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 minsObjectives:Show understanding that thermal energy is transferred from a region of higher temperature to a region of lower temperatureShow understanding that regions of equal temperature will be in thermal equilibriumDescribe 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 areaApply 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 canDo 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 ExplanationMetal 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.
8 Transfer of thermal energy to achieve thermal equilibrium High temperatureLow temperatureby processConductionConvectionRadiationbybybyEmission ofInfra-red radiation1. Vibration of atomsor molecules2. Movement of freeelectronsDensity changesrate affected bycolour and texture of the surfacesurface temperaturesurface areahaveEveryday applications
9 Contents Sect A Thermal Equilibrium Sect F Worksheet / Class note Sect ConductionSectGSectCExplorationConvectionSectDQuitNowQuitRadiationSectEQuiz
10 thermal equilibriumWhat 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 energymolecules have lower kinetic energyfinal tempMolecules have same kinetic energymolecules in hot object lose kinetic energy by slowing downmolecules in cold object gain energy by speeding upTransfer 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 transmittedConductionConvectionRadiationDoes 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 viewConduction takes place by molecular vibration or free electron diffusion.
15 direction of heat transfer along the rod conductionMolecular VibrationIn 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 rodClick 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 animateThese 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 conductionDemonstrationThis 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 reachedno net movement of particles during conduction, the particles merely vibrate about its rest positionClick 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 gasesin solids, the particles are closer together than in liquids and gaseskinetic energy is transferred more quicklyClick on the following link:Conduction in solidsConduction in liquidConduction in gasComparison of conduction in all three states of matter
20 conduction rate of conduction Metals conduct heat much faster than non-metalsheat 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 moleculesHeat conduction in a non metalAttention: 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 conductionheat transfer by electrons is faster than by vibrationcollision between moving electrons transfer some energymetals are good thermal conductors of heat but are poor insulatorsGases 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 ConductivityThe 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.
24 a saucepan makes use of good conductors and insulators Applicationuses of good conductorsGood conductors are used in situations where heat has to be transmitted quickly.insulatorsgood conductorsa saucepan makes use of good conductors and insulatorsA 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 pipescavity wall insulationFloor insulationVacuum flasksDouble glazingThick curtainUse of fiberglass, felt and expanded polystrene foam as insulators in the wall of houses
27 transfer of heat by convection convection in liquids and gasesA process by which heat is transmitted from one place to another by the movement of heated particles of a gas or liquid.liquid or gasoccurs in fluids (liquids and gases), but not in solidsdensity changes cause convection currents; hot fluids rise and cold fluids sinkcool(higher density)Look under convectionhot(lower density)transfer of heat by convection
28 Click on image to animate convectionWhen 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 convectionWhen 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 convectionconvection in liquidssmall flamewaterpotassium permanganate crystalspurple streakflaskThe 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 airsmouldering papersmokeglass windowglass cylindersboxlighted candleClick 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 risingboilerhot water storage tankexpansion pipeball valvecold tankcold water drawn downClick to understand how the water heater separates hot water from cold water in the tank.
34 copper tubes with cooling fins Applicationcooling system in a car – the radiatorWhen 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 radiatorcopper tubes with cooling finshot watercool waterpumpengineWhen 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 sinkswarm air which is less dense risescycle repeats until room air temperature is the same as the temperature set on the thermostat of the air-conditionerwarm airair-conditionercold 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 risescool air above land moves to the seaDo 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 risescool air above sea rushes inDo 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 mediumcan take place in a vacuumHeat 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.Radiationabsorbed 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 slowlyemitted by a shiny boxRadiation is quicklyemitted by a black box
42 which side feels hotter? radiationemission of radiationboiling waterdull blackshinywhich side feels hotter?the hotter an object is, the more energy it radiatesdull black surfaces are good emitters or radiators than shiny onesthe 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 weatherthe greater the surface area and temperature of the object, the faster is the rate of heat transfer from it
44 radiation absorption of radiation polisheddull blackwax melts firstcork held by waxcork falls offdull black surfaces are good absorbers of radiation than shiny onesin general, good emitters or also good absorbers
45 cooling fins at the back of a refrigerator painted dull black Applicationuses of good and poor emittersGood emitters are used in situations where heat has to be quickly emitted.refrigeratorGood EmittersPoor EmittersCooling fins at the back of a refrigerator painted dull blackA shiny metal teapotcooling 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 AbsorbersPoor AbsorbersSolar heating panels are painted in dull black paintHouses in hot countries and factory roofs are painted in white, light-coloured paint, aluminium paintLight colours are chosen for clothes and cars in hot weathersolar panels
47 glass silvered surfaces Applicationvacuum flaskkeeps hot liquids hot and keeps cold liquids coldplastic capvacuumhot liquidglass silvered surfacesfoam plastic supportouter caseminimize heat losses by conduction, convection, radiation and evaporationClick here to view design features of a vacuum flaskClick here to return to main menu
49 Design features of a vacuum flask Prevent heat gain/lost due to ConductionConvectionRadiationEvaporationVacuum between the wallsBoth walls are silvered on the vacuum sideSupported on foam plasticPlastic 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 byA. collisions of molecules in random motionB. collisions of vibrating moleculesC. convection currentsD. radiation2) Metals are better conductors of heat than non-metals because in metals there areA. many free electrons that are in motionB. many free electrons that are in vibrationC. less free electrons that are in motionD. less electrons that are in vibration
51 3) Water in the electric heater gets heated up by A. conductionB. convectionC. conduction and convectionD. none of the aboveClick 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 ExplorationYou 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 insulationFiberglass insulationHeat and temperature
55 You are about to end this lesson. QuitYou are about to end this lesson.Are you sure?Click Yes to exit.Click No to return to the main menuYesNo