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# Ch09 o com_pwb

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'O' Level Workbook answers for chapter 9

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### Ch09 o com_pwb

1. 1. 9 CHAPTER Transfer of Thermal Energy This chapter is organised into the sections as follows: A. Conduction, Convection and Radiation B. Challenging Questions SECTION A Conduction, Convection and Radiation 1 A cup of hot tea is left in the same room as a bowl of ice-cream. What is likely to happen to both of them after some time, if the two objects are both placed on the dining table? A The tea becomes as cold as the ice-cream. B The ice-cream becomes warmer than the tea. C The tea cools to room temperature, the ice-cream melts and warms to room temperature. D The tea becomes colder than before, but not as cold as the air in the room. [ C ] 2 When a piece of plastic is warmed on one end, the heated plastic molecules transfer thermal energy by A streaming from the heated end to the colder end. B vibrating more vigorously and knocking into their neighbours. C radiating infra-red rays to the colder end. D moving to the less dense region. [ B ] 3 Min Min steps on the carpeted ﬂoor and notices that it feels warmer than the marble ﬂoor. This is because A the carpet is darker in colour than the marble ﬂoor. B there are layers of trapped air between the ﬁbres of the carpet. C she has a very low metabolic rate. D the marble ﬂoor is white and shiny so it is a bad absorber. [ B ] 4 The diagram shows a metal pot containing some water which is placed on a hot plate. The metal base is heated up quickly mainly by A movement of electrons. B movement of solid molecules. C molecular vibration. D emission of infra-red radiation. [ A ] 59
2. 2. 5 (a) Describe, in terms of particles and electrons, how energy is transferred in the process of conduction 9 in (i) wood; [2] Transfer of Thermal Energy When heated, the wood particles nearer to the heat source increase their energy of vibrations, and pass this excess energy to the neighbouring molecules by increased vibration. (ii) copper wire. [2] When heated, the free electrons nearer to the heat source speed up and increase their collisions with the particles in the copper wire. The increased particle vibration is passed along the wire. This makes the copper particles further away vibrate more and the wire becomes hotter. (b) Explain why: (i) a cooking pot may have a copper base and a plastic handle; [1] Copper material is a good conductor while plastic material is a poor conductor of heat. (ii) wearing a few layers of T-shirt can keep you warm during cold nights. [1] The layers of T-shirt help trap air, which is a poor conductor. 6 In an investigation to compare the thermal conductivity of four metals, Jocelyn coats the four different metal rods with a thin layer of wax, and attaches each rod to a container of boiling water. boiling water rod coated with a thin layer of wax when cold steel iron melted wax aluminium solid wax copper (a) (i) What can you deduce from Jocelyn’s investigation? [1] Copper conducts heat the fastest. 60
3. 3. (ii) Explain your answer. [2] The four metal rods are heated up equally, but the wax on the copper rod melts more 9 Transfer of Thermal Energy within a shorter time compared to the remaining three metal rods. So more energy must have been conducted by the copper rod to the wax at a faster rate. (b) State what Jocelyn should do to ensure a fair test in her investigation. [2] 1. She should coat each metal with same thickness of wax. 2. The metal rods used should be of equal size and length. 7 An unlit match stick is placed on top of a lighted candle. The match bursts into ﬂame because it is heated up mainly by A convection. B convection and radiation. C conduction and convection. D conduction, convection and radiation. [ A ] 8 A beaker of water is heated up by convection. Why does the heated water rise? A The water molecules expand and become less dense. B The water contracts and its density increases. C The water expands and its density decreases. D The water expands and its density increases. [ C ] 9 The diagram below shows two vacuum ﬂasks containing cold tea and hot tea respectively. hot air rises cold air sinks cold tea hot tea (a) In the case of the hot tea, draw the likely path of a convection current in the air in the container. [1] (b) Explain why it is essential to place a stopper over the hot tea to keep it warm, but not necessary to do so for the cold tea to keep it cold. [2] Hot air heated by the hot tea will rise and escape into the air, to be replaced by cold air which sinks, cooling the hot tea by convection. However, cold air above the cold tea has no tendency to rise due to its higher density than the warmer air above. Hence cannot set up convection current. Cold tea is not signiﬁcantly warmed. 61
4. 4. 10 Julius switches on his table lamp and settles down to study for his Physics test. While he is at it, he 9 notices that the paper on the pin-board above the table lamp ﬂips away from the board, and remains at this position until he switches off the table lamp. Transfer of Thermal Energy (a) What is the cause of this strange behaviour of the paper on the pin-board? [2] The table lamp heats up the air immediately above the lamp, causing the heated region above it to expand. The heated air becomes less dense and rises. This upward movement of hot air ﬂips the paper away from the pin-board. (b) (i) Comment on the difference in observation if Julius’s table lamp has a silver, shiny shade. [1] It will take a slightly longer time for the paper to ﬂip away from the pin board. (ii) Explain your comment. [2] Shiny, silver surface is a poor radiator Hence it takes a longer time to heat up the layer of air above it by radiation. 11 (a) Describe, in terms of ﬂuid movement, how a beaker of water is heated up by convection. [4] – When heated, water molecules will gain energy and move more vigorously and further apart. – Water expands, causing a drop in its density. – Heated water rises, to be replaced by colder water on the higher layer. – The movement of water of due to density difference of the heated and cold water causes convection current. 62
5. 5. (b) Explain why: (i) the heating element of the electric kettle is placed at the bottom; [2] 9 Transfer of Thermal Energy The water heated by the heating element will expand and decrease in density. The cooler air will cause the heated water to rise and be replaced by the colder water above, resulting in convection current hence the water is heated up quickly. If placed at the top, the top layer of hot water will not be able to move down by convection, and since water is a poor conductor of heat, water cannot be efﬁciently heated up. (ii) the freezer compartment in a refrigerator is placed on top. [2] Cooler air from the freezer compartment is denser. The cooler air will tend to sink to the lower shelf and hence cools the food below the freezer compartment more efﬁciently than if the freezer compartment is to be placed at the bottom. 12 A block of heated metal cools down by emitting . A ultraviolet rays B infra-red radiation C heat waves D heat particles [ B ] 13 In a vacuum ﬂask, which of the following energy transfers cannot be prevented? A Conduction and convection B Convection and radiation C Radiation D Convection [ C ] 63
6. 6. 14 A vacuum ﬂask has two shiny glass walls. The purpose of this feature is to reduce energy loss by 9 Transfer of Thermal Energy A convection and conduction. B conduction. C radiation and convection. D radiation. [ D ] 15 As Mrs Lee’s stove has broken down, she is considering on heating up a small amount of water by ﬁlling a plastic bag with tap water and hanging it in a sunny place. What is the colour of the plastic bag she should use in order to heat up the bag of water in the shortest possible time? You may assume that Mrs Lee’s coloured plastic bags are made of the same material. A clear, colourless B black C blue D any colour will produce the same heating effect [ B ] 16 The diagram illustrates the structure of a solar panel used to heat water. water pipes in solar panel roof solar panel hot water out cold water in Which of the following materials would be the most suitable for the pipes in the solar panel? A Black plastic B White plastic C Black painted copper D Polished steel [ C ] 64
7. 7. 17 Kim Seng sets up the apparatus as shown in the diagram below to investigate thermal energy transfer. 9 Transfer of Thermal Energy dull polished black surface wooden cork surface a thin layer of wax a thin layer of wax (a) Kim Seng would like to gather reliable data on thermal energy transfer through this investigation. [1] State one action he can take to ensure that the experiment is fair. Possible answers: 1 He can arrange the two surfaces to be of equal distance from the Bunsen ﬂame. 2 He should use identical material for the two surfaces, although one is to be painted black. 3 The layer of wax use should be of equal thickness. 4 The wooden cork used should be identical. 5 The surface should have the same area. (b) State and explain Kim Seng’s observation when he lights the Bunsen ﬂame. [2] The wooden cork attached to the dull, black surface will drop off ﬁrst. The wooden cork on the left gets heated up faster by radiation, as dull black surface is a better absorber of thermal energy. SECTION B Challenging Questions 1 The diagram shows a hot-water storage tank that Mr Tan has built. metal tank electric heater hot water out cold water in 65
8. 8. (a) Using his Physics knowledge acquired while he was a young man, he gave the exterior of the 9 storage tank a shiny, silver coat of paint. State and explain whether Mr Tan has applied his Physics knowledge correctly here. [2] Transfer of Thermal Energy Mr Tan has applied his understanding of radiation correctly. As silver, shiny surface is a bad radiator, the thermal energy in the storage tank will take a longer time to be transferred outwards. Hence water in the tank keeps warm longer. (The effect would have been better if he had painted the interior part silver as well.) (b) Mr Tan realises that to conserve the energy in the storage tank, he also needs to insulate the tank. (i) Suggest one material he can use as insulator. [1] Use ﬁbre ﬁll or styrofoam (ii) Explain your choice. [1] Such materials are bad conductors. They reduce energy loss by conduction through layers of air trapped between the materials. (c) However, Mr Tan makes a mistake in his design of the hot-water storage tank. (i) Identify his mistake. Explain the consequence of this mistake. [2] He places the heating element too high up in the storage tank. The water in the tank is mainly heated up by conduction when heater is placed at this position. Since water is a bad conductor, it will take a very long time to heat up the whole tank. (ii) How can his mistake be corrected? [1] He should lower the heating element to the bottom of the tank. (iii) Explain your answer to (c)(i). [3] With the heating element positioned at the bottom of the tank, the water at the bottom is heated up, expands and becomes less dense. The less dense water will rise, and the denser, colder water will sink. In this way, convection current can occur, heating the whole water tank quickly. 66