1. Unit 8: Transfer of Thermal Energy
Discover PHYSICS
Unit 8: Transfer of Thermal Energy
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2. 8.1 Transfer of Thermal Energy
Learning Outcomes
In this section, you’ll be able to:
Understand that thermal energy is transferred from a region of higher temperature to a region of lower temperature
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3. 8.1 Transfer of Thermal Energy
What causes transfer of thermal energy?
Thermal energy is transferred only when there is a difference in temperature.
Thermal energy always flows from a region of higher temperature to a region of lower temperature.
There is no transfer of heat at thermal equilibrium.
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4. 8.1 Transfer of Thermal Energy
How is thermal energy transferred?
Thermal energy is transferred by:
Conduction
Convection
Radiation
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5. 8.1 Transfer of Thermal Energy
Key Ideas
Transfer of thermal energy takes place when there is a temperature difference. Thermal energy is always transferred from a hotter region to a colder region.
When thermal equilibrium is reached between two bodies (i.e. both bodies are at the same temperature), there is no net flow of thermal energy between them.
There are three different processes of thermal energy transfer: conduction, convection and radiation.
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6. 8.1 Transfer of Thermal Energy
Test Yourself
1. During winter, it is common for people to say ‘keep the cold out of the house’. Is this statement correct? Comment.
Answer:
The statement ‘keep the cold out of the house’ seems to suggest that ‘the cold tends to move into the house’ which is not true.
In fact, it is the transfer of heat energy from the inside of the house to the outside that causes the temperature in the house to drop.
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7. 8.2 Conduction Learning Outcomes In this section, you’ll be able to:
Describe how energy transfer occurs in solid.
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8. Definition: 8.2 Conduction
What is conduction?
Definition:
Conduction is the process of thermal energy transfer without any flow of the material medium.
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9. 8.2 Conduction Experiment 8.1 Objective: Apparatus: Procedure:
To investigate the transfer of thermal energy through solids
Apparatus:
bath, rods of the same dimensions but of different materials, stopwatch
Procedure:
1. Coat the parts of the rods that are on the outside of the tank evenly with melted wax (see figure).
2. Pour boiling water into the bath, so that the ends of the rods are submerged.
3. Record the length of wax that melts in a given interval of time for each of the four rods.
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10. Observation: The wax melts the furthest along the copper rod, followed by iron, glass and wood.
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11. 8.2 Conduction Experiment 8.1 Two important conclusions can be drawn:
1. Thermal energy flows through the material of the rods without any flow of the material itself. This process is called conduction.
2. Different materials conduct heat at different rates. Those that conduct faster are called good conductors (e.g. copper) and those slower are called poor conductors (e.g. wood).
Note:
Poor conductors are also known as insulators.
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12. 8.2 Conduction How does conduction work?
Conductors and insulators have different mechanisms to transfer of thermal energy.
All solids are made up of tiny particles called atoms or molecules.
Metals contain free electrons which move randomly between the atoms and molecules.
Non-metals do not have free electrons.
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13. 8.2 Conduction How does conduction work?
When thermal energy is supplied to one end of a rod, the particles (atoms and molecules) at the hot end vibrate vigorously.
These particles collide with neighbouring particles, making them vibrate as well.
Kinetic energy of vibrating particles at the hot end is transferred to neighbouring particles.
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14. 8.2 Conduction How does conduction work? Good Conductor
In metals, another much faster mechanism of thermal energy transfer takes place at the same time -free electron diffusion.
The free electrons gain kinetic energy and move faster.
The fast-moving electrons then diffuse into cooler parts of the metal.
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15. 8.2 Conduction How does conduction work? Insulators
In insulators, the transfer of thermal energy is solely the results of vibrating atoms and molecules.
There is no free electrons.
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16. 8.2 Conduction Conduction in liquids and gases
Thermal energy can be conducted from a hotter to a cooler region.
Process of conduction is inefficient.
Liquid particles are further apart and collisions of particles are less frequent and even lesser in gases.
Thus, transfer of kinetic energy from fast-moving molecules to neighbouring molecules is slower.
Hence air is poor conductor of heat compared to water, which is in turn is a poor conductor compared to most solids.
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17. 8.2 Conduction Experiment 8.1 Objective: Apparatus: Procedure:
To test conduction of thermal energy in water
Apparatus:
test-tube, ice, metal gauze, Bunsen burner,water
Procedure:
1. Wrap a piece of ice with metal gauze and place it at the bottom of a test-tube.
2. Fill the test-tube with tap water till it is almost full.
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18. 8.2 Conduction
3. Heat the test-tube at the upper end, as shown in the figure.
4. Observe the water being heated and the ice below it.
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19. 8.2 Conduction
Key Ideas
Conduction is the transfer of thermal energy without any flow of the material medium.
The two mechanisms for conduction are atomic or molecular vibrations (for both metals and non-metals) and free electron diffusion (for metals only).
Liquids and gases are poor conductors of heat compared to solids.
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20. 8.2 Conduction Test Yourself
1. Give an account of thermal energy conduction in metals and non-metals.
Answer:
In metals, conduction of heat is due mainly to the diffusion of free electrons from a hotter region to a colder region. Conduction of heat can also take place with molecular vibrations.
In non-metals, conduction of heat only takes place due to molecular vibrations, where the K.E. of the vibrating molecules at the hot end is transferred to the neighbouring molecules.
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21. 8.2 Conduction Test Yourself
2. Why are good conductors of thermal energy also good conductor of electricity?
Answer:
Good conductors such as metals have free electrons. It is the presence of free electrons that enable metals to conduct both thermal energy as well as electricity.
Conduction of electric current is the flow of electric charges such as electrons.
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22. 8.2 Conduction Test Yourself
3. Is the heat transferred from a barbecue fire to a person standing in front of it a good example of heat transfer by conduction? Explain.
Answer:
A person standing in front of a barbecue fire and feeling hot is not a good example of conduction since air is a poor conductor of heat.
In fact, we will learn later that we feel the hotness of the barbecue fire due to radiation of the heat energy.
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23. 8.3 Convection Learning Outcomes In this section, you’ll be able to:
Describe how energy transfer occurs in fluids.
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24. Definition: 8.3 Convection
What is convection?
Definition:
Convection is the transfer of thermal energy by means of currents in a fluid (liquids or gases).
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25. 8.3 Convection Experiment 8.3 Objective Apparatus Procedure
To show convection in water
Apparatus
Large, round-bottomed flask, potassium permanganate crystals, Bunsen burner
Procedure
Fill the flask with water. Carefully place some potassium permanganate crystals at the bottom of the flask.
Place a Bunsen burner with a small flame under the flask and observe the crystals.
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26. Unit 8.3: Convection Experiment 8.4 Objective Apparatus Procedure
To show convection in air
Apparatus
large box with two chimneys on top, a piece of clear glass on one side, candle, matches
Procedure
Place the candle below one of the chimneys. Light the candle.
Introduce smoke into the other chimney by placing a piece of smouldering paper over it and observe the movement of the smoke.
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27. 8.3: Convection How does convection work?
When fluids (liquids and gases) are heated, they expand and become less dense.
The less dense fluids tend to rise from the heating source.
Cooler fluids, being more dense, sink to replace the less dense fluids.
This movement of fluid due to a difference in its density sets up a convection current.
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28. 8.3: Convection How does convection work?
Convection currents occur only in fluids such as liquids and gases but not in solids.
Convection involves the bulk movement of the fluids which carry with them thermal energy.
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29. 8.3: Convection
Key Ideas
Convection is the transfer of thermal energy by means of currents in a fluid (liquid or gas).
A convection current is the movement of fluid caused by the change in density in various parts of the fluid.
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30. 8.3: Convection Test Yourself
1. Why does it feel hot when you put your hands above a small burning candle?
Answer:
The hand feels hot because of convection. The air around the flame is being heated and becomes less dense and rises.
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31. 8.3: Convection Test Yourself
2. Describe briefly the mechanism for the transfer of thermal energy in fluids.
Answer:
When fluids are heated, they expand and become less dense. The less dense fluid rises.
The cooler, denser fluids will replace the less dense fluids.
This sets up a convection current.
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32. 8.4: Radiation Learning Outcomes In this section, you’ll be able to:
Explain energy transfer of a body by radiation.
State the factors affecting the rate of energy transfer by radiation.
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33. Definition: 8.4: Radiation
What is radiation?
Definition:
Radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium.
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34. 8.4: Radiation What is radiation?
Radiation does not require a medium for energy transfer.
It can take place in vacuum.
For example, the Sun is a major source of radiant heat.
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35. 8.4: Radiation What is radiation? The sun emits electromagnetic waves.
Part of this electromagnetic waves, called infrared waves, make us feel warm.
Thermal energy from infrared waves is called radiant heat.
All objects emit some radiant heat.
The hotter the object, the greater the radiant heat emitted.
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36. 8.4: Radiation Absorption of infrared radiation
Infrared radiation is absorbed by all objects and surfaces.
The absorption of radiant heat causes a temperature rise.
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37. 8.4: Radiation Emission of infrared radiation
Infrared radiation is emitted by all objects and surfaces.
This emission causes the temperature of the objects themselves to fall.
In general, good emitter of radiant heat is also a good absorber of radiant heat.
Conversely, poor emitter of radiant heat is also a poor absorber of radiant heat.
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38. Unit 8.4: Radiation Experiment 8.6 Objective
To investigate the emission of infrared radiation
Apparatus
Two temperature sensors, data logger, two identical tins (one black and one shiny), boiling water from two electric kettles
Procedure
Connect the temperature sensors A and B to the data logger
Set the sampling rate to ten seconds
Pour boiling water into both tins at the same time until both are filled to the brim.
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39. Experiment 8.6
4. Place the lid and the temperature sensors onto the tins. Temperature sensor A will monitor the temperature of the black tin, while temperature sensor B records the temperature of the shiny tin.
5. Start recording the temperature. Observe the temperature time graph of both sensors.
6. Stop recording after ten minutes.
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40. 8.4: Radiation Factors affecting rate of infrared radiation
Colour and texture of the surface
Dull, black surfaces are good absorbers of infrared radiation than shiny, white surfaces
Dull, black surfaces are better emitters of infrared radiation.
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41. Unit 8.4: Radiation Factors affecting rate of infrared radiation
2. Surface temperature
Rate of infrared radiation also depends on surface temperature
The higher the temperature of the surface of the object relative to the surrounding temperature, the higher the rate of infrared radiation.
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42. Unit 8.4: Radiation Factors affecting rate of infrared radiation
3. Surface area
The larger surface area will emit infrared radiation at a higher rate.
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43. Unit 8.4: Radiation Key Ideas
Radiation is the continual emission of thermal energy in the form of infrared waves.
Radiation is emitted from the surface of all bodies and does not require a medium of thermal transfer.
Dull, black surfaces are better emitters of infrared radiation than shiny, white surface.
The factors affecting rate of energy transfer by radiation are: colour, and texture of the surface, surface temperature and surface area.
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44. Unit 8.4: Radiation Test Yourself
Give two everyday examples of thermal energy transfer by radiation.
Answer:
Feeling the hotness of Sun’s radiation – Sun’s thermal energy reaches earth by radiation.
Standing next the a BBQ fire will make you feel hot. The thermal energy reaches you by radiation.
Placing your hand next to a hot object such a jar of hot water. Thermal energy reaches your hand by radiation.
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45. Unit 8.4: Radiation Test Yourself
State briefly how thermal energy is transferred by radiation.
Answer:
Hot objects emit thermal energy in the form of infrared radiation, which is a type of electromagnetic waves.
The hotter the object, the higher the rate of radiation.
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46. Unit 8.4: Radiation Test Yourself
State three factors that affect the rate of transfer of thermal energy by radiation.
Answer:
Colour and texture of the surface.
Surface temperature.
Surface area.
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47. Unit 8.5: Applications of Thermal Energy Transfer
Learning Outcomes
In this section, you’ll be able to:
Understand and identify how thermal energy is transferred by conduction, convection and radiation in everyday life.
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48. Unit 8.5: Applications of Thermal Energy Transfer
Common applications of conduction
Uses of good conductors of heat
1. Cooking utensils – made of metals eg. Stainless steel or aluminium
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49. Unit 8.5: Applications of Thermal Energy Transfer
Common applications of conduction
Uses of good conductors of heat
2. Soldering iron rods – the tip is made of copper
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50. Unit 8.5: Applications of Thermal Energy Transfer
Common applications of conduction
Uses of good conductors of heat
3. Heat exchanges.
A heat exchanger transfers thermal energy from hot dirty water to cold clean water. Copper tubes are used to aid rapid transfer of thermal energy from the hot dirty water to the cold clean water.
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51. Unit 8.5: Applications of Thermal Energy Transfer
Common applications of conduction
Uses of bad conductors of heat (insulators)
1. Handles of appliances and utensils – made of plastics or wood
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52. Unit 8.5: Applications of Thermal Energy Transfer
Common applications of conduction
Uses of bad conductors of heat (insulators)
2. Table mats – made of cork
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53. Unit 8.5: Applications of Thermal Energy Transfer
Common applications of conduction
Uses of bad conductors of heat (insulators)
3. Sawdust.
Used to cover ice blocks because of its insulating property
Wooden ladles
Useful for stirring or scooping hot soup.
Woolen clothes
Used to keep body warm
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54. Unit 8.5: Applications of Thermal Energy Transfer
Common applications of conduction
Uses of bad conductors of heat (insulators)
6. Fiberglass, felt and expanded polystyrene foam.
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55. Unit 8.5: Applications of Thermal Energy Transfer
Common applications of convection
1. Heating water in Electric kettles – the heating coil is placed at the bottom to aid the heating of water by convection.
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56. Unit 8.5: Applications of Thermal Energy Transfer
Common applications of convection
2. Household hot water system – the heater is located at the bottom of the system.
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57. Unit 8.5: Applications of Thermal Energy Transfer
Common applications of convection
3. Air conditioners.
Air conditioners are installed near to the ceiling of rooms to facilitate setting up convection currents as cooler air sinks
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58. Unit 8.5: Applications of Thermal Energy Transfer
Common applications of convection
4. Refrigerators
Freezing unit is placed at top to cool the air and facilitate the setting up of convection currents.
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59. Unit 8.5: Applications of Thermal Energy Transfer
Common applications of radiation
1. Teapots
Shiny teapots can keep tea warm for a longer time than black teapots.
It can also keep cold liquids cool for a longer time than black containers.
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60. Unit 8.5: Applications of Thermal Energy Transfer
Common applications of radiation
2. Greenhouses – infrared radiation emitted by the contents in the greenhouse is trapped in the greenhouse.
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61. Unit 8.5: Applications of Thermal Energy Transfer
Common applications of radiation
3. Vacuum flasks
Stopper is made of poor conductor.
The vacuum between the double-glass wall minimises conduction and convection.
The glass walls are silvered and highly reflective to minimise heat loss due to radiation.
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62. Unit 8.5: Applications of Thermal Energy Transfer
Worked Example 8.2
The figure shows a typical vacuum flask designed to keep liquids hot. Part of the vacuum flask is enlarged. State and explain the function of each of the parts labelled A to C
Solution
A: Thin silvering wall to minimise
thermal energy loss by radiation
Since shiny surfaces are poor
absorbers of radiant heat.
B: Vacuum to prevent thermal energy
loss by conduction and convention
(both require material medium for energy transfer)
C: Hollow plastic stopper. Plastic is a poor conductor of heat, minimising thermal energy loss by conduction.
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63. Unit 8.5: Applications of Thermal Energy Transfer
Key Ideas
Some everyday applications of thermal energy transfer involving conduction include cooking utensils and table mats.
Some everyday applications of thermal energy transfer involving convection include household hot water systems and electric kettles.
Some everyday applications of thermal energy transfer involving radiation include vacuum flasks and greenhouses.
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64. Unit 8.5: Applications of Thermal Energy Transfer
Test Yourself
1. A saucepan with a thick copper base contains water and is placed on a flat electric hot plate.
a. State the process by which energy is
i. transferred from the hot plate to the water,
ii. spread through the water.
b. The sides of a saucepan are often polished. How does this reduce energy loss?
Answer:
a(i) Conduction.
Convection.
b. The sides are polished to reduce heat loss due to radiation. Polished and shiny surfaces are poor emitters of radiation
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65. Unit 8: Transfer of Thermal Energy
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