Presentation on theme: "Transfer of Thermal Energy"— Presentation transcript:
1 Transfer of Thermal Energy Name: ________________Class: _________________Index: ________________
2 Learning Objectivesshow understanding that thermal energy is transferred from a region of higher temperature to a region of lower temperature.describe, in molecular terms, how energy transfer occurs in solids.describe, in terms of density change, convection in fluids.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.apply the concept of thermal energy transfer to everyday applications.
3 In general, heat travels from higher temperature region to lower temperature region. There are three processes by which heat may transmitted:Conduction,Convection, andRadiation
5 ConductionConduction is the process by which heat is transmitted through a medium from oneparticle to another.
6 Conduction Heat travels through some material better than others. Pins are hold by waxMaterial Time taken for pin to drop(s)wood did not dropcoppersteelaluminium
7 A good conductor has a high conductivity. Metals are good conductors of heat. Silver and copper have highest conductivity.Non-metals, such as plastic and air, are poorConductor (insulators).Liquids and gases are usually poor conductorsThe poorest conductor is vacuum.
8 Test Yourselves the water at the top starts boiling, the ice cube Heat a test tube of water near the top with a‘weighted’ ice cube near the bottom. Even whenthe water at the top starts boiling, the ice cubedoes not melt. Why ?Ans.: Water is a poorconductor of heat.
9 Insulators Insulators are used to lag pipes, lofts, hot water tanks and manyother objects.Insulators are very poorconductors.Wrapped round hot objects,they act as barrier, reducingthe heat loss.Fibre glass insulation is apopular choice for insulatingthe loft of homes.
10 InsulationLagging in a cavity wallprovides extra insulation
12 Convection Convection is the process by which heat is transmitted from one place to another bythe movement of heated particles of a gasor liquid due to difference in density.
13 Convection in Liquids To demonstrate convection in water, drop a few tiny crystals of potassium permanganateinto a flask filled with water.Gently heat the flask, purple streaks of waterwill rise upwards and then fan outwards.The water becomes uniformly purplish aftersome time.The circulation of a liquid in this matter iscalled a convection current.Thinking :Why hot water rises and cold water sinks ?
15 Convection in GasesThe figure below shows a simple demonstration of convection of gas. The hot gases from the burning candle go straight up the chimney above the candle. Cold air is drawn down the other chimney to replace the air leaving the room.
16 Concept on ventilation of a room with fire place
18 Sea BreezeDuring the day the sun heats the land must faster than the sea.(Why ?)The air above the land is heated, expands and rises.Cold air from the sea moves inland to take its place.Hence, sea breeze is obtained.Discussion : How land breeze is produced ?
19 Test Yourselves 1. Usually, it is better to install air conditioner in the higher portion of the wall. Pleaseexplain this statement with reasons.2. How are winds forms ?3. Why is the heating coil of an electric kettleplaced near the bottom of the vessel ?
21 RadiationRadiation is a method of heat transfer that does not require any medium.It can take place in a vacuum.In radiation, heat transmits energy in the form ofwaves.
22 Heat Radiation All objects emit heat radiation. Heating an object up make it radiate more energy.A dull dark surfaces is a better emitter or radiator than a shiny one.
23 What type of surface is the best absorber of heat Fig. below shows one way to test different surfaces.Results from this type of test show that,a dull black surface is the best absorber of radiation,a shiny silvery surface is the worst absorber of radiation.
24 What type of surface is the best absorber of heat
25 A black body is defined as a body that absorbs all radiation that falls on its surface. Actual black bodies don't exist in nature - though its characteristics are approximated by a hole in a box filled with highly absorptive material.A black body is a hypothetic body that completely absorbs all wavelengths of thermal radiation incident on it. Such bodies do not reflect light, and therefore appear black if their temperatures are low enough so as not to be self-luminous. All blackbodies heated to a given temperature emit thermal radiation.
26 The radiation energy per unit time from a blackbody is proportional to the fourth power of the absolute temperature and can be expressed with Stefan-Boltzmann Law asq = σ T4 A (Not Tested)whereq = heat transfer per unit time (W)σ = (W/m2K4) - The Stefan-Boltzmann ConstantT = absolute temperature Kelvin (K)A = area of the emitting body (m2)Therefore, the rate of energy transfer increases with temperature and surface area.
28 Vacuum Flask The flask is a double-walled glass bottle. The space between the twowalls is a vacuum. This canstop energy transfer out byconduction and convection.The shiny bright silveringcoated glass surfaces canreduced the heat lost byradiation. (Why ?)
29 Test Yourselves 1. Brightly polished kettle do not lose much energy by radiation. Why ?2. The cooling fins on the back of a refrigerator,in a car radiator should be dull black. Why ?