Download - Chapter 9 Transfer of Thermal Energy - · PDF fileChapter 9 Transfer of Thermal Energy 2. Discuss with students uses of good and poor conductors. 3. We should be glad that air is a

Chapter 9 Transfer of Thermal Energy


Learning Outcomes:

After completing this chapter, students should be able to:

1. show understanding that thermal energy is transferred from a region of higher temperature to a region of lower temperature

2. describe, in molecular terms, how energy transfer occurs in solids 3. describe, in terms of density changes, convection in fluids 4. explain that energy transfer of a body by radiation does not require a material medium and the rate of

energy transfer is affected by: (i) colour and texture of the surface (ii) surface temperature (iii) surface area

5. apply the concept of thermal energy transfer to everyday applications

Introduction page 139 L Take two beakers and insulate them with tissue paper. Fill one of them with cold water and the other

with hot water. Dip a thermometer into the cold water and let the students see the mercury level or temperature. Remove the thermometer and place it into the hot water. Ask students to observe the mercury level and then answer the following questions.

(a) Compare the temperature of the thermometer with that of the hot water just before the thermometer touches the hot water.

Answer: The temperature of the thermometer is lower than that of the hot water.

(b) What changes do you see from the instant the thermometer is placed in the hot water?

Answer: The mercury level rises and then stops after a short while.

(c) Give possible reasons for the changes observed in question (b).

Answer: The mercury level rises due to expansion in volume of mercury after receiving thermal energy from the hot water. The mercury level then stops rising because there is no net transfer of thermal energy to the mercury.

(d) What inferences can you make from the observations?

Answers: Thermal energy is transferred from a body of higher temperature to that of lower temperature. There is no net transfer of thermal energy when the two bodies in contact are of the same temperature. Or Heat flows from a body of higher temperature to that of lower temperature. There is no heat flow when the two bodies in contact are of the same temperature.

9.1 Conduction page 139 I. Describe how to distinguish between good and bad conductors of thermal energy.

For example (a) Stir hot tea with plastic, wooden, ceramic, aluminium, stainless steel and silver spoons. (b) Poke a fire with iron, brass and copper rods.

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2. Discuss with students uses of good and poor conductors.

3. We should be glad that air is a poor conductor of heat. If air is a good conductor of heat, we would feel quite chilly on a cold night (or very warm on a hot day), even inside the house.

4. The poor conductivity of air does not itself explain insulation, as most buildings are surrounded by many metres of air. Most thermal energy is transferred to or from buildings and rooms by convection in the air. The principle of most insulation mechanisms is to suppress convection. This is done by trapping the air.

5. Use a model of a solid structure (balls joined by springs) to describe in molecular terms how thermal energy transfer occurs in solids. Show that shaking one end leads to vibrations at the other.

6. In a metal, a vibrating atom can propel electrons a very long distance at high speed. They collide with other atoms far off and set them vibrating. This is a much faster way of transferring thermal energy by molecular vibration.

7. Analogy of conduction: Imagine we want to send a ball from point A to point B. One way of doing is to get the students line up from point A to point B and the ball is passed from one student to the next student until it reaches B.

8. Experiment to show water is a poor conductor of thermal energy Prepare a test tube full of cold water with chunks of ice at the bottom. Hold the bottom end of the test tube in your hand. Heat the top part in a flame until it boils. The fact that you can still hold the bottom of the test tube and the water above can be brought to a boil without melting the ice shows that glass and water are poor conductors of thermal energy.

Answers to Section Review questions page 143

1. For all solids (metals and non-metals), conduction takes place when thermal energy is transferred from one particle (atom or molecules) to the next. When one end of a rod is heated, the molecules there gain energy and vibrate faster. These molecules collide with their less energetic neighbours. Some of their energy is transferred to these neighbouring molecules which in turn gain kinetic energy. In this way, thermal energy is passed along the rod by the vibrating molecules. There is no net movement of molecules during the process of conduction.

2. In metal, other than the conduction through vibrating molecules, the free electrons in a metal are also responsible for the conduction of thermal energy. When heated, the free electrons gain energy and move faster. These electrons are free to travel in the metal through spaces between the atoms for some distance before colliding with atoms and transferring some of their energy to them.

3. Pans, kettles and other cooking utensils which have to be heated directly are made of good conductors such as aluminium, copper and steel. Refer to text for other examples.

4. Kettles, saucepans and electric irons usually have plastic or wooden handles because plastic and wood are poor conductors. Refer to text for other examples.

9.2 Convection page 144 1. Use a Bunsen burner to heat a beaker of water at one side, on the bottom. The convection current can

be seen using a few tiny crystals of potassium permanganate at the bottom.

2. There is a clear series of events which take place in convection: hot water expands -7 its density decreases -7 it rises -7 it pushes away the liquid above it and sucks in the liquid next to it -7 a circulation is set up.



3. illustrate convection with specific examples: wind, convection rain, ice-box in a refrigerator cools the area below it, air-conditioners positioned high for effective cooling, and heaters positioned low for effective heating.

4. Compare the temperature of an inland area with one that is near the sea. Both of them are at the same latitude. [Answer: The temperature of the inland area is controlled by the amount of solar energy received and hence can be extreme in temperatures during winter and summer. The coastal area, on the other hand, experiences land and sea breezes, which keep it warmer in winter and cooler in summer.]

5. Analogy of convection: Imagine we want to send a ball from point A to point B. One way of doing is to get the student carrying the ball at point A to move to point B.

Answer to Think Time question page 146

Water can still be heated up relatively quickly because of convection currents set up.


1. Convection is the process by which thermal energy is transmitted from one place to another by the movement of heated particles of a gas or liquid.

2. The operation of a hot water system for a house, cooling system in a car - the radiator, refrigerator, sea breeze and land breeze.

3. This is the most efficient arrangement to heat up the water by convection, in which the heated water rises and the cold water sinks to the bottom to be heated by the heating coil.

4. The denser cold air remains at the lower level, and the warm air at the higher level remains warm. Hence, the room is not completely cooled.

5. Refer to 'Land and Sea Breezes' on page 146 and page 147.

9.3 Radiation page 148 1. Hot objects emit infra red radiation. The hotter the object, the more radiation it emits. At thermal

equilibrium, an object absorbs just as much as it emits.

2. Examples of radiation include: energy from the Sun to the Earth, electric fues, thermal imaging.

3. Be careful to distinguish between absorption and emission. When the body ' s temperature is higher than the surrounding, there is a net emission. When the body's temperature is lower than the surrounding, there is a net absorption.

4. Describe how to distinguish between good and bad emitters and good and bad absorbers of infra-red radiation. This will depend on the colour and texture of the surface.

5. Ask the students what is the difference if they were to wear a black T-shirt and a white T-shirt in direct sunlight. [Answer: A black coloured object is a good absorber of infra red radiation, thus it will get heated up very fast.]

6. Analogy of radiation: Imagine we want to send a bail from point A to point B. One way of doing is to get throw the ball at point A to point B.

7. There was the belief that once a vacuum flask is used to store cold water, it cannot be used to keep hot water warm anymore. Ask the students to check the truth of this belief. [This is not true.]

Answer to Think Time question page 150

At room temperature, a good emitter of radiation will be a good absorber of radiation. Since all the radiations (including light) are absorbed, the object wiil appear black.

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1. Radiation is the transfer of thermal energy by infra-red waves . Heat radiates from a hot object in the form of infra-red waves .

2. At the speed of light, i.e. 3.0 x 108 m S- I.

3. There is a vacuum between the Earth and the Sun, yet the thermal energy can be transferred by radiation from the Sun to the Earth.

4. The cooling fins at the back of a refrigerator and the heat sink of electronic equipments are painted dull black so that they emit radiation more quickly.

5. Shiny metal teapot is a poor emitter, this helps to keep the tea hot for a longer period. Shiny aluminium foil helps keep food dishes warm before serving.

6. Solar heating panels are deliberately painted dull black in order to absorb as much radiation from the sun as possible.

7. Houses in hot countries are often painted white to keep them cooler. Windows have coatings that prevent some heat from reaching a building's interior, which reduces air-conditioning use, but they also trap heat inside during cold periods. Factory roofs are sometimes coated with aluminium paint. This reduces absorption of radiation during the day.

Physics in Society: Secrets of the Sun page 153 Answers to Q

1. One sees the destructive effects of the atomic bomb while the other sees the potential deterrent of the bomb to further conflicts between nations.

2. Capture the solar energy with effective solar cells.

3. Hydrogen is the lightest element in the whole universe. It is possible to compact hydrogen to a density many times greater than that of lead without the formation of heavier nuclei.

4. The circulation of convection current from the convection zone to the photosphere.

Physics and the Environment: The Greenhouse Effect . page 154 Answers to Q

1. To reduce the greenhouse effect and reduce global warming of the Earth.

2. Reduce consumption of fuel like petrol, gasoline and fuel. Limit de-forestation and protect the natural environment.

Answers to Misconception Analysis page 156

1. True 2. False. 3. True 4. True 5. True 6. False. 7. True 8. True 9. False. 10. False.

There is still conduction but liquids and gases are not good conductor of thermal energy.

Radiation is the transfer of thermal energy by infra-red radiation.

A silvery surface is a poor emitter and good reflector of heat. Though the aluminium is a good reflector of heat, in this case food is kept warm because aluminium is a poor emitter of heat.

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Answers to Multiple Choice Questions . page 156, 157

l.D 2. B 3. C 4.B 5.C 6. B 7. A 8.A 9.C 10. C

Answers to Structured Questions page 157, 158

1. (a) Conduction through the base of the saucepan, convection of hot air and radiation (b) Convection of water

2. (a) Air is a very poor conductor. The tiny pockets of air trapped among the loosely packed leaves reduce the thermal energy loss from the food .

(b) If the leaves are tightly packed, there will be fewer air pockets. The material of leaves is a better conductor than air, thus more thermal energy will be lost from the food.

3. (a) Expanded polystyrene is a bad conductor of thermal energy. When it is inserted in the cavity walls of buildings, it reduces transfer of thermal energy.

(b) Wood is a bad conductor of thermal energy and so the wooden handle will not be too hot. (c) This reduces the heat lost through radiation by reflecting the heat back into the flask.

4. Yes, because the surface of polished chrome is a bad radiator, so less energy will be lost to the surroundings. When the saucepan is covered, energy lost by convection above the water surface is much reduced.

5. (a) In a refrigerator, energy is extracted from the ice box placed at the top of the compartment. The air near the ice box gets cold. The denser cold air sinks to the bottom and causes warmer air to rise to the top to be cooled. The location of the ice box makes the transfer of thermal energy by convection very efficient.

(b) The cooling fins are painted black so that they emit radiation more quickly.

6. (a) (i) Radiation (ii) Conduction

(b) Copper is used because it is a good conductor of thermal energy. The pipes are painted black so that they are good absorber of radiation.

7. (a) The metal base is a good conductor. This will conduct thermal energy to heat up the food quickly. The plastic handle is a poor conductor, so it will not get heated up.

(b) Air-conditioner is placed near the ceiling so that cold air with higher density sinks and warm air with lower density rises. Thermal energy is transferred by convection and the room can be cooled effectively.

(c) Water is a better conductor of heat than air. A wet cloth will conducts thermal energy to the hands more quickly.

(d) At night, the surrounding temperature is lower than the temperature of the person. Energy is emitted by radiation. Black colour object is a better emitter of radiation.

Answers to Critical Thinking Questions page 158

1. A stone floor feels colder than the mat, even though both materials are at the same temperature. This is because stone is a better conductor of heat than mat, and so thermal energy is more readily conducted out of the foot touching the stone.

2. The wire gauze is used to provide even heating to the base of any beaker placed on it as metal is a very good conductor.

3. Trapped air can only transfer thermal energy by conduction but air surrounding a hot cup of coffee is able to transfer thermal energy by convection.

4. The air trapped in the material of the blanket is a poor conductor of thermal energy and thus reduces energy loss to the surrounding.

5. One feels warmer wearing a darker coloured shirt due to its higher absorption power of radiant heat.

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However, if you stay out in the sun long enough, it does not matter whether you are wearing a dark coloured shirt or a light coloured shirt.

6. There is no particle to carry the energy.

7. When the fingers are placed beside a candle flame, the transfer of thermal energy is mainly by radiation. When the fingers are placed above the flame, the transfer of thermal energy is mainly by convection. The transfer of thermal energy by convection is much more than by radiation.

8. A good radiator is also a good absorber of heat. Since transparent bodies are not good absorbers of radiation, they are bad radiators.

Answers to Extension page 158

Search the Internet for "built solar heater" Suggested sites: http://www.thesietch.org/projects/solarthermalpane12/index.htm http://www.motherearthnews.comllibraryI1979 _September_ October/ A_Homemade_Solar_ Water_Heater


Download - Chapter 9 Transfer of Thermal Energy - · PDF fileChapter 9 Transfer of Thermal Energy 2. Discuss with students uses of good and poor conductors. 3. We should be glad that air is a

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