Presentation on theme: "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."— Presentation transcript:
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
~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
~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
Introduction This dessert tastes hot on the outside and on biting into the bread; you will taste the cold, unmelted ice cream. Have you ever heard or tried this dessert before? Can you think of two reasons why the ice cream did not melt while it was fried with the bread in the hot oil?
Watch the following demo …. 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 explanationexplanation Plastics vs metal can
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.
by process Low temperature Transfer of thermal energy to achieve thermal equilibrium ConductionRadiation 1. Vibration of atoms or molecules or molecules 2. Movement of free electrons electrons Convection by High temperature Density changes by Everyday applications Emission of Infra-red radiation colour and texture of the surfacecolour and texture of the surface surface temperaturesurface temperature surface areasurface area by rate affected by have
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.here thermal equilibrium
Heat travels from a place of higher temperature to a place of lower temperature. molecules in hot object lose kinetic energy by slowing down molecules in cold object gain energy by speeding up molecules have higher kinetic energy molecules have lower kinetic energy final temp Molecules have same kinetic energy 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.same temperature
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.
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 Heat travels from a place of higher temperature to a place of lower temperature. Three processes by which heat may be transmitted Does the hand receives heat from the electric iron via the three processes of heat transfer? 1.Conduction 2.Convection 3.Radiation
conduction how conduction works Conduction is the process by which heat is transmitted through a medium from one particle to another. Conduction takes place by molecular vibration or free electron diffusion.molecular vibration free electron diffusion Click to view
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
conduction Free electron diffusion In free electron diffusion, free electrons at the heated end move faster as a result of having more kinetic energy. 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. Click on the picture to animate
conduction Demonstration heat travels from the hot end of the metal pipe to the cooler end 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………..
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 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
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 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 Conduction in solids Conduction in liquid Conduction in gas Comparison of conduction in all three states of matter Click on the following link:
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 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 metal 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 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. rate of conduction
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.
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
uses of good conductors Application Good conductors are used in situations where heat has to be transmitted quickly. A stone floor feels cool to a bare feet, but a floor mat in the same room feels warm. Why? good conductors insulators a saucepan makes use of good conductors and insulators
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?
How can we save energy? Application Insulators can reduce transfer of energy, therefore using insulation results in the saving of energy. 1.lagging hot water tanks and pipes 2.cavity wall insulation 3.Floor insulation 4.Vacuum flasks 5.Double glazing 6.Thick curtain 7.Use of fiberglass, felt and expanded polystrene foam as insulators in the wall of houses Following are examples of insulation to save energy cost.
convection convection in liquids and gases occurs in fluids (liquids and gases), but not in solids density changes cause convection currents; hot fluids rise and cold fluids sink occurs in fluids (liquids and gases), but not in solids density changes cause convection currents; hot fluids rise and cold fluids sink A process by which heat is transmitted from one place to another by the movement of heated particles of a gas or liquid. transfer of heat by convection hot (lower density) liquid or gas cool (higher density) Look under convection
Click on image to animate When a portion of water is heated by direct contact, it expands and decreases in density. Therefore, warm water rises.
convection The surrounding cold water (being relatively denser) moves in to take its place. This process is repeated and the water gradually becomes hot. 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
convection in liquids convection The circulation of a liquid in this manner is called a convection current. small flame water potassium permanganat e crystals purple streak flask
convection Convection occurs more readily in gases than in liquids because they expand much more than liquids when their temperature rises. convection in gases smouldering paper smoke glass window glass cylinders box lighted candle convection currents in air Click here for explanation for above experimenthere
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.
hot water system Application 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.http://www.howstuffworks.com/water-heater.htm
cooling system in a car – the radiator Application 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 pumpengine 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.
Application Air-conditioners are best positioned high, near the ceiling of a room. air-conditioners cold air warm air air-conditioner 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 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
Application Click below for an animation on land and sea breeze. Click for an explanation of land breeze and sea breeze.land breeze sea breeze You may also visit the following website: 1903page01.cfm?chapter_no=visualization
land breeze Application sea cools slower than the land warm air above sea rises cool air above land moves to the sea 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?
sea breeze Application land heats up faster than the sea air above land heats up and rises cool air above sea rushes in 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?
does not require any medium can take place in a vacuum does not require any medium can take place in a vacuum radiation A method of heat transfer by which a heat source transmits infra-red (IR) electromagnetic waves. Heat from the sun reaches the Earth by the process of 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 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 quickly emitted by a black box Radiation is slowly emitted by a shiny box radiation
emission of radiation radiation 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 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 boiling water dull black shiny which side feels hotter?
emission of radiation radiation the greater the surface area and temperature of the object, the faster is the rate of heat transfer from it the big ears of the African elephant provide a large surface area for the giant mammal to cool off quickly in hot weather
absorption of radiation radiation dull black polished cork held by wax cork falls off wax melts first dull black surfaces are good absorbers of radiation than shiny ones in general, good emitters or also good absorbers dull black surfaces are good absorbers of radiation than shiny ones in general, good emitters or also good absorbers
uses of good and poor emitters Application Good emitters are used in situations where heat has to be quickly emitted. Good Emitters Poor Emitters Cooling fins at the back of a refrigerator painted dull black A shiny metal teapot refrigerator cooling fins at the back of a refrigerator painted dull black
uses of good and poor absorbers Application 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 Good absorbers are used in situations where heat has to be quickly absorbed. solar panels
keeps hot liquids hot and keeps cold liquids cold vacuum flask plastic cap vacuum hot liquid glass silvered surfaces foam plastic support outer case Application minimize heat losses by conduction, convection, radiation and evaporation Click here to view design features of a vacuum flaskhere Click here to return to main menuhere
Design features of a vacuum flask
Prevent heat gain/lost due to ConductionConvectionRadiationEvaporation Vacuum between the walls Both walls are silvered on the vacuum side Supported on foam plastic Plastic cap
Quiz 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 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 There are three questions in this section. Try them and check the answer provided on the next slide.
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 answerhere
The answers are as follows: 1)B 2)A 3)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.here Answer for Quiz
Worksheet / Lecture note You should print out a copy of the above worksheet and class note to accompany you through this e-lesson
Exploration How does insulation work for us? Clothing and insulation Fiberglass insulation Heat and temperature How does insulation work for you? You may visit the following websites to discover more about the application of heat transfer in our daily life.
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