St. Anthony's Canossian Secondary School Sec 3NA Science (Physics) Chapter 7 Transfer of Thermal Energy. Name: ( ) Class: Sec Date:

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1 St. Anthony's Canossian Secondary School Sec 3NA Science (Physics) Chapter 7 Transfer of Thermal Energy Name: ( ) Class: Sec Date: Candidates should be able to: (a) show understanding that thermal energy is transferred from a region of higher temperature to a region of lower temperature (b) describe, in molecular terms, how energy transfer occurs in solids (c) describe, in terms of density changes, convection in fluids (d) 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 (e) apply the concept of thermal energy transfer to everyday applications CONDUCTION 1. What is heat? Heat is a form of which flows from a region of temperature to a region of temperature. 2. How is heat transferred from a region of higher temperature to lower temperature? Heat can be transferred in THREE ways: a. Conduction b. Convection c. Radiation - Takes place mainly in - Takes place mainly in Takes place mainly in - Slowly in and or 1

2 3. How do we explain conduction in molecular terms? When one end of the rod is heated,! The solid particles gain and faster.! As they vibrate more vigorously, they with their less energetic neighbouring particles and transfer some of their energy to them. The process continues until the end of the rod is heated.! In this way, heat is conducted from the end to the end. 4. What is the definition of conduction? Conduction is the transfer of heat in which energy from the particles in the warmer region is passed on to the particles in the cooler region without of the medium (solid). Think! Can conduction take place in a vacuum (empty space)? Why or why not? 2

3 5. Do different materials conduct heat differently? Materials which allow heat to flow through it quickly are called. Materials which do not allow heat to flow through quickly are. Good conductors " Poor conductors " Worst conductor Solid Liquid, Gas Vacuum Metal Non metals 6. Why are solids better heat conductors than gases? The particles in solids are than those in gases. For the same volume, there are particles in solids to collide against each other. The of the particles is therefore passed on to neighbouring particles faster in solids than in gases. Hence solids are better heat conductors than gases. 7. Why are metals good conductors of heat? Metals have vibrating atoms and. When these free electrons gain, they around faster and more with neighbouring electrons and atoms.they help to pass on the energy more quickly than vibrating atoms. Applications of heat conduction 8. What are some applications in which heat conduction is increased? Pots, pans and base of electric irons are made of to transfer heat quickly by conduction. However, their ladles are made of heat such as plastic or wood. 9. What are some applications in which heat conduction is reduced? 3

4 Good reduce heat transfer by conduction. For example, Styrofoam cups, cork mat. 10. Why do people wear wollen clothings to keep themselves warm? Wool traps air and is a good insulator of heat. Hence it reduces the transfer of heat from our body to the surroundings by conduction. Fur and feathers of animals trap air and. Check point 1 1. Metals are good conductors of heat whereas air is a poor conductor of heat What are the two ways in which heat is conducted in metals? 2. Explain why a saucepan has a body usually made of copper. 3. Explain why a saucepan has a handle usually made of plastic? 4. The diagram shows the inside of a refrigerator. Explain why a layer of insulation is needed. ICE BOX insulation outer case 4

5 CONVECTION 11. Why are heating coils placed at the bottom of the electric heater? When water at the bottom of a pot gains, it and becomes. # The hot water to the top. # The cooler water at the top is denser and hence to the bottom. # The cooler water is heated by the heat source when it reaches the bottom. # It becomes and rises while cooler water continues to sink to the bottom and is heated. # This circulation of water is called. It enables heat to be transferred rapidly from the bottom to the top of a fluid. Water is a poor of heat. It will take a long time for the water to be heated if we only depend on conduction. Hence we heat water from the so that heat can be transferred quickly through CONVECTION. 12. Definition of convection Convection is the transfer of heat by the of currents from one region to another in a or a. 5

6 13. Where can we observe convection currents? Air conditioners positioned near the ceiling of a room # Cold air, which is, sinks # Warm air which is, rises # Cycle repeats until room air temperature is the same as the pre-set temperature. Think! What happens if the air-con is placed near the floor? Refrigerators # The freezers of refrigerators are usually located at the of the refrigerator. # The cold air from the freezer to the bottom of the refrigerator to cool things in it while warmer air from the bottom to the freezer where it is cooled. Natural ventilation at home # Windows are installed closer to the door to allow cooler and fresh air to flow into the room. # Ventilation outlets are installed up to allow warm, stale air to leave the room. 6

7 Checkpoint 2 1. The diagram shows some rice being boiled in a pan. boiling water aluminium pan rice plastic handle heat (a) Name the process by which heat passes through the bottom of the pan. (b) Why is the handle made of plastic? (c) Explain how the water at the top of the pan gets hot. 2. The diagram shows the position of an air-conditioner in a room. Air-conditioner (a) Draw arrows to show the convection current set up in the air in the room. (b) Explain how the air-conditioner cools the room. 3. In an air-conditioned room, the metal door knob feels colder than the wooden door. Yet both of them are at the same temperature. Why is this so? 7

8 RADIATION 14. Comparing conduction, convection and radiation! Conduction and convection require a to transfer thermal energy. Radiation is the third mechanism of thermal energy transfer that does not require a medium, it can take place in a. For example, energy from the Sun reaches us via. Conduction and convection cannot take place because there is between the Sun and the Earth. 15. Definition of radiation? Radiation is the transfer of by electromagnetic waves. These waves are part of the electromagnetic spectrum radiation are known as infra-red radiation. In fact, all objects emits radiation Draw 3 ways in which your hands can feel the heat from the fire through conduction, convection and radiation. 8

9 16. What is the rate of radiation dependent on? Surface Temperature Colour and Texture of surface Surface Area The hotter the object, the energy it radiates per unit time. Black, dull surfaces are good and of thermal energy. Silver, smooth surfaces are poor and of thermal energy. Silver surfaces are also good of thermal energy. The greater the surface area, the energy it radiates per unit time. 17. A black and white can both contains hot water. Which can contains water at a higher temperature? Explain your answer. A) The black can B) The white can C) Not enough information 9

10 18. What is the colour of the solar heating panels? Why? 19. Why are the cooling fins of the refrigerators painted dull black? 20. Why are metal plates attached to the cooling fins? 21. Why are the metals teapots silvery and polished? 22. Explain why the wax at the dull black surface falls off first. 10

11 23. Vacuum Flask A vacuum flask is designed to keep hot liquids hot, and cold liquids cold. Transfer of thermal energy is by conduction, convection and radiation. The plastic cap reduce heat transfer via because. It stops and. Vacuum reduce heat transfer via and because The inner silvered surface reduce heat transfer via because The outer silvered surface reduce heat transfer via because The foam plastic reduce heat transfer via because 11