body to another body due to temperature difference between them is called heat. It is an invisible energy which causes the sensation of hot and cold. The direction of flow of heat energy is from hot object to cold object.
The degree of hotness or coldness of a body is called temperature. Or a reliable measure of the hotness of an object is called its temperature. It gives the measurement of heat energy.
Q.2. Name the device which is used to measure the temperature?
Ans. Thermometer is used to measure the temperature.
Q.3. What is the difference between the clinical and lab thermometer?
Ans. Clinical Thermometer Laboratory thermometer
It is used to measure body temperature. It is used to measure the temperature of objects
It ranges from 350 C to 420 C. It ranges from -100 C to 110 0 C.
It is provided with a kink which checks It is not provided with any kink.
the backward flow of mercury
Q.4. Why is mercury used in the thermometer?
Ans. Mercury is used in the thermometers because of the following reasons.
1. It is opaque and shining. Hence, it is easily visible through the glass.
2. It does not wet the glass.
3. It has a low freezing point ( -390 C) and a high boiling point (3570C). Hence, it can be used over a wide range of temperature.
4. It is a good conductor of heat, so it quickly absorbs heat to attain the temperature of the object.
5. It has a wide range of temperature expansion.
6.
Q.5. What is the use of kink in a clinical thermometer?
Ans. Kink is the bent portion of the thermometer. It is used in a clinical thermometer because it prevents the backward flow of mercury into the bulb due to which the readings of the mercury stays constant.
Q.6. What are the different methods of transfer of heat in matter?
Ans. The different method of transfer of heat in mater are:-
1. Conduction
2. Convection
3. Radiation
Q.7. Define conduction, convection and radiation.
Ans. a) Conduction: The process by which heat is transferred in solids from hot object to cold object without the actual movement of particles (atoms or molecules) of the solid is called conduction. It takes place in solids from one end to another by the vibrations of the particles. This method of transfer of heat is called conduction. In this process of transfer of heat a solid material medium is required.
b) Convection: The process by which heat is transferred in liquids and gases by the actual movement of particles is called convection. In this process of transfer of heat a liquid or gaseous material medium is required. For example, when we heat water in the container, the water gets hot due to convection.
c) Radiation: The process of transfer of heat without the help of a material medium is called radiation. It does not require any medium for transfer of heat. For example, the rays from the sun directly reach on the earth traveling through the space due to radiation.
Q.8. Differentiate between conductors and insulators.
Conductors Insulators
Conductors are the materials which allow heat Insulators are the materials which does not allow heat to pass through them.to pass through them very easily.
All the metals such as steel, iron, copper etc Wood, plastic, foam etc are the examples are the examples of conductors. of insulators.
Q.9. Write an experiment which shows i) conduction in solids ii) convection in liquids.
Ans. Conduction in solids: Take a rod or flat strip of a metal, say of aluminium or iron. Fix a few small wax pieces on the rod. These pieces should be at nearly equal distances. Put one end of the rod in between bricks. Now, heat the other end of the rod and observe. It can be seen that the wax pieces began to fall. The first piece nearer to the flame falls first. Heat is transferred from one end nearest to the flame to the other end.
Convection in liquids: Take a round bottom flask. Fill it two-thirds with water. Place it on a tripod or make some other arrangements, so that it can be heated by placing a candle below it. Wait till the water in the flask is still. Place a crystal potassium permanganate at the bottom of the flask gently using a straw. Now, heat the water by placing the candle just below the crystal.
Q.10. Explain about the formation of sea breeze and land breeze.
a) Sea breeze: During day time, sun shines equally on the land and the sea. However, the temperature of the land rises more rapidly as compared to the seas as the specific heat capacity of the land is far less than sea water. Thus, the air above the land gets heated. This hot air expands and rises up. This results in fall of pressure over the surface of the land. To make up for this fall in pressure, the cold air from the sea starts blowing towards the land, giving rise to sea breeze.
b) Land breeze: During night, land as well as the sea loses heat. However, the land loses its heat more rapidly as compared to the sea on account of its low specific heat capacity. Thus, the air above the land becomes colder than the sea. The warm air above the sea expands and rises up. Thus, the air pressure falls over the sea. To make up for this fall in pressure, the cold air from the land starts blowing towards the sea giving rise to the land breeze.
Q.11. Give reasons for the following statements.
a) It is better to wear light coloured dress in summer.
Ans. Light coloured dresses are poor absorbers of heat energy and good reflector of heat energy. So it is better to wear light coloured clothes in summer, so that they can absorb less heat energy and can reflect more heat energy to protect us from heat.
b) Utensils are made of metals. Why handles are made of insulators.
Ans. Utensils are made of metals because they are good conductors of heat energy. They absorb heat energy very quickly due to which food is cooked very quickly. But the handles of utensils are made of plastic or any other insulators because they are very poor conductors /good insulators of heat energy. So when we use the handles to lift the utensils, they protect our hand from burning.
c) Two thin sweaters feel warmer than one thick sweater.
Ans. Two thin sweaters are warmer than one thick sweater because the air between the two sweaters is a poor conductor of heat and does not allow heat energy of our body to escape. In case of one thick sweater there is no air trapped in it due to which there is no insulator and the heat does escapes from our body.
Exercises
Q.1. State similarities and difference between the lab and clinical thermometers.
Ans. Similarities between the lab and clinical thermometers
1. Lab thermometer is use to read the temperature. Clinical thermometer is also used to measure temperature.
2. Mercury is used in both lab and clinical thermometers.
3. Both lab and clinical thermometers are made of capillary tube.
Differences between the lab and clinical thermometers
Lab Thermometer clinical thermometer
1. It is used to measure the temperature of objects. It is used to measure the body temperature.
2. It ranges from -10 0 C to 1100C. It ranges from 350C to 420C
3. It is not provided with a kink. It is provided with a kink
Q.2. Give the examples of each of conductors and insulators of heat.
Ans. The materials which allow heat to pass through them easily are conductors of heat. Two examples of conductors are aluminium, iron and copper
The materials which do not allow heat to pass through them easily are poor conductors of heat or insulators. The examples of insulators are plastic and dried wood.
Q.3. Fill in the blanks.
a) The hotness of an object is determined by its temperature.
b) Temperature of boiling water cannot be measured by a clinical thermometer.
c) Temperature is measured in degree Celsius, fahrenheit
d) No medium is required for transfer of heat by the process of radiation.
e) A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of conduction.
f) Clothes of dark colours absorb heat better than clothes of light colours.
Q.4. Match the following
i) Land breeze blows during night
ii) Sea breeze blows during day
iii) Dark coloured clothes are preferred during winter
iv) Light coloured clothes are preferred during summer
Q.5. Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.
Ans. Wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing because the air between the layers of clothes is a poor conductor of heat and does not allow heat energy of our body to escape. In case of just one thick piece of clothing, there is no air trapped in it due to which there is no insulator and the heat does escapes from our body.
Q.6. When we are boiling water in a utensil, with the help of a flame, heat is transferred by which process.
Ans. Conduction.
Q.7. In places of hot climate it is advised that the outer walls of houses be painted white. Explain.
Ans. White colour is a poor absorber of heat energy and good reflector of heat energy. In summer, outer walls which are painted white to absorb less heat energy and can reflect more heat energy, so that the dwellers can feel less heat. So, in places of hot climate, it is advised that the outer walls of houses be painted white.
Q.8. One litre of water at 300 C is mixed with one litre of water at 500 C. The Temperature of the mixture will be
i) 800 C ii) more than 500 C but less than 800 C
iii) 200 C iv) between 300 C and 500 C
Q.9. An iron ball at 400 C is dropped in a mug containing water at 400 C. The heat will
a) flow from iron ball to water
b) not flow from iron ball to water or from water to iron ball
c) flow from water to iron ball d) increase the temperature of both
Q.10. A wooden spoon is dipped in a cup of ice cream. Its other end
a) becomes cold by the process of conduction
b) becomes cold by the process of convection
c) becomes cold by the process of radiation
d) does not become cold.
Q.11. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that
a) copper bottom makes the pan more durable. b)such pans appear colourful
c) copper is a better conductor of heat than the stainless steel.
d) copper is easier to clean than the stainless steel.
PRIVATE BOOK ( HEAT FLOW AND TEMPERTURE )
I. Fill in the blanks.
1. The degree of hotness is called temperature.
2. Temperature of boiling water is 100 degree Celsius.
3. Shining bodies are poor radiators of heat energy.
4. Conduction is not possible in liquid and gases
5. Radiant heat can easily pass through vacuum.
II. Statements given below are incorrect. Correct them.
1. During conduction the heat energy travels by the actual visible movement of particles.
Ans. During conduction heat energy travels without the actual movement of the molecules of the solid.
2. Black bodies are good absorbers, but poor radiators of heat.
Ans. Black bodies are good absorbers and good radiators of heat.
3. During heat radiation, the medium in between the hot body and the cold body becomes hot.
Ans. During heat radiation the medium in between the hot body and the cold body is not heated.
4. Sea breeze blows during night time.
Ans. Sea breeze blows during day time.
5. Copper is an example of good insulator of heat.
Ans. Copper is an example of good conductor of heat.
III. Match the statements.
1. Transmission of heat energy with actual movement of the Convection
Particles of a medium
2. A device which measures the degree of hotness Thermometer
3. The thermometric scale use in laboratory thermometer Celsius
4. A process of transmission of heat which does not require Radiation
any medium
5. A process of transmission of heat in which particles of a Conduction
Medium do not move towards the source of heat
6. A device used for measuring temperature of human body Clinical thermometer
IV. Write true or false
1. Shining bodies are good absorbers and good radiators of heat. FALSE
(Shining bodies are poor absorbers and poor radiators of heat)
2. Convection easily takes place in gases. TRUE
(Convection easily takes place in liquids and gases)
3. Radiation can easily take place through vacuum TRUE
4. Conduction can easily take place in liquid. FALSE
(Conduction takes place only in solids)
5. Wood is a good insulator of heat. TRUE
6. White clothes are cool in summer. TRUE
(White colour is poor absorber of heat energy.)
V. Answer the following questions.
1. What is the temperature of a normal human being on the Celsius scale?
Ans The temperature of a normal human being on the Celsius scale is 370 C.
In Fahrenheit scale it is 98.40 F.
2. Name the thermometer used for measuring the temperature of human body?
Ans. The thermometer used for measuring the temperature of human body is called clinical thermometer. It ranges from 350C to 420C on Celsius scale, as the temperature of human body does not fall below 350C nor does it rise above 420 C. On Fahrenheit scale the range of a clinical thermometer is from 940 F to 1080 F. It is provided with a kink.
3. State the precautions for using laboratory thermometer.
Ans. 1. The Laboratory thermometer should be kept upright.
2. The bulb of the thermometer should be completely surrounded by the liquid.
3. The bulb of the thermometer should not touch the sides or base of the container.
4. The stem of the thermometer should not be dipped into the liquid.
4. What is the function of kink in a clinical thermometer?
Ans. Kink is the bent portion of the thermometer. It is used in a clinical thermometer because it prevents the backward flow of mercury into the bulb due to which the readings of the mercury stays constant.
5. Why is the handle of a pressure cooker covered with thick plastic?
Ans. The handle of a pressure cooker is covered with thick plastic because it is a bad conductor of heat. Hence, the heat from the hot pressure cooker does not flow to the handle and prevents our hand from burning.
6. How does a blanket keep you warm in winter, but prevents ice from melting?
Ans. A layer of air trapped under the blanket, which act as an insulator does not allow the heat from the body to escape. So it keeps us warm in winter. Again the layer of air trapped under the blanket, which acts as an insulator does not allow the heat from outside to flow to the ice. So it prevents ice from melting.
7. Why do the mud houses with thatched roof keep cool in summer and warm in winter?
Ans. The mud houses with thatched roof keep cool in summer and warm in winter because the thatched roof contains a large amount of trapped air. Again mud is a bad conductor of heat. Hence, in summer, heat from outside does not flow in and in winter heat from inside does not flow out. So, mud houses with thatched roof keep cool in summer and warm in winter.
8. Define convection as a mode of transmission of heat.
Ans. Convection: It is the process by which transfer of heat in liquid and gases. It is caused by the actual movement of particle. In this process of transfer of heat a liquid or gaseous material medium is required. For example, when we heat water in the contained the water gets hot due to convection.
9. Why is convection not possible in solids?
Ans. The molecules of a solid are held strongly due to intermolecular forces. As these molecules cannot travel to the source of heat energy, convection is not possible in solids.
1. Not required
2. Why do people prefer to wear dull clothes in winter?
Ans. Dull clothes are good absorbers of heat. Because of which dull clothes keep the body warm in winter. Hence, the people prefer to wear dull clothes in winter to feel warmth.
a) Why do people prefer to wear white clothes in summer?
Ans. White colour is a poor absorber of radiant heat energy. So, people prefer to wear white colour clothes in summer as they absorb less radiant energy and keep the body cool.
12. While sitting around burning wood, by which mode of transmission do you receive heat energy? Why do other modes of transmission not play a significant role?
Ans. While sitting around burning wood, we receive heat energy by radiation. Radiation is not possible here because the air between the burning wood and us is a poor conductor of heat energy. Convection is also not possible because hot air rises upward rather then sideways.
13. Which of the following containing same amount of boiling hot water is likely to cool first and why?
(a) A beaker whose surface is covered with aluminium foil.
(b) A beaker whose surface is painted black.
Ans. A beaker whose surface is painted black will cool first because black and dull bodies are good radiators of radiant heat. Hence, it will radiate heat quickly and will cool first.
SOME IMPORTANT THINGS TO REMEMBER
1. Laboratory thermometer rages from -100 C to 1100 C.
2. Clinical thermometer ranges from 350 C to 420 C ( on Celsius scale)
3. Clinical thermometer ranges from 940 F to 1080 F.
4. The temperature of a normal human is 370 C ( 98.40 F)
5. The thermometer consists of a very fine glass tube called capillary tube.
6. Capillary tube is protected by a thick glass tube called stem.
7. Bimetallic thermometers are used to measure high temperature.
8. Maximum and Minimum thermometers are used to measure maximum and minimum temperatures of previous day.
9. Black and dull bodies are good absorbers and good radiators of radiant heat.
10. White & shining bodies are poor absorbers and poor radiators of heat.
11. White & shining bodies are good reflectors of heat.
Heat: The energy transferred from one body to another body due to temperature difference between them is called heat. It is an invisible energy which causes the sensation of hot and cold. The direction of flow of heat energy is from hot object to cold object.
Temperature: The degree of hotness or coldness of a body is called temperature. Or a reliable measure of the hotness of an object is called its temperature. It gives the measurement of heat energy.
Thermometer: A device used for measuring the temperature of different objects are called thermometer.
Clinical thermometer: A device which measures the temperature of human body is called clinical thermometer. It is marked from 350 C to 420 C.
Q.1 What do you mean by the terms heat and temperature?
Ans. The energy transferred from one body to another body due to temperature difference between them is called heat. It is an invisible energy which causes the sensation of hot and cold. The direction of flow of heat energy is from hot object to cold object.
The degree of hotness or coldness of a body is called temperature. Or a reliable measure of the hotness of an object is called its temperature. It gives the measurement of heat energy.
Q.2. Name the device which is used to measure the temperature?
Ans. Thermometer is used to measure the temperature.
Q.3. What is the difference between the clinical and lab thermometer?
Ans. Clinical Thermometer Laboratory thermometer
It is used to measure body temperature. It is used to measure the temperature of objects
It ranges from 350 C to 420 C. It ranges from -100 C to 110 0 C.
It is provided with a kink which checks It is not provided with any kink.
the backward flow of mercury
Q.4. Why is mercury used in the thermometer?
Ans. Mercury is used in the thermometers because of the following reasons.
1. It is opaque and shining. Hence, it is easily visible through the glass.
2. It does not wet the glass.
3. It has a low freezing point ( -390 C) and a high boiling point (3570C). Hence, it can be used over a wide range of temperature.
4. It is a good conductor of heat, so it quickly absorbs heat to attain the temperature of the object.
5. It has a wide range of temperature expansion.
6.
Q.5. What is the use of kink in a clinical thermometer?
Ans. Kink is the bent portion of the thermometer. It is used in a clinical thermometer because it prevents the backward flow of mercury into the bulb due to which the readings of the mercury stays constant.
Q.6. What are the different methods of transfer of heat in matter?
Ans. The different method of transfer of heat in mater are:-
1. Conduction
2. Convection
3. Radiation
Q.7. Define conduction, convection and radiation.
Ans. a) Conduction: The process by which heat is transferred in solids from hot object to cold object without the actual movement of particles (atoms or molecules) of the solid is called conduction. It takes place in solids from one end to another by the vibrations of the particles. This method of transfer of heat is called conduction. In this process of transfer of heat a solid material medium is required.
b) Convection: The process by which heat is transferred in liquids and gases by the actual movement of particles is called convection. In this process of transfer of heat a liquid or gaseous material medium is required. For example, when we heat water in the container, the water gets hot due to convection.
c) Radiation: The process of transfer of heat without the help of a material medium is called radiation. It does not require any medium for transfer of heat. For example, the rays from the sun directly reach on the earth traveling through the space due to radiation.
Q.8. Differentiate between conductors and insulators.
Conductors Insulators
Conductors are the materials which allow heat Insulators are the materials which does not allow heat to pass through them.to pass through them very easily.
All the metals such as steel, iron, copper etc Wood, plastic, foam etc are the examples are the examples of conductors. of insulators.
Q.9. Write an experiment which shows i) conduction in solids ii) convection in liquids.
Ans. Conduction in solids: Take a rod or flat strip of a metal, say of aluminium or iron. Fix a few small wax pieces on the rod. These pieces should be at nearly equal distances. Put one end of the rod in between bricks. Now, heat the other end of the rod and observe. It can be seen that the wax pieces began to fall. The first piece nearer to the flame falls first. Heat is transferred from one end nearest to the flame to the other end.
Convection in liquids: Take a round bottom flask. Fill it two-thirds with water. Place it on a tripod or make some other arrangements, so that it can be heated by placing a candle below it. Wait till the water in the flask is still. Place a crystal potassium permanganate at the bottom of the flask gently using a straw. Now, heat the water by placing the candle just below the crystal.
Q.10. Explain about the formation of sea breeze and land breeze.
a) Sea breeze: During day time, sun shines equally on the land and the sea. However, the temperature of the land rises more rapidly as compared to the seas as the specific heat capacity of the land is far less than sea water. Thus, the air above the land gets heated. This hot air expands and rises up. This results in fall of pressure over the surface of the land. To make up for this fall in pressure, the cold air from the sea starts blowing towards the land, giving rise to sea breeze.
b) Land breeze: During night, land as well as the sea loses heat. However, the land loses its heat more rapidly as compared to the sea on account of its low specific heat capacity. Thus, the air above the land becomes colder than the sea. The warm air above the sea expands and rises up. Thus, the air pressure falls over the sea. To make up for this fall in pressure, the cold air from the land starts blowing towards the sea giving rise to the land breeze.
Q.11. Give reasons for the following statements.
a) It is better to wear light coloured dress in summer.
Ans. Light coloured dresses are poor absorbers of heat energy and good reflector of heat energy. So it is better to wear light coloured clothes in summer, so that they can absorb less heat energy and can reflect more heat energy to protect us from heat.
b) Utensils are made of metals. Why handles are made of insulators.
Ans. Utensils are made of metals because they are good conductors of heat energy. They absorb heat energy very quickly due to which food is cooked very quickly. But the handles of utensils are made of plastic or any other insulators because they are very poor conductors /good insulators of heat energy. So when we use the handles to lift the utensils, they protect our hand from burning.
c) Two thin sweaters feel warmer than one thick sweater.
Ans. Two thin sweaters are warmer than one thick sweater because the air between the two sweaters is a poor conductor of heat and does not allow heat energy of our body to escape. In case of one thick sweater there is no air trapped in it due to which there is no insulator and the heat does escapes from our body.
Exercises
Q.1. State similarities and difference between the lab and clinical thermometers.
Ans. Similarities between the lab and clinical thermometers
1. Lab thermometer is use to read the temperature. Clinical thermometer is also used to measure temperature.
2. Mercury is used in both lab and clinical thermometers.
3. Both lab and clinical thermometers are made of capillary tube.
Differences between the lab and clinical thermometers
Lab Thermometer clinical thermometer
1. It is used to measure the temperature of objects. It is used to measure the body temperature.
2. It ranges from -10 0 C to 1100C. It ranges from 350C to 420C
3. It is not provided with a kink. It is provided with a kink
Q.2. Give the examples of each of conductors and insulators of heat.
Ans. The materials which allow heat to pass through them easily are conductors of heat. Two examples of conductors are aluminium, iron and copper
The materials which do not allow heat to pass through them easily are poor conductors of heat or insulators. The examples of insulators are plastic and dried wood.
Q.3. Fill in the blanks.
a) The hotness of an object is determined by its temperature.
b) Temperature of boiling water cannot be measured by a clinical thermometer.
c) Temperature is measured in degree Celsius, fahrenheit
d) No medium is required for transfer of heat by the process of radiation.
e) A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of conduction.
f) Clothes of dark colours absorb heat better than clothes of light colours.
Q.4. Match the following
i) Land breeze blows during night
ii) Sea breeze blows during day
iii) Dark coloured clothes are preferred during winter
iv) Light coloured clothes are preferred during summer
Q.5. Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing.
Ans. Wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing because the air between the layers of clothes is a poor conductor of heat and does not allow heat energy of our body to escape. In case of just one thick piece of clothing, there is no air trapped in it due to which there is no insulator and the heat does escapes from our body.
Q.6. When we are boiling water in a utensil, with the help of a flame, heat is transferred by which process.
Ans. Conduction.
Q.7. In places of hot climate it is advised that the outer walls of houses be painted white. Explain.
Ans. White colour is a poor absorber of heat energy and good reflector of heat energy. In summer, outer walls which are painted white to absorb less heat energy and can reflect more heat energy, so that the dwellers can feel less heat. So, in places of hot climate, it is advised that the outer walls of houses be painted white.
Q.8. One litre of water at 300 C is mixed with one litre of water at 500 C. The Temperature of the mixture will be
i) 800 C ii) more than 500 C but less than 800 C
iii) 200 C iv) between 300 C and 500 C
Q.9. An iron ball at 400 C is dropped in a mug containing water at 400 C. The heat will
a) flow from iron ball to water
b) not flow from iron ball to water or from water to iron ball
c) flow from water to iron ball d) increase the temperature of both
Q.10. A wooden spoon is dipped in a cup of ice cream. Its other end
a) becomes cold by the process of conduction
b) becomes cold by the process of convection
c) becomes cold by the process of radiation
d) does not become cold.
Q.11. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that
a) copper bottom makes the pan more durable. b)such pans appear colourful
c) copper is a better conductor of heat than the stainless steel.
d) copper is easier to clean than the stainless steel.
PRIVATE BOOK ( HEAT FLOW AND TEMPERTURE )
I. Fill in the blanks.
1. The degree of hotness is called temperature.
2. Temperature of boiling water is 100 degree Celsius.
3. Shining bodies are poor radiators of heat energy.
4. Conduction is not possible in liquid and gases
5. Radiant heat can easily pass through vacuum.
II. Statements given below are incorrect. Correct them.
1. During conduction the heat energy travels by the actual visible movement of particles.
Ans. During conduction heat energy travels without the actual movement of the molecules of the solid.
2. Black bodies are good absorbers, but poor radiators of heat.
Ans. Black bodies are good absorbers and good radiators of heat.
3. During heat radiation, the medium in between the hot body and the cold body becomes hot.
Ans. During heat radiation the medium in between the hot body and the cold body is not heated.
4. Sea breeze blows during night time.
Ans. Sea breeze blows during day time.
5. Copper is an example of good insulator of heat.
Ans. Copper is an example of good conductor of heat.
III. Match the statements.
1. Transmission of heat energy with actual movement of the Convection
Particles of a medium
2. A device which measures the degree of hotness Thermometer
3. The thermometric scale use in laboratory thermometer Celsius
4. A process of transmission of heat which does not require Radiation
any medium
5. A process of transmission of heat in which particles of a Conduction
Medium do not move towards the source of heat
6. A device used for measuring temperature of human body Clinical thermometer
IV. Write true or false
1. Shining bodies are good absorbers and good radiators of heat. FALSE
(Shining bodies are poor absorbers and poor radiators of heat)
2. Convection easily takes place in gases. TRUE
(Convection easily takes place in liquids and gases)
3. Radiation can easily take place through vacuum TRUE
4. Conduction can easily take place in liquid. FALSE
(Conduction takes place only in solids)
5. Wood is a good insulator of heat. TRUE
6. White clothes are cool in summer. TRUE
(White colour is poor absorber of heat energy.)
V. Answer the following questions.
1. What is the temperature of a normal human being on the Celsius scale?
Ans The temperature of a normal human being on the Celsius scale is 370 C.
In Fahrenheit scale it is 98.40 F.
2. Name the thermometer used for measuring the temperature of human body?
Ans. The thermometer used for measuring the temperature of human body is called clinical thermometer. It ranges from 350C to 420C on Celsius scale, as the temperature of human body does not fall below 350C nor does it rise above 420 C. On Fahrenheit scale the range of a clinical thermometer is from 940 F to 1080 F. It is provided with a kink.
3. State the precautions for using laboratory thermometer.
Ans. 1. The Laboratory thermometer should be kept upright.
2. The bulb of the thermometer should be completely surrounded by the liquid.
3. The bulb of the thermometer should not touch the sides or base of the container.
4. The stem of the thermometer should not be dipped into the liquid.
4. What is the function of kink in a clinical thermometer?
Ans. Kink is the bent portion of the thermometer. It is used in a clinical thermometer because it prevents the backward flow of mercury into the bulb due to which the readings of the mercury stays constant.
5. Why is the handle of a pressure cooker covered with thick plastic?
Ans. The handle of a pressure cooker is covered with thick plastic because it is a bad conductor of heat. Hence, the heat from the hot pressure cooker does not flow to the handle and prevents our hand from burning.
6. How does a blanket keep you warm in winter, but prevents ice from melting?
Ans. A layer of air trapped under the blanket, which act as an insulator does not allow the heat from the body to escape. So it keeps us warm in winter. Again the layer of air trapped under the blanket, which acts as an insulator does not allow the heat from outside to flow to the ice. So it prevents ice from melting.
7. Why do the mud houses with thatched roof keep cool in summer and warm in winter?
Ans. The mud houses with thatched roof keep cool in summer and warm in winter because the thatched roof contains a large amount of trapped air. Again mud is a bad conductor of heat. Hence, in summer, heat from outside does not flow in and in winter heat from inside does not flow out. So, mud houses with thatched roof keep cool in summer and warm in winter.
8. Define convection as a mode of transmission of heat.
Ans. Convection: It is the process by which transfer of heat in liquid and gases. It is caused by the actual movement of particle. In this process of transfer of heat a liquid or gaseous material medium is required. For example, when we heat water in the contained the water gets hot due to convection.
9. Why is convection not possible in solids?
Ans. The molecules of a solid are held strongly due to intermolecular forces. As these molecules cannot travel to the source of heat energy, convection is not possible in solids.
1. Not required
2. Why do people prefer to wear dull clothes in winter?
Ans. Dull clothes are good absorbers of heat. Because of which dull clothes keep the body warm in winter. Hence, the people prefer to wear dull clothes in winter to feel warmth.
a) Why do people prefer to wear white clothes in summer?
Ans. White colour is a poor absorber of radiant heat energy. So, people prefer to wear white colour clothes in summer as they absorb less radiant energy and keep the body cool.
12. While sitting around burning wood, by which mode of transmission do you receive heat energy? Why do other modes of transmission not play a significant role?
Ans. While sitting around burning wood, we receive heat energy by radiation. Radiation is not possible here because the air between the burning wood and us is a poor conductor of heat energy. Convection is also not possible because hot air rises upward rather then sideways.
13. Which of the following containing same amount of boiling hot water is likely to cool first and why?
(a) A beaker whose surface is covered with aluminium foil.
(b) A beaker whose surface is painted black.
Ans. A beaker whose surface is painted black will cool first because black and dull bodies are good radiators of radiant heat. Hence, it will radiate heat quickly and will cool first.
SOME IMPORTANT THINGS TO REMEMBER
1. Laboratory thermometer rages from -100 C to 1100 C.
2. Clinical thermometer ranges from 350 C to 420 C ( on Celsius scale)
3. Clinical thermometer ranges from 940 F to 1080 F.
4. The temperature of a normal human is 370 C ( 98.40 F)
5. The thermometer consists of a very fine glass tube called capillary tube.
6. Capillary tube is protected by a thick glass tube called stem.
7. Bimetallic thermometers are used to measure high temperature.
8. Maximum and Minimum thermometers are used to measure maximum and minimum temperatures of previous day.
9. Black and dull bodies are good absorbers and good radiators of radiant heat.
10. White & shining bodies are poor absorbers and poor radiators of heat.
11. White & shining bodies are good reflectors of heat.