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Transfer of Thermal Energy

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Presentation on theme: "Transfer of Thermal Energy"— Presentation transcript:

1 Transfer of Thermal Energy
A bird can reduce the heat loss from its body during cold weather by fluffing up its feathers. What are the processes of heat transfer? Click to proceed

2 ~microLESSON Information~
Level: Secondary Three (Special) Time: 60 mins Objectives: Show understanding that thermal energy is transferred from a region of higher temperature to a region of lower temperature Show understanding that regions of equal temperature will be in thermal equilibrium Describe in molecular terms, how energy transfer occurs in solids

3 ~microLESSON Information~
Objectives: ( continue…) Describe, in terms of density changes, convection in fluids. Show understanding 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 Apply the concept of thermal energy transfer to everyday applications

4 Introduction Have you ever heard or tried this dessert before?
This dessert tastes hot on the outside and on biting into the bread; you will taste the cold, unmelted ice cream. Can you think of two reasons why the ice cream did not melt while it was fried with the bread in the hot oil?

5 Watch the following demo ….
Plastics vs metal can Do you know why the part where the paper is wrapped around the plastic container burnt faster than the paper wrapped around the metal can? Click to see explanation

6 Explanation Metal can is a good conductor of heat, it conducts away the heat faster hence, taking the paper a longer time to catch fire as opposed to the part just wrapped around the plastic container that catches fire faster.

7 CONCEPT MAP

8 Transfer of thermal energy to achieve thermal equilibrium
High temperature Low temperature by process Conduction Convection Radiation by by by Emission of Infra-red radiation 1. Vibration of atoms or molecules 2. Movement of free electrons Density changes rate affected by colour and texture of the surface surface temperature surface area have Everyday applications

9 Contents Sect A Thermal Equilibrium Sect F Worksheet / Class note Sect
Conduction Sect G Sect C Exploration Convection Sect D Quit Now Quit Radiation Sect E Quiz

10 thermal equilibrium What happens if a hot object is placed next to a cold object? Well, we expect the two objects to finally come to the same final temperature after sometime. We say that the objects now are in thermal equilibrium with each other. How does this happen? Click here.

11 Molecules have same kinetic energy
Heat travels from a place of higher temperature to a place of lower temperature. molecules have higher kinetic energy molecules have lower kinetic energy final temp Molecules have same kinetic energy molecules in hot object lose kinetic energy by slowing down molecules in cold object gain energy by speeding up Transfer of energy stops when molecules of both objects have the same average kinetic energy. The objects are said to be in thermal equilibrium and they are at the same temperature.

12 Temperature is a measure of average kinetic energy of molecules in an object.
Two objects in thermal contact have the same temperature are in thermal equilibrium because the molecules have the same average kinetic energy.

13 transfer of thermal energy
Heat travels from a place of higher temperature to a place of lower temperature. Three processes by which heat may be transmitted Conduction Convection Radiation Does the hand receives heat from the electric iron via the three processes of heat transfer?

14 conduction how conduction works
Conduction is the process by which heat is transmitted through a medium from one particle to another. Click to view Conduction takes place by molecular vibration or free electron diffusion.

15 direction of heat transfer along the rod
conduction Molecular Vibration In molecular vibration, the molecules of the substance being heated vibrate faster. They jostle their neighbouring molecules, making them vibrate as well. In this way, heat flows one point to another. direction of heat transfer along the rod Click on the picture to animate

16 conduction Free electron diffusion
In free electron diffusion, free electrons at the heated end move faster as a result of having more kinetic energy. Click on the picture to animate These fast moving, energy carrying electrons spread to the cooler parts of the metal and then transfer their kinetic energy to other electrons by colliding with them. This usually occurs in metals and is faster than the vibration process.

17 heat travels from the hot end of the metal pipe to the cooler end
conduction Demonstration This demonstration shows how heat travels along a metal pipe by conduction.Match sticks are attached to the pipe with wax. The heat melts the wax and this causes the match sticks to fall. Loading video clip……….. heat travels from the hot end of the metal pipe to the cooler end

18 conduction how conduction works
this transfer of energy from particle to particle continues until thermal equilibrium is reached no net movement of particles during conduction, the particles merely vibrate about its rest position Click on the following link: Look under conduction & click on Start heating

19 conduction rate of conduction
Solids are better conductors of heat than liquids and gases in solids, the particles are closer together than in liquids and gases kinetic energy is transferred more quickly Click on the following link: Conduction in solids Conduction in liquid Conduction in gas Comparison of conduction in all three states of matter

20 conduction rate of conduction
Metals conduct heat much faster than non-metals heat is conducted in metals by ‘free’ electrons and by the vibration of molecules whereas heat is conducted in non-metals only by the vibration of molecules Heat conduction in a non metal Attention: in the video clip, the rod is a non metal and not a metal as indicated in the clip. Heat conduction in a metal

21 rate of conduction heat transfer by electrons is faster than by vibration collision between moving electrons transfer some energy metals are good thermal conductors of heat but are poor insulators Gases are very poor conductors of heat as their molecules are too far apart to affect each other much. This means that air is a terrible conductor of heat energy.

22 Thermal Conductivity The ability to transfer heat within an object. It varies for different materials. Good conducting materials are those materials where there is little space between the particles - like most metals. These are called heat conductors. Poor conductors, like glass and wood are called heat insulators.

23 Materials containing trapped air:
conduction conductors and insulators Good Conductors Poor Conductors gold silver copper iron brass aluminium glass stone water plastics wood Materials containing trapped air: polystyrene foam wool fibreglass

24 a saucepan makes use of good conductors and insulators
Application uses of good conductors Good conductors are used in situations where heat has to be transmitted quickly. insulators good conductors a saucepan makes use of good conductors and insulators A stone floor feels cool to a bare feet, but a floor mat in the same room feels warm. Why?

25 Application uses of poor conductors (insulators)
Poor conductors (insulators) are used in situations where unwanted heat has to be kept away or to prevent heat loss. How does the fur of a polar bear help to minimise heat loss from the body?

26 Application How can we save energy?
Insulators can reduce transfer of energy, therefore using insulation results in the saving of energy. Following are examples of insulation to save energy cost. lagging hot water tanks and pipes cavity wall insulation Floor insulation Vacuum flasks Double glazing Thick curtain Use of fiberglass, felt and expanded polystrene foam as insulators in the wall of houses

27 transfer of heat by convection
convection in liquids and gases A process by which heat is transmitted from one place to another by the movement of heated particles of a gas or liquid. liquid or gas occurs in fluids (liquids and gases), but not in solids density changes cause convection currents; hot fluids rise and cold fluids sink cool (higher density) Look under convection hot (lower density) transfer of heat by convection

28 Click on image to animate
convection When a portion of water is heated by direct contact, it expands and decreases in density. Therefore, warm water rises. Click on image to animate

29 Click on image to animate
convection When a portion of water is heated by direct contact, it expands and decreases in density. Therefore, warm water rises. The surrounding cold water (being relatively denser) moves in to take its place. This process is repeated and the water gradually becomes hot. Click on image to animate

30 potassium permanganate crystals
convection convection in liquids small flame water potassium permanganate crystals purple streak flask The circulation of a liquid in this manner is called a convection current.

31 convection convection in gases
Convection occurs more readily in gases than in liquids because they expand much more than liquids when their temperature rises. convection currents in air smouldering paper smoke glass window glass cylinders box lighted candle Click here for explanation for above experiment

32 Explanation for experiment
A candle is lit below one chimney. The heat from this initiates convection current. The hot air flows out of this chimney. Cold air flows in through the other chimney. This is tested by holding a piece of smouldering paper over the top of the chimney through which the cold air enters the box and observing the path of the smoke. This simple method of air circulation was once used to ventilate underground mines.

33 Application hot water system
A hot water system makes use of the principle of convection. hot water rising boiler hot water storage tank expansion pipe ball valve cold tank cold water drawn down Click to understand how the water heater separates hot water from cold water in the tank.

34 copper tubes with cooling fins
Application cooling system in a car – the radiator When a car engine is running for a long time, a lot of heat energy is produced. It is necessary to cool the engine so that it does not overheat. air into radiator copper tubes with cooling fins hot water cool water pump engine When water in the water jacket gets heated, it flows into copper tubes and cool the water in them. The cooled water flows down and back into the engine through a hose at the bottom.

35 Application air-conditioners
Air-conditioners are best positioned high, near the ceiling of a room. cold air which is denser sinks warm air which is less dense rises cycle repeats until room air temperature is the same as the temperature set on the thermostat of the air-conditioner warm air air-conditioner cold air

36 Application Click below for an animation on land and sea breeze.
Click for an explanation of land breeze and sea breeze. You may also visit the following website:

37 Application land breeze sea cools slower than the land
warm air above sea rises cool air above land moves to the sea Do you know why the sea cools slower than the land?

38 Application sea breeze land heats up faster than the sea
air above land heats up and rises cool air above sea rushes in Do you know why the land heats up faster than the land?

39 Heat from the sun reaches the Earth by the process of radiation
A method of heat transfer by which a heat source transmits infra-red (IR) electromagnetic waves. does not require any medium can take place in a vacuum Heat from the sun reaches the Earth by the process of radiation

40 radiation Infra-red radiation is absorbed by all objects and surfaces.
This absorption causes heat gain and a temperature rise. Dull,black surfaces absorb infra-red rays very quickly. Radiation absorbed by the box

41 radiation Infra-red radiation is emitted by all objects and surfaces.
This emission causes the temperature to fall as heat is lost. Dull, black surfaces are the best radiators. Radiation is slowly emitted by a shiny box Radiation is quickly emitted by a black box

42 which side feels hotter?
radiation emission of radiation boiling water dull black shiny which side feels hotter? the hotter an object is, the more energy it radiates dull black surfaces are good emitters or radiators than shiny ones the greater the surface area and temperature of the object, the faster is the rate of heat transfer from it

43 radiation emission of radiation
the big ears of the African elephant provide a large surface area for the giant mammal to cool off quickly in hot weather the greater the surface area and temperature of the object, the faster is the rate of heat transfer from it

44 radiation absorption of radiation
polished dull black wax melts first cork held by wax cork falls off dull black surfaces are good absorbers of radiation than shiny ones in general, good emitters or also good absorbers

45 cooling fins at the back of a refrigerator painted dull black
Application uses of good and poor emitters Good emitters are used in situations where heat has to be quickly emitted. refrigerator Good Emitters Poor Emitters Cooling fins at the back of a refrigerator painted dull black A shiny metal teapot cooling fins at the back of a refrigerator painted dull black

46 Application uses of good and poor absorbers
Good absorbers are used in situations where heat has to be quickly absorbed. Good Absorbers Poor Absorbers Solar heating panels are painted in dull black paint Houses in hot countries and factory roofs are painted in white, light-coloured paint, aluminium paint Light colours are chosen for clothes and cars in hot weather solar panels

47 glass silvered surfaces
Application vacuum flask keeps hot liquids hot and keeps cold liquids cold plastic cap vacuum hot liquid glass silvered surfaces foam plastic support outer case minimize heat losses by conduction, convection, radiation and evaporation Click here to view design features of a vacuum flask Click here to return to main menu

48 Design features of a vacuum flask

49  Design features of a vacuum flask Prevent heat gain/lost due to
Conduction Convection Radiation Evaporation Vacuum between the walls Both walls are silvered on the vacuum side Supported on foam plastic Plastic cap

50 Quiz There are three questions in this section.
Try them and check the answer provided on the next slide. 1) Heat is conducted through a plastic rod by A. collisions of molecules in random motion B. collisions of vibrating molecules C. convection currents D. radiation 2) Metals are better conductors of heat than non-metals because in metals there are A. many free electrons that are in motion B. many free electrons that are in vibration C. less free electrons that are in motion D. less electrons that are in vibration

51 3) Water in the electric heater gets heated up by
A. conduction B. convection C. conduction and convection D. none of the above Click here for answer

52 Answer for Quiz The answers are as follows: B A C
Good! Did you get all the answers correct? Now you may click here to proceed to section G to further explore on this topic.

53 Worksheet / Lecture note
You should print out a copy of the above worksheet and class note to accompany you through this e-lesson

54 Exploration You may visit the following websites to discover more about the application of heat transfer in our daily life. How does insulation work for us? How does insulation work for you? Clothing and insulation Fiberglass insulation Heat and temperature

55 You are about to end this lesson.
Quit You are about to end this lesson. Are you sure? Click Yes to exit. Click No to return to the main menu Yes No


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