Thermometry And Thermal Expansion

Transcription

1 Thermometry And Thermal Expansion 1. Define heat (a) on conventional basis (b) on the basis of kinetic model. Ans. (a) On the conventional basis heat is the from of energy which causes in us sensation of warmth. (b) On the basis of kinetic model, heat is the sum total of kinetic and potential energies of all the molecules of a substance. 2. Define temperature (a) on conventional basis (b) on the basis of kinetic model. Ans. (a) On the conventional temperature the degree of hotness or coldness of a body. (b) On the basis of kinetic model, temperature is the average kinetic energy of all the molecules of a substance. 3. State three differences between heat and temperature. Ans. (1) Heat is the sum total of potential and kinetic energies of all the molecules of a substance, whereas temperature is the average kinetic energy of all the molecules of a substance. (2) Heat is an energy and is measured in joules, whereas temperature is degree of hotness and is measured in degrees on a suitable scale. (3) The amount of heat in a body depends upon the number of molecules, whereas temperature of a body depends upon its average kinetic energy of molecules. 4. State four effects, which heat energy can bring about. Ans. (i) Heat brings about change in temperature. (ii) Heat brings about change in dimensions of a body. (iii) Heat brings about change in state of a body. (iv) Heat brings about chemical reactions in large number of bodies. 1

2 5. (a) Name the instrument for measuring temperature. (b) Name two liquids which are commonly used as thermometric liquids. (c) State the principle on which common laboratory thermometer works. Ans. (a) Thermometer. (b) Mercury and alcohol. (c) Liquids expand on heating and contract on cooling. 6. State four characteristics of an ideal thermometric liquid. Ans. (i) The liquid should have low specific heat capacity. (ii) The liquid should have wide range i.e., high melting point and low freezing point. (iii) The liquid should have uniform rate of expansion. (iv) The liquid should have large expansion for small rise in temperature, such that it is visible to unaided eye. 7. State four reasons for using mercury as thermometric liquid. Ans. Reasons for using mercury as thermometric liquid : (a) It has a very low specific heat capacity. (b) It has a uniform rate of expansion. (c) It has a wide range, I,e., its b.p. is 357ºC and freezing point 39 ºC. (d) It dose not stick to the sides of glass capillary tube. (e) It is a good conductor of heat. (f) It is opaque and shining and can be easily seen in glass. 8. State two disadvantages of mercury thermometer. Ans. 1. It cannot be used below 39 C as mercury freezes at this point. 2. The expansion of mercury per C rise in temperature is very small. Thus, fractional divisions of one degree are not possible. 2

3 9. State two advantages of alcohol thermometer over mercury thermometer. Ans. 1. Expansion of alcohol is fairly large for one degree Celsius rise in temperature as compared to mercury. Thus, fractional divisions of a degree are possible. 2. It does not freeze up to 100 C. Thus, it can be used to record very low temperature. 10. State two disadvantages of alcohol thermometer over mercury thermometer. Ans. 1. Alcohol is a very poor conductor of heat. Thus, alcohol thermometers are not quick in action. 2. Alcohol thermometer cannot be used beyond 78 C for recording higher temperatures. 11. State six reasons for not using water as a thermometric liquid. Ans. Reasons for not using water as thermometric liquid : (a) It has the highest specific heat capacity of 4.2 jg 1 ºC 1. (b) Its rate of expansion is not uniform. (c) It boils at 100 ºC and freezes at 0ºC and hence, does not have a wide range. (d) It is transparent and is not visible in glass. (e) It sticks to the sides of glass capillary tube. (f) It is a bad conductor of heat. 12. (a) What do you understand by the term standard point? (b) Define the following and state their magnitude in (i) degrees Celsius (ii) degrees Fahrenheit (i) Lower standard point (ii) Upper standard point. Ans. (a) The melting point of pure ice and the boiling point of pure water at a pressure of 76 cm of mercury are called standard points of a thermometer. (b) (i) The melting point of pure ice at a pressure of 76 cm of mercury is called lower standard point. Magnitude of lower standard point is 0 C or 32 F. 3

4 (ii) The boiling point of pure water at a pressure of 76 cm of mercury is called upper standard point. Magnitude of upper standard point is 100 C or 212 F. (ii) Refer to point 11, Unit I. Magnitude of upper standard point is 100 C or 212 F. 13. Name two thermometric scales, commonly used for laboratory thermometers. State the relation for converting temperature from one scale to another scale. Ans. (i) Celsius scale or centigrade scale (C) (ii) Fahrenheit scale (F). C F 32 C F 32 Relation : = or = (a) Draw a neat labelled diagram for clinical thermometer. (b) Answer the following questions regarding clinical thermometer. (i) Why is its stem thin and short? (ii) Why is its stem made triangular? (iii) Why is the scale on it marked from 95 F to 110 F? (iv) Why is its capillary tube provided with constriction? (v) Why is its bulb made fairly large? (vi) Why this thermometer is washed with water and jerked before use? (vii) What does it indicate when temperature recorded by it is (a) below 98.4 F (b) above 98.4 F. Ans. (a) Diagram is given below. 4

5 (b) (i) The stem is made short as it is a specialised thermometer which records temperature between 95 F to 110 F or 35 C to 43 C. The stem is made thin so that it can easily fit in mouth cavity. (ii) Total internal reflection takes place in triangular stem, Thus, a thin thread of mercury in the capillary tube appears as a broad band and hence the temperature can be easily read. (iii) It is because temperature of human body does not fall below 95 F or rise above 110 F as in either case death occurs. Thus, in order to keep thermometer sensitive, its stem is marked between 95 F to 110 F. (iv) The constriction in capillary tube does not allow the mercury to flow back into bulb, when thermometer is taken out from the mouth of patient. Thus, it helps in recording accurate temperature of the patient. (v) Larger the bulb, more is the amount of mercury in it and hence more is the expansion of mercury for 1 C rise in temperature. As the expansion of mercury increases per degree rise in temperature, therefore divisions on the stem can be marked far apart. This helps in making thermometer sensitive. (vi) It is washed with water to remove any germs or bacteria sticking to it. It is jerked so that mercury flows back to bulb and thermometer is ready for use. (vii)if the temperature recorded is below 98.4 F then person is considered to be suffering from some kind of weakness. If the temperature is above 98.4 F then person is suffering from fever. 5

6 15. State two ways to make a thermometer (a) Quick in action (b) Sensitive. Ans. (a) To make thermometer quick in action. 1. The bulb should be made from very thin glass. 2. The size of bulb should be decreased. (b) To make thermometer sensitive. 1. The bore of capillary tube should be made as fine as possible. 2. The bulb should be made of large volume. 16. Criticize the action of a nurse who sterilised clinical thermometer in boiling water. Ans. Nurse did not realise that the maximum temperature a clinical thermoteter can record is 110 F, whereas temperature of boiling water is 212 F. Thus, when clinical thermometer is placed in boiling water, it is likely to break, because there is no space for the expanding mercury. Numerical Problems 17. Express 350 C into Fahrenheit scale. Ans. F = 9 5 C = = 662 F. 18. Express 250 C into Fahrenheit scale. Ans. F 32 C = 9 5 F = 9 5 C + 32 = 9 ( 250) + 32 = = 418 F. 5 6

7 19. Express 112 F into Celsius scale Ans. Temperature in Fahrenheit scale = 112 F. C F 32 = 5 9 C = 5 9 (F 32) = 5 9 (112 32) = 5 9 = C. 20. Express 58 F into Celsius scale. Ans. Temperature in Fahrenheit scale = 58 F Temp. in Celsius, C = 5 9 (F 32) = 5 9 ( 58 32) = = 50 C. 21. The temperature of human body rises by 1 C. Calculate the corresponding rise in temperature on Fahrenheit scale. Ans. Degrees between standard points on Celsius scale = (100 0) = 100 Degrees between standard points on Fahrenheit scale = =180. Now, 100 C = 180 F 1 C = 180 = 1.8 F The average temperature of human body is 98.6 F, Calculate the equivalent temperature in Celsius scale. Ans. Temperature in Celsius scale C = 5 (F 32) 9 5 = ( ) = 66.6 = 37 C. 9 7

8 23. The mercury thread rises by 3/5 parts between the standard points on Celsius scale when the bulb of thermometer is placed in hot water. Calculate the temperature (a) in Celsius scale (b) Fahrenheit scale. Ans. (a) Number of divisions between standard points on Celsius scale = of Celsius scale = C = 60 C. 5 (b) Temp. on Fahrenheit scale, F = 9 5 C + 32 = = = 140 F Mercury thread falls by 9/16 parts between two standard points on Celsius scale when boiling water at 100 C is allowed to cool. Calculate the temperature in (a) Celsius scale (b) Fahrenheit scale. Ans. (a) Number of divisions between standard points on Celsius scale = of Celsius scale = 9 = C 16 Fall in temperature = C Temperature on Celsius scale = = C. (b) Temperature on Fahrenheit scale, F = 9 5 C + 32 = = = F. 8

9 25. The mercury thread falls by 7/8 parts between two standard points on Fahrenheit scale when pure boiling water at 76 cm pressure is allowed to cool to room temperature. Calculate the room temperature in (a) Fahrenheit scale (b) Celsius scale. Ans. (a) Number of divisions between standard points on Fahrenheit scale = 180 of Fahrenheit scale = = F. 8 Fall in temperature = F Temperature on Fahrenheit scale = ( ) F = 54.5 F. Temperature on Celsius scale, C = 5 9 (F 32) = ( ) = = 12.5 C. 26. The mercury thread rises by 11/16 parts between two standard points on Fahrenheit scale when placed in hot milk. What is the correct temperature in (a) Fahrenheit scale (b) Celsius scale. Ans. (a) Number of divisions between standard points on Fahrenheit scale = of Fahrenheit scale = = F Rise in temperature = F. Temperature on Fahrenheit scale = ( ) = F. (b) Temperature on Celsius scale, C = 5 9 ( F 32) = 5 9 = ( ) = = C. 9 9

10 27. At what temperature, numerical value of Celsius and Fahrenheit thermometers is same? Ans. Let the common value be x. Now, = C 5 = F 32 9 x 5 = x x = 5x x = 160 x = 40 Thus, 40 C and 40 F are common values on both scales. 28. When will Celsius thermometer read twice as much as Fahrenheit thermometer? Ans. Let the temperature on Fahrenheit scale = x The temperature on Celsius scale = 2x C Now, 5 = F x 5 = x x = 5x x = 160. x = = F. Fahrenheit temp. = F Celsius temp. = 2 ( 12.30) = C. 29 What do you understand by the term linear expansion of solids? Ans. If the surface area of a material is too small, such that it can be neglected, then increase in length of material on heating is called linear expansion. 10

11 30. Describe an experiment to prove that solids expand on heating. Ans. Take Gravesand s ring and ball apparatus. Check that ball passes through ring at room temperature. Heat the ball strongly on a flame for five minutes and try to pass the ball through ring. It is observed that ball does not pass through ring. Thus, experiment proves that solids expand on heating. 31. Name three factors which determine the thermal (linear) expansion of solids Ans. Factors on which linear expansion depends : (i) Linear expansion in a solid is directly proportional to its initial length. (ii) Linear expansion in a solid is directly proportional to rise in temperature. (iii) Linear expansion in a solid depends upon nature of material. 31. What do you understand by the following terms? (i) Superficial expansion (ii) Cubical expansion : Ans. (i) Superficial expansion, If the material is in the form of a plate, such that its thickness is too small as compared to surface area and hence, can be neglected then increase in surface area on heating is called superficial expansion. (ii) Cubical expansion : Increase in volume of a material on heating is called cubical expansion. 33 State two examples in which thermal expansion of solids is put to good use. Ans. (i) During rivetting of metal plates the expansion of solids is put to good use. (ii) In mounting iron rims over wooden wheels expansion of solids is put to good use. 11

12 34. Give two examples where expansion of solids is hindrance. Ans. (i) The expansion of solids is a hindrance in laying electric transmission wires as they sag in summer and snap in winter due to expansion and contraction. (ii) The expansion of steel rails during summer is a hindrance as it tends to bend the rails. 35. (a) What is a bimetallic strip? (b) Name two devices which use bimetallic strip? Ans. (a) The thin plates of two different metals, which are riveted together at number of points in such a way that they cannot slids on being heated or cooled is called bimetallic strip. (b) (i) Thermostat (ii) Fire alarm. 36. How bimetallic strip is used in the construction of thermostat? Illustrate your answer by diagram. Normally, the contact points are in contact with each other and hence, current flows through heating coils. When the temperature rises to some particular value, the bimetallic strip bends downward. This breaks the electrical circuit and hence, temperature stops rising. When the temperature. falls, the contact points rejoin and again current starts flowing. Thus, a constant temperature is maintained. 12

13 37. Diagram alongside shows a steel washer. Explain how the following are affected when washer is heated to 100 C. (i) Internal diameter (ii) External diameter (iii) Mass. (iv) Density (v) Volume. Ans. (i) Internal diameter of washer increases. (ii) External diameter of washer increases. (iii) Mass of washer remains same. (iv) Density of washer decreases. (v) Volume of washer increases. 38. A compound bar of brass and invar is shown in diagram. Copy the diagram and on it show the changes which occur, when bar is heated. Indicate by completing the diagram, how the bar could be used to operate a fire alarm system at a particular temperature. Ans. When the temperature rises to some pre-determined value, the bimetallic strip bends and makes contact with the contact screw. This in turn completes electrical circuit and hence bell starts ringing. 13

14 39 The diagram shows a bimetallic strip as used in a simple fire alarm. (a) Name suitable materials for A and B. (b) What adjustments would you make to the arrangement shown, to make the alarm operate at lower temperature? Give reasons for your answer. Ans. (a) A is invar and B is brass. (b) The contact points are brought closer to one another. This makes the bimetallic strip to make contact at lower temperature and hence, bell starts ringing. 40 Diagram alongside shows a bimetallic strip, a battery B and two bulbs L 1 and L 2. Copy the diagram and complete the electric circuit such that: (a) L 1 glows when temperature falls 20 C, below the room temperature. (b) L 2 glows when temperature rises 50 C, above the room temperature. 14

15 41. Explain the following : (a) Why are tonga drivers seen to pour water over wooden wheels in summer? (b) Why are telegraph wires not fixed very tightly in winter? (c) Why are small gaps left between rails of a railway track? (d) Why does a thick glass tumbler crack when boiling water is poured in it? (e) Why are the griders supporting bridges mounted on roller? (f) Why does a platinum wire fuse more easily in glass than copper wire? (g) Why is pitch filled in between the cement blocks of pavement? (h) Why are rivets heated to red hot temperature during riveting? (i) Why does borosil glass not cracks on heating directly on a flame? (j) Why does a hot glass chimney cracks when drop of water falls on it? (k) Why do we put hot water on the jammed metal cap of an inkpot? Ans. (a) The iron rims mounted on wooden wheel expand in summer due to heat. There is a possibility that iron rim may expand to such an extent that they may slip from wooden wheel. To prevent such a slippage cold water is poured over the wheels which does not allow the rims to expand. (b) This is to allow for contraction of wires in winter. If we do not keep the wires some what loose, then in winter they will contract to such an extent that they may snap. (c) This is to allow for expansion of rails in summer as otherwise when the rails expand, they will bend. This in turn will cause derailment. 15

16 (d) The glass is a bad conductor of heat. Thus, when boiling water is poured in glass tumbler, the inner surface suddenly expands, but not the outer surface. This uneven expansion builds a lot of strain within the glass and hence breaks it. (e) This is to allow for expansion of girders in summer. When the girders increase in length they move forward on roller, without exerting any strain on supporting pillars. (f) The cubical expansion of platinum and glass is almost same. Thus, when platinum wire is fused in glass, it does not loosen. However, the cubical expansion of copper is very large. Thus, when copper wire is fused in glass, on cooling it contracts far more than glass. Hence it loosens out. (g) The cement blocks like any other solid expand on heating. If no space is left in between the blocks, the force of expanding block will produce cracks in them. Thus, to avoid such a situation, gaps are left between blocks and then filled with pitch. The pitch being soft gets squeezed out when blocks expand. (h) On heating the rivets expand. When the ends of rivets are hammered after placing them in a hole in metal plate, their length decreases. On cooling, they further decrease in length and hence hold the metal plates very tightly. (i) Borosil is a special kind of glass, whose cubical expansion is very very small. Thus, when heated there is not much difference in the expansion of external and internal surface. Hence, no strain builds up in the vessel of borosil. Thus, it does not crack on directly heating on flame. (j) The point where drop of water falls, suddenly contracts. However, rest of the chimney stays in expanded position. This results in uneven expansion, which develops a strain within the glass and hence it cracks. 16

17 (k) The cubical expansion of metal cap is very large as compared to glass. Thus, on pouring boiling hot water, the cap expands far more than glass. Thus, cap loosens and hence can be easily removed. 42. How will you prove experimentally that (i) liquids expand on heating? (ii) liquids expand more than solids for the same rise in temperature? Ans. Take a test tube completely filled with coloured water, such that in its mouth is fixed a stopper containing a long glass tubing. Let the level of liquid in glass tubing be at A. Place the test tube in a beaker, containing boiling hot water. It is observed that first the level of water falls to B and then rises up to C. The fall in level of liquid can be attributed to the fact that initially the glass tube expands, but not the water. However, the level of liquid rises to C, when liquid starts expanding. Thus, experiment proves: (1) Liquids expand on heating. (2) Liquids expand more than solids for the same rise in temperature. 43. What do you understand by the term anomalous expansion of water? Ans. It means that water does not behave like normal liquids when heated or cooled. It has been found that water has minimum volume at 4 C. If the water is heated or cooled from 4 C, it always expands. 17

18 44. Draw a graph between volume and temperature when 5 cm3 of ice at 10 C is heated to form water at 10 C. Ans. 45 How do fishes survive in frozen lakes? Ans. Only the top surface of lake freezes due to anomalous expansion of water. Water below the surface of ice stays between 0 C to 4 C. Thus, fishes can easily survive in water, below the surface of ice. 46. A deep pond of water has its top layer frozen during winter. What will be the temperature of water layer (i) just in contact with ice? (ii) At the bottom of pond? Ans. (i) The temperature of water just in contact with ice is 0 C. (ii) The temperature of water at the bottom of pond is 4 C. 47. Adjacent figure shows a hard glass test tube containing coloured water, such that level of water is up to point A. The test tube is placed in a large beaker containing boiling hot water. It is observed that level of coloured water first drops to B and then rises up to C. Answer the following questions: (a) Why is there a drop in the level of water? 18

19 (b) Why does the level of water start rising after some time? (c) State two important deductions which can be made regarding the action of heat on liquids from the above observations. (d) If the test tube is placed in ice cold water, instead of boiling hot water, state your observations with reasons. Ans. (a) The test tube expands first, but not the coloured water. Due to the increase in volume of test tube, the level of water drops from A to B. (b) It is because liquid after gaining heat expands and hence, level of liquid rises to C. (c) (i) Liquids expand on heating. (ii) Liquids expand more than solids (glass). (d) Initially, the level of liquid rises in glass tubing and then falls down. It is because initially the test tube contracts but not coloured water. Thus, level of water rises up. However, when coloured liquid contracts the level falls down. 48. The adjacent figure shows Hope s apparatus, used to study anomalous expansion of water. Answer the following questions: (a) Amongst A and B, which thermometer will register fall in temperature first and why? (b) Which thermometer will stop registering fall in temperature after some time? State the magnitude of lowest temperature recorded by this thermometer. (c) Give an explanation of your answer in (b). (d) Why does the second thermometer goes on recording fall in temperature? (e) What conclusions can be drawn about the nature of water from the above experiment? (f) State one natural consequence of the above property of water. 19

20 Ans. (a) Thermometer B. It is because, as the water in the region of freezing mixture, falls in temperatrue, its volume decreases, its density increases and hence it sinks down. (b) Thermometer B. It is because, when water attains a temperature of 4 C, its density is maximum. Thus, it just settles down and cannot rise up. Thus, the temperature of thermometer B stops registering any fall in temperature. (c) Refer to answer in 7(b). (d) It is because, when the tem-perature falls below 4 C, the water starts expanding and hence moves upward. Thus, ther-mometer A continues registering fall in temperature. (e) (i) Water has maximum density at 4 C. (ii) Water has minimum volume at 4 C. (iii) Water has anomalous expansion i.e., it expands, if heated or cooled from 4 C. (f) When the water bodies (such as lakes; rivers; seas), freeze in cold countries in winter, only the upper surface freezes to form ice. Water below ice stays at 4 C and hence, water animals can survive. 49. Draw a diagram showing the temperature of various layers of water in an ice covered pond. Ans. 50. Why are soft drink bottles not completely filled? Ans. All soft drinks contain water. When soft drink bottles are chilled in sub-zero temperatures, the water on account of its anomalous expansion expands. Thus, to provide space for expanding water, soft drink bottles are not completely filled as otherwise they will burst. 20

21 51. Why do water pipes burst during severe frost? Ans. When the temperature falls below zero degree Celsius, the water in the pipes expands dueto anomalous expansion. However, if there is no space for expansion, then it exerts tremendous pressure on the pipes and bursts them. 52 Why does a glass bottle filled completely with water and tightly corked, break in a freezer chamber of fridge? Ans. When the temperature of water within the glass bottle falls below 4 C, it starts expanding due to anomalous expansion. Since, there is no space for expansion, it exerts tremendous pressure and bursts open the bottle. 53. Why are exposed water pipes lagged with straw in hilly regions during winter? Ans. When the pipes are lagged, the temperature of water within the pipes does not fall below 4 C. Thus, due to anomalous expansion, water will not exert force on pipes and hence, escape bursting. 54. Why are the taps left dripping in sub-zero temperatures during winter? Ans. The dripping taps will always create some space for expansion. Thus, when water in the pipes freeze in sub-zero temperatures, due to its anomalous expansion, it expands. This expanding water always finds space for expansion as tap is dripping. Thus, water pipes do not burst. 55. Why do vegetables and fruits get damaged during severe winter? Ans. When the water in the cell sap freezes, it expands due to anomalous expansion. This expanding water breaks the cell walls, which in turn damages fruits. 56. A partly filled balloon is placed close to an electric heater. After some time, it is noticed that balloon gets fully inflated. Explain. Ans. It is because, gases expand on heating. The increase in volume of gas in turn inflates the balloon. 21