Presentation on theme: "Learning outcomes Appreciate how thermal energy is transferred form a higher temperature region to one of lower temperature Describe how energy transfer."— Presentation transcript:
1 Learning outcomesAppreciate how thermal energy is transferred form a higher temperature region to one of lower temperatureDescribe how energy transfer occurs through molecules in solidsDescribe density changes in fluids leading to convectionExplain that radiation does not require a material medium and 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
2 9.1 ConductionConduction is the process by which heat is transmitted through a medium from one particle to another
3 9.1 Conduction How Conduction Works Solids: conduction occurs when thermal energy transferred from one particle to another.When one end of a rod is heated, molecules there gain energy and vibrate faster.These molecules collide with less energetic neighbors.Some of their energy is transferred to neighboring molecules which thus gains kinetic energy.
4 9.1 ConductionThermal energy is passed along the rod by the vibrating molecules.Vibration transfers energy from molecule to molecule until cold end reaches same temperature as hot endNo net movement of molecule during conduction.Solids are better conductors of heat than liquids and gases as molecules are closer together.Kinetic energy transferred more quickly.
5 9.1 ConductionConduction takes place through vibrating molecules as well as free electrons in metals.When heated, free electrons gain energy and move faster.They move in the spaces between atoms before colliding with some atoms and transferring energy to them.Much faster than conduction by vibration of molecules in non-metals.Metals conduct heat much faster then non-metals which have no free electrons.
6 Poor conductors (insulators) 9.1 ConductionExamples of good and poor heat conductors (insulators)Good conductorsPoor conductors (insulators)GoldSilverCopperIronBrassAluminiumGlassStoneWaterPlasticsWoodPolystyrene foam*Wool*fibreglass**Materials containing trapped air
7 9.1 Conduction Applications of Conduction Uses of Good Conductors Used where thermal energy has to be quickly transmittedE.g. Pans, kettles, cooking utensils are made of metals like aluminium, copper, steelCopper pipes for heat exchangersHeat sinks in electronic equipment
8 9.1 Conduction Uses of Poor Conductors (Insulators) E.g. Cloth, plastics, woodHelp keep unwanted heat awayHelp prevent heat loss
9 9.1 Conduction Air is good insulator, reduces conduction of heat Felt material used as insulators to trap air pocketsPrevent?
10 Thermal energy of a body is the total kinetic energy of the atoms or molecules in the body. Heat is energy in transit from a high temperature object to a lower temperature object. It is recognised only as it crosses the boundary of a system.Conduction is the process by which thermal energy is transmitted through a medium from one particle to another.Conduction in metals is far better than in other solids because other than the vibrating atoms, the free electrons in a metal are also responsible for the conduction of thermal energy.Liquids and gases are poor thermal conductors.
12 9.2 ConvectionConvection is the process by which heat is transmitted from one place to another by the net movement of heated particles of a gas or liquid.Convection takes place in liquids and gases where atoms or molecules move from one place to anotherTransfer of thermal energy depends on transfer of heated matter.
13 9.2 Convection Convection in Liquids Add potassium permanganate crystals into a flask of waterCrystals sink to bottom of the flaskHeat flask gently just below the crystalsPurple-streaked water rise upwards and fan outwards.Water becomes uniformly purplish after a while.
14 9.2 Convection Circulation of a liquid called a convection current Due to heating at the bottom of liquidWhen a liquid is heated, it expands, becomes less dense and floats upwards.Cold and denser liquid moves down to replace the rising liquid masses.This cold liquid, in turn, gets heated up.
15 9.2 Convection A Hot Water System based on convection principle Consists of a boiler, water storage tank and cold tank interconnected by pipesConvection currents drive hot water from the top of boiler into hot water storage tank.Cold water from storage tank is drawn down to the boiler where it in turn becomes heated.
16 9.2 Convection Car Radiator Car engines are kept cool by a water jacketWhen water in the water jacket gets heated, it flows into copper tubes which include many cooling fans.A fan causes air to flow past the tubes and cool water in them.Cooled water flows down and back into engine through a hose at the bottom.
17 9.2 Convection Convection in Gases To show convection currents in air Fit two glass chimneys to the top of a wooden box with plane glass windowLight a candle below one chimneyHeat from this initiates a convection currentHot air flows out of this chimneyCold air flows in through other chimneyUsed to ventilate underground mines
18 9.2 Convection Air-conditioners Cold air blown in sinks because it’s denser then warm air.Warm air displaced by cold air rises and is drawn into air-conditioner to be cooled.RefrigeratorCold air below freezer sinksSet up convection currentsToday’s fridge, freezer below??
19 9.2 Convection Ventilation in a Kitchen Hot air rises through chimney Sets up a convection currentCool air drawn into kitchen
20 9.2 Convection Sea and Land Breezes In the day, land heats up faster than the seaAir above land expands and risesCool air above sea moves inlandResult in sea breeze
21 9.2 Convection At night, the land cools faster than the sea Warm air above sea risesCool air above land moves out to take its placeResult in land breeze
22 The process of thermal energy transfer most significant in fluids (liquids and gases) is convection. Convection is the process by which heat is transmitted from one place to another by the movement of heated particles of a gas or liquid.Convection currents are caused by density changes.Hot fluids rise and cold fluids sink.
24 9.3 RadiationRadiation is the transfer of energy by electromagnetic wavesEmission of RadiationThe rate of energy transfer by radiation is affected by:(a) surface temperature(b) colour and texture of the surface(c) surface area
25 9.3 Radiation Uses of Poor Emitters of Radiation Shiny surfaces are poor radiatorsE.g. Teapots and aluminum foilUses of Good Emitters of RadiationDull surfaces radiate quicklyE.g. Cooling fins of refrigerators and heat sinks
26 9.3 Radiation Absorption of Radiation Dull black surfaces better absorbers than shiny surfacesGood emitter is also good absorberShiny surfaces good reflectors of heat
27 9.3 Radiation Uses of Good and Poor Absorbers of Radiation Houses in hot countries painted white to keep them cooler.Factory roofs coated with aluminium to reduce radiation absorption.Solar control films allow light to pass through but keep out infrared radiation.Dark clothes absorb radiation readily.Solar panels painted black to absorb as much radiation from sun as possible.
28 9.3 Radiation Vacuum Flask Reduces transfer of thermal energy by conduction, convection and radiationConsists of a double-walled glass container with a vacuum in betweenSilvered walls on vacuum sideContainer supported by foam plastic, a poor conductor
29 9.3 RadiationNo heat can enter or leave flask by conduction or convection across the vacuumInner silvered surface reflects radiation from hot fluids back into flaskOuter silvered surface reflects radiation in external surroundings away from flaskFoam plastic support and plastic cup minimises heat transmitted by conduction through thin glass wallsPlastic cup stops convection and evaporation
30 Radiation is the transfer of energy by infra-red electromagnetic waves. Dull black surfaces are good emitters and absorbers of radiation.The greater the surface area of the object, the faster is the rate of thermal transfer from it.The greater the temperature of an object, the faster is the rate of thermal energy transfer from it.