HEAT and HEAT CAPACITY

Transcription

1 Grade VIII Sub:Physics HEAT and HEAT CAPACITY I. Tick ( ) the most appropriate answer. 1. The coolant used in car radiators is: (a) alcohol (b) benzene (c) water (d) petrol 2. During a sea breeze there is a fall in pressure over the surface of: (a) water (b) land (c) air (d) none of these 3. The heat required to raise the temperature through 1 C is called: (a) specific heat (b) thermal energy (c) thermal capacity (d) calorie 4. The conversion of ice into water is an example of: (a) evaporation (b) melting (c) freezing (d) vaporisation 5. The temperature at which a liquid gets converted into vapour states is called its: (a) boiling point (b) melting point (c) freezing point (d) dew point 6. The SI unit of latent heat is: (a) J/g (b) J/kg. (c) J/kg C (d) J/g C 7. Land breeze blows: (a) only during the day (b) only during the night (c) throughout the night and day(d) none of these 8. Cooling is caused during: (a) vaporisation (b) heating (c) evaporation (d) none of these 9. Which of these substances has the highest specific heat? (a) aluminium (b) copper (c) water (d) alcohol 10. The specific latent heat of fusion of ice is: (a) 4.2 J/g C (b) 4200 J/kg C (c) 336 J/g (d) J/g 11. During snow storm, the atmosphere gets: (a) warm (b) cold (c) very cold (d) very hot. 12. Sea breeze blows: (a) only during day (b) only during night (c) throughout day and night (d) none of these Class-VIII Physics 1

2 Ans. 1. (c) 2. (b) 3. (c) 4. (b) 5. (a) 6. (b) 7. (b) 8. (c) 9. (c) 10. (c) 11. (a) 12. (a) II. Fill in the blanks. 1. has the lowest specific heat amongst all liquids. 2. Water is used in a hot water bottle for. 3. Water is used as a in thermal power stations. 4. The cold air moving from land towards the sea is called kilocalorie = joules. 6. The amount of heat required to change the state of a substance without any change in temperature is called the. 7. The boiling point of water is. 8. The temperature at which the solid state of a substance gets converted into liquid state is called its. 9. At equilibrium, no heat flows from the body at higher temperature to that at lower temperature. 10. The SI unit of heat capacity is. 11. Heat energy always flows from a body at temperature to a body at lower. 12. When two bodies at different temperatures on coming in contact with one another attain same temperature, then heat given out by body is equal to heat absorbed by body. 13. The heat energy required to raise the temperature of of substance through is called specific heat capacity. 14. The unit of is J C During the change in state, the temperature of a substance remains. 16. The temperature at which a solid starts changing into liquid state, is called. 17. Rise in temperature of a body is an of heat. 18. When two bodies at different temperatures are brought into contact, the body gains heat. 19. Exchange of heat between two bodies can take place only if. Class-VIII Physics 2

3 20. The temperature of a body gives a measure of. 21. The effect of same quantity of heat on equal masses of different substances is. 22. The SI unit of heat is. 23. is a characteristic of the material of which a substance is made of calories = joules. 25. The specific heat capacity of substance A is double that of substance B. Therefore, the heat capacity of substance B will substance A J is the quantity of heat required to convert to at the same temperature. 27. The quantity of heat that has to be supplied to melt ice at 0 C to water 0 C is. 28. The greater the mass of a substance the its heat capacity. 29. are good conductors of heat. 30. The amount of heat energy supplied to a liquid at its boiling point such that it changes into gaseous state at some fixed temperature, without any in temperature is called latent heat of. 31. The process due to which a rapidly changes into gaseous state at some fixed temperature with the of heat is called vaporisation. Ans. 1. mercury 2. fomentation purposes 3. coolant 4. land breeze latent heat C 8. melting point 9. thermal 10. J/ C 11. higher, temperature 12. hot, cold g, 1 C 14. heat capacity 15. constant 16. melting point 17. example 18. colder 19. there exists a temperature difference between them 20. heat 21. different 22. joule 23. specific heat half of Class-VIII Physics 3

4 26. water, steam J/g 28. greater 29. metals 30. rise, vaporisation 31. liquid, absorption III. Write true or false for each statement given below. 1. The SI unit of specific heat capacity is J/kg C. 2. More is the specific heat capacity of a substance, more is the rise in temperature for given amount of heat. 3. Heat energy required to melt 1 g of ice at 0 C to form water at 0 C is 2260 J. 4. Water at 100 C causes more severe burns than steam at 100 C. 5. Land and sea breezes are caused due to high specific heat capacity of water calorie is equal to 2.4 J. 7. The unit of specific heat capacity of vaporisation is Joule/Kilogram. 8. The temperature at which a liquid change to solid is called solidification point.. Ans. 1. True 2. False 3. False 4. False 5. True 6. False 7. False 8. True IV. Write true or false for each statement. Rewrite the false statements correctly. 1. We feel refreshed under a tree during summers. Ans. True 2. Boiling takes place at all temperatures. Ans. False Correct statement: Boiling takes place at a fixed temperature. 3. Evaporation takes place over the entire mass of the liquid. Ans. False Correct statement: Evaporation takes place over the upper surface of the liquid 4. Boiling water at 100 C gives out more heat than an equal amount of steam at 100 C. Ans. False Class-VIII Physics 4

5 Correct statement: Steam at 100 C gives out more heat than an equal amount of boiling water at 100 C 5. When heat energy is supplied to a substance, its intermolecular space increases. Ans. True 6. The process due to which a liquid on heating changes to vapour state at some fixed temperature is called condensation. Ans. False Correct statement: The process due to which a liquid on heating changes to vapour state at some fixed temperature is called boiling. 7. The rise in temperature or quantity of heat absorbed by a substance depends on the mass of the substance taken. Ans. True 8. In cold countries, juice bottles are kept under water. Ans. True 9. The more the temperature of a hot body, the more slowly the heat flows from it into the cold body. Ans. False Correct statement: The more the temperature of a hot body, the more rapidly the heat flows from it into the cold body. 10. The heat energy required to raise the temperature of a given mass of water through 1 C is 4.2 J. Ans. False Correct statement: The heat energy required to raise the temperature of 1 gram of water through 1 C is 4.2 J. 11. The temperature at which a liquid changes into gaseous state rapidly is called melting point. Ans. False Correct statement: The temperature at which a liquid changes into gaseous state rapidly is called boiling point. 12. Heat required to raise the temperature of 1 g of a substance through 1 C is called thermal capacity of the substance. Ans. False Class-VIII Physics 5

6 Correct statement: Heat required to raise the temperature of 1 g of a substance through 1 C is called specific heat capacity of the substance. 13. The specific heat capacity of water is 2.4 Jg 1 C 1. Ans. False Correct statement: The specific heat capacity of water is 4.2 Jg 1 C 1. V. Match the column A with column B. Column A Column B 1. Latent heat of fusion of ice (a) 100 C 2. Latent heat of vaporisation of water (b) 4200 J/kg C 3. Specific heat of water (c) J/kg 4. Specific heat of aluminium (d) 0 C 5. Boiling point of water (e) 900 J/kg C 6. Melting point of ice (f) J/Kg Ans. 1. (f), 2. (c), 3. (b), 4. (e) 5. (a), 6. (d) VI. Find the odd one out. Give reasons for your choice. 1. Quantity of heat absorbed, heat supplied, mass, nature, density of the substance. Ans. The quantity of heat absorbed by a substance depends on the heat supplied, mass of the substance and nature of the substance. But it does not depend on the density of the substance. Hence density of the substance is odd one among quantity of heat absorbed, heat supplied, mass, nature and density of the substance. 2. Kilojoules, joules, calories, newton. Ans. Newton is the unit of force while kilojoules, joules and calories are the units of heat energy. VII. Differentiate between the following. 1. Heat and temperature. Ans. Heat is an invisible energy, which causes in us the sensation of hotness or coldness while the degree of hotness or coldness of a body is called temperature. In SI system heat is measured in joule whereas Class-VIII Physics 6

7 temperature is measured in Kelvin. 2. Specific heat and latent heat. Ans. The quantity of heat required to raise the temperature of unit mass of a substance by 1 C is called its specific heat. Its SI unit is J/ kg C while the amount of heat required to change the state of a substance without any change in the temperature is called the latent heat. Its SI unit is J/kg. 3. Melting point and boiling point. Ans. The temperature at which a solid starts changing into liquid state is called melting point, while the temperature at which a liquid changes into gaseous state rapidly is called boiling point. 4. Conductors and insulators of heat. Ans. The materials which allow the heat energy to pass through them quickly are called conductors of heat while the materials which do not allow the heat energy to pass through them easily are called insulators of heat. Metals like silver, copper, etc. are conductors of heat while wool, glass, etc. are insulators of heat. 5. Calorie and joule. Ans. The amount of heat energy required to raise the temperature of one gram of water by 1 C is called one calorie. Calorie is the practical unit of heat while joule is the SI unit of heat 1 Calorie = 4.2 joules 6. Latent heat of fusion and latent heat of vaporisation. Ans. Latent heat of fusion of a substance is the quantity of heat required to convert unit mass of the substance from the solid state to the liquid state at its melting point without any change of temperature, while latent heat of vaporisation is the quantity of heat required to convert unit mass of the substance from the liquid state to the vapour state at its boiling point without any change of temperature. VIII. Give reasons for the following. 1. On supplying the same amount of heat to equal quantities of water and milk, their temperature rises by different amounts. Class-VIII Physics 7

8 Ans. On supplying the same amount of heat to equal quantities of water and milk, their temperature rises by different amounts. It is because the quantity of heat absorbed depends upon the nature of material. 2. Water cannot be used as a thermometric liquid. Ans. Water has the highest specific heat and a small amount of heat cannot raise the temperature of water. So water cannot be used as thermometric liquid 3. During summer, people use earthern pots to get cool water. Ans. When water comes out of the tiny pores of the earthern pots, it gets evaporated. It requires a lot of heat to change from liquid to vapour state. This heat is drawn from the surroundings and from the water itself. Thus water gets cooled. So people use earthern pots to get cool water during summer. 4. Kolkata has an equitable climate. Ans. Kolkata has an equitable climate because it is situated near sea and is affected by land and sea breezes which are due to high specific heat capacity of water. 5. We feel refreshed under a fan after perspiring. Ans. We feel refreshed under a fan after perspiring because when the sweat evaporates, it takes the heat required for evaporation from the body, thereby cooling it and maintaining the temperature of the body. 6. Water from lakes, rivers, etc. does not vaporise rapidly. Ans. Water from lakes, rivers, etc. does not vaporise rapidly because water has very high specific heat so a large amount of heat energy is required for evaporation of water. 7. An ice-cube is used for cooling a drink. Ans. An ice-cube is used for cooling a drink because water has the highest specific heat capacity and it absorbs a large amount of heat from drink to change an ice-cube into water and the drink gets cooled. 8. Water is used as a coolant in automobiles. Ans. Water is used as a coolant in automobiles because it has the highest specific heat capacity and absorbs a large amount of heat energy produced by the automobiles. Class-VIII Physics 8

9 9. It takes much lesser time to heat 10 g of mercury to 50 C than to heat 10 g of water to the same temperature. Ans. It takes much lesser time to heat 10 g of mercury to 50 C than to heat 10 g of water to the same temperature because the quantity of heat absorbed depends upon the chemical composition or nature of the substances i.e., specific heat of the substance. Specific heat of mercury is 136 J/kg C whereas the specific heat of mercury is 4200 J/kg C. 10. Steam causes more severe burns than boiling water. Ans. Steam causes more severe burns than boiling water because steam at 100 C releases additional heat which is equal to latent heat, which makes it to cause more severe burns. 11. A hot water bottle is a more effective warming device than a block of wood having the same mass and at the same temperature. Ans. A hot water bottle is a more effective warming device than a block of wood having the same mass and at the same temperature because water in hot water bottles has very high specific heat capacity and can store a large amount of heat energy at comparatively low temperature as compared to a block of wood. 12. We generally feel cold after bath. Ans. We generally feel cold after bath because when water evaporates, it takes the heat required for evaporation from the body which makes the body feel cold. 13. It is generally colder after a hail storm than during and before the hailstorm. Ans. It is generally cold after a hailstorm than during and before the hail storm because water has high specific heat capacity and the hail requires a lot of heat to convert into liquid state which it takes from the surroundings and makes the environment cool. 14. In summer after a rainfall, the heat is generally very oppressive. Ans. In summer after a rainfall, the heat is generally very oppressive because, the air is almost saturated with water vapour. Thus, the evaporation of sweat from our body drops suddenly and hence the Class-VIII Physics 9

10 body stops losing heat. This in turn gives a very oppressive sensation. 15. In summer, dogs hang out their tongues. Ans. During summer, saliva of dog s tongue get evaporated by taking heat from dog s body so, they hang out their tongues. 16. Tea gets cooled when sugar is added to it. Ans. Tea gets cooled when sugar is added to it because sugar particles take some heat from tea to get dissolve and make the tea cool. 17. Ice-cream appears colder to the mouth as compared to water at 0 C. Ans. Ice-cream appears colder to the mouth than water at 0 C due to its high latent heat of fusion. Every gram of ice-cream absorbs 336 joules of heat energy more than water at 0 C from the mouth. 18. Damp khus-khus screens are used to cool air in the room in summers. Ans. Damp khus-khus screens are used to cool air in the room in summers. The reason is that when water evaporates from the khus-khus, latent heat of vapourization is taken from the air entering the room, as a result the air of the room gets cool. 19. A person with high fever is often advised to put a wet cloth on his/ her forehead. Ans. A person with high fever is often advised to put a wet cloth on his/ her forehead, the reason is that when water from wet cloth put on patients forehead evaporates, it takes away the necessary heat from the body of the patient and thus lowers the temperature of his/her body. IX. Answer the following questions. 1. (a) Define heat energy. (b) How does heat energy flow from one body to another body? Ans. (a) Heat is an invisible energy which causes in us the sensation of hotness or coldness. (b) Heat energy always flows from a body at a higher temperature to a body at lower temperature till both the bodies attain same temperature. Class-VIII Physics 10

11 2. (a) Define (i) calorie (ii) kilocalorie. (b) Name the unit in which heat energy is measured these days. (c) How many joules are equal to one calorie? Ans. (a) (i) Calorie: The amount of heat energy required to raise the temperature of one gram of water through 1 C is called one calorie. (ii) Kilocalorie: The amount of heat energy required to raise the temperature of 1 kilogram of water through 1 C is called one kilocalorie. (b) These days, heat energy is measured in terms of Joules. (c) 1 calorie = 4.2 joules 3. State three factors which determine the quantity of heat energy in a body. Ans. The three factors which determine the quantity of heat energy in a body are: (a) Mass of the substance (b) Nature of the substance (c) Amount of heat supplied 4. Define the term specific heat capacity and state its units. Ans. The amount of heat energy required to raise the temperature of one gram of a substance through 1 C is called specific heat capacity of the substance. The S.I. unit of specific heat capacity is J kg 1 C 1. However, the unit J g 1 C 1 is commonly used. 5. What do you understand by the statement that specific heat capacity of copper is 400 J kg 1 C 1? Ans. The statement specific heat capacity of copper is 400 J kg 1 C 1 means that 400 Joules of heat energy is required to raise the temperature of one kilogram of copper through 1 C. 6. What do you understand by the term thermal capacity? State its units? Ans. The amount of heat energy required to raise the temperature of a given mass of substance through 1 C is called its heat capacity or thermal capacity. Its unit is J/ C in S.I. system. 7. Why do we prefer warm water in hot water bottles for fomentation? Class-VIII Physics 11

12 Ans. Owing to its high specific heat capacity of 4.2 J/ C, water can store large amount of heat energy at comparatively low temperature as compared to other substances so, it is used in hot water bottles for fomentation purposes. 8. Explain the formation of land and sea breezes. Ans. Sea Breeze: During day time, the temperature of the land rises more rapidly as compared to the sea. It is because the specific heat of land is less than the sea water. The air above the land gets heated, and rises up. It results in the fall of pressure on the land. To make up for this fall in pressure, the cold air from the sea starts blowing towards the land. It gives rise to sea breeze. Land Breeze: During night, the land loses its heat more rapidly as compared to sea on account of its low specific heat capacity. So 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 land breeze. 9. Why is water used as a coolant in the radiators of motor car engines? Ans. Water is used as a coolant in the radiators of motor car engines as it has the highest specific heat capacity of 4.2 J/g C. Thus it can absorb a large amount of excess heat energy produced by the automobile engines, but itself does not rise to a very high temperature. 10. What do you understand by the term latent heat of fusion? Support your answer with an example. Ans. The amount of heat energy supplied to a solid at its melting point, such that it changes into liquid state without any rise in temperature, is called latent heat of fusion. Class-VIII Physics 12

13 For example, 1 kg of ice at 0 C requires J of energy to change into water at the same temperatures. This is known as latent heat of fusion of ice. 11. What do you understand by the term latent heat of vaporisation? Support your answer with an example. Ans. Latent heat of vaporization of a substance is the quantity of heat required to convert unit mass of a substance from liquid to vapour state at its boiling point without any change of temperature. For example, 1 g of water at 100 C requires 2260 J of energy to change into vapour at the same temperature. 12. Under similar conditions, which will cool faster 1 kg of aluminium at 400 C and 1 kg of lead at the same temperature? Ans. Specific heat of aluminium (900 J/kg C) is more than that of lead (130 J/kg C). Hence lead will cool faster. 13. State three factors on which the evaporation of a liquid depends? Ans. Evaporation of a liquid depends upon the following three factors. (a) The nature of the liquid (b) Area of exposed surface (c) The temperature of the liquid (d) Humidity or the amount of water vapour present in air 14. What is evaporation? Ans. Evaporation is the process in which a liquid changes into its vapour, from its surface, at a temperature below its boiling point. 15. State three differences between evaporation and boiling. Ans. Evaporation Boiling 1. It is a slow process. It is a fast process. 2. It takes place at the surface of It takes place throughout the the liquid. whole mass of the liquid. 3. It takes place at all It takes place at a definite and temperature. constant temperature which is called boiling point of the liquid. Class-VIII Physics 13

14 16. Explain how the human body maintains a constant temperature of about 98.4 F even when the outside temperature may be as 110 F. Ans. Evaporation helps in maintaining the body temperature at 98.4 F. When the atmospheric temperature rises, the body perspire. When the sweat evaporates, it takes the heat required from the body, thereby cooling it and maintaining the temperature of the body. 17. Define the following terms. 1. Fusion Ans. Fusion: The process due to which a solid changes into liquid state at some fixed temperature, by the absorption of heat energy is called fusion. 2. Solidification Ans. Solidification: The process due to which a liquid changes into solid state at some fixed temperature by the liberation of heat energy is called solidification. 3. Melting point Ans. Melting point: The temperature at which a solid starts changing into liquid state is called melting point. 4. Freezing point Ans. Freezing point: The temperature at which a liquid starts changing into solid state is called freezing point. 5. Vaporisation Ans. Vaporisation: The process due to which a liquid rapidly changes into gaseous state at some fixed temperature due to absorption of heat energy is called vaporisation. 6. Condensation Ans. Condensation: The process due to which vapour changes into liquid state at some fixed temperature due to liberation of heat energy is called condensation. 7. Boiling point Ans. Boiling point: The fixed temperature at which a liquid changes into gaseous state rapidly is called boiling point. 8. Condensation point. Class-VIII Physics 14

15 Ans. Condensation point: The fixed temperature at which a vapour changes into liquid state rapidly, is called condensation point. X. Practice for numerical problems. 1. Express 2268 joules in calories. Ans. 1 calorie = 4.2 joule 1 joule = 1 calorie joule = 2268 calorie 4.2 = 540 calorie calories of heat is supplied to 200 g of water. Find the rise in temperature. Ans. Mass of water, m = 200 g = 0.20 kg Heat supplied = 2000 cal. = J Specific heat of water (s) = 4200 J/kg C Heat supplied, Q = m s t = t t = = 10 C Therefore the rise in temperature is 10 C. 3. A lump of iron at 170 C is cooled to 20 C. If the mass of iron is 150 g and the heat radiated is J, find the specific heat capacity of iron. Ans. Change in temperature (t) = 170 C 20 C = 150 C Mass (m) = 150 g = kg Heat radiated (Q) = J Specific heat capacity (s) =? Q = m s t Class-VIII Physics 15

16 s = = = Q m t J/ kg C = 460 J/kg C 4. Calculate the latent heat of fusion of a solid if 25 g of it requires 8400 J of heat energy for melting. Ans. The energy required to melt 25 g solid is 8400 J The energy required to melt 1g of solid = = 336 J/g Therefore the latent heat of fusion of solid is 336 J/g. 5. Calculate the heat energy liberated by 45g of steam so as to form water at 100 C (latent heat of vaporisation of water is 2260 J/g.) Ans. Heat released by 1 g of steam to form water at 100 C = 2260 J/g Heat released by 45 g of steam = 2260 J/g 45 g = J 6. Calculate the thermal capacity of 1.5 kg of water. Ans. Specific heat capacity of water = 4200 J/kg C Thermal capacity of 1.5 kg of water = 1.5 kg 4200 Kg C = 6300 J/ C 7. Calculate the amount of heat energy required to raise the temperature of 2 g of mercury of specific heat capacity 0.12 Jg 1 C 1 from 10 C to 30 C. Ans. Rise in temperature (t) = (30 10) C = 20 C Specific heat capacity (s) = 0.12 Jg 1 C 1 Mass (m) = 2 g Amount of heat energy required = m s t J Class-VIII Physics 16

17 20 C = 4.8 joule g of copper at 200 C is cooled to 20 C. If the specific heat capacity of copper is 0.4 J g 1 C 1, calculate the heat energy given out. Ans. Mass of copper m = 200 g J = 2 g 0.12 g C Change in temperature (t) = 200 C 20 C = 180 C Specific heat capacity of copper (s) = 0.4 J/g C Amount of heat energy given out = m s t = 200g 0.4 g J C 180 C = joules g of water at 10 C is heated by supplying J of heat energy. If specific heat capacity of water is 4.2 Jg 1 C 1, calculate the final temperature of water. Ans. Mass of water (m) = 75 g Amount of heat energy (Q) = J Specific heat capacity (s) = 4.2 J g 1 C 1 Q Rise in temperature t = m s = = 80 C Therefore final temperature of water = 80 C + 10 C = 90 C 10. Calculate the thermal capacity of a solid of mass 0.8 kg and Ans. specific heat capacity 0.4 Jg 1 C 1. Mass of solid = 0.8 kg = 800 g Specific heat capacity = 0.4 J/ g C Thermal capacity = mass specific heat capacity Class-VIII Physics 17

18 = 800 g 0.4 g C = 320 J/ C J of heat energy is supplied to a solid of specific heat capacity 0.85 J g 1 C 1 to raise its temperature by 1 C. What is the mass of solid? Ans. Amount of heat energy supplied (Q) = J Specific heat capacity (s) = 0.85 J/ g C Rise in temperature (t) = 1 C Q Mass of solid = s t = = 125 g 12. Find the specific heat of gold if 108 J of heat is required to raise 9 g of the metal from 0 C to 100 C. Ans. Mass of gold (m) = 9 g Amount of heat required (Q) = 108 J Change in temperature (t) = 100 C 0 C = 100 C Q Specific heat of gold = m t 108 = = 0.12 J/g C J Class-VIII Physics 18