Hewitt Suchocki Hewitt Conceptual Physical Science Fourth Edition Chapter 7 HEAT TRANSFER AND CHANGE OF PHASE Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley This lecture will help you understand Conduction Convection Radiation Newton s Law of Cooling Gl b l W Global Warming i and d th the G Greenhouse h Eff Effectt Heat Transfer and Change of Phase Boiling Melting and Freezing Energy and Change of Phase Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Heat Transfer Processes of thermal energy transfer Conduction Convection C ti Radiation Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Conduction Conduction Transfer of internal energy gy by y electron and molecular collisions within a substance Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Heat Transfer Conduction Conduction occurs predominately in solids where the molecules remain in relatively restricted locations When you stick a nail into ice does cold flow from the ice to your hand or heat f from your hand h d tto the th ice i Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Conduction Insulation Doesn t p prevent the flow of internal energy gy Slows the rate at which internal energy flows E Example l Rock wool or fiberglass between walls slows the transfer of internal energy from a warm house to a cool exterior in winter winter and the reverse in summer Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Conduction Application Snow patterns on the roof of a house show areas of conduction and insulation Bare parts show where heat from inside has conducted through the roof and melted the snow Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Heat Transfer Conduction G d conductors Good d t Composed of atoms with loose outer electrons Known K as poor insulators i l t Examples all metals to varying degrees Poor conductors Delay the transfer of heat Known as good insulators Examples wood E l d wool l straw t paper St Styrofoam f cork liquid gases air or materials with trapped air Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Co duct o Conduction Dramatic example Author John Suchocki walks barefoot without burning his feet on red hot red hot coals due to poor conduction between the coals and his feet Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Convection Convection Transfer of heat involving only bulk motion of fluids Examples Visible shimmer of air above a hot stove or above asphalt on a hot dayy Visible shimmers in water due to temperature difference Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Convection Cooling by expansion Opposite to the warming that occurs when air is compressed Example The cloudy region above hot steam issuing from the nozzle of a pressure cooker is cool to the touch a combination of air expansion and mixing with cooler surrounding air Careful the part at the nozzle th t you can t that t see is i steam ouch Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Convection Currents Convection currents produced by unequal heating of land and water During the day warm air above the land rises and cooler air over the water moves in to replace it At night the direction of air flow is reversed Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Convection Reason warm air rises Warm air expands p becomes less dense and is buoyed upward Air rises until its density equals that of the surrounding air E Example l Smoke S ffrom a campfire f rises and blends with the surrounding cool air Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Radiation Radiation Transfer of energy gy via electromagnetic g waves that can travel through empty space Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Heat Transfer Radiation Wavelength of radiation is related to the frequency q y of vibration Low frequency vibrations long waves Hi h frequency High f vibrations ib ti short waves Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Radiation Emission of radiant energy Everyy object j above absolute zero radiates Sun s surface comes From the Sun s light or solar radiation From the Earth Earth s s surface is terrestrial radiation in the form of infrared waves below our threshold of sight Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Wave Frequency Temperature a A low temperature cool source emits primarily low frequency long wavelength waves b A medium temperature source emits primarilyy medium frequency c A high temperature high temperature source emits primarily high frequency high frequency short wavelength waves Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Radiation E i i off radiant Emission di t energy Peak frequency of radiation is proportional to the absolute temperature of the source f T Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Emission and Absorption The surface of any material both absorbs and emits radiant energy gy When a surface absorbs more energy than it emits it is a net absorber emits absorber and temperature tends to rise When a surface emits more energy than it absorbs it is a net emitter and temperature tends to fall Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Emission and Absorption Absorption of Radiant Energy The ability of a material to absorb and radiate thermal energy is indicated by its color Good absorbers and good emitters are dark in color P Poor absorbers b b and d poor emitters are reflective or li ht iin color light l Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Emission and Absorption Whether a surface is a net absorber or net emitter depends p on whether its temperature is above or below that of its surroundings surroundings A surface hotter than its surroundings will be a net emitter and tends to cool A surface colder than its surroundings will be a net absorber and tends to warm Copyright 2008 Pearson Education Inc publishing as Pearson Addison Wesley Radiation ad at o Reflection of radiant energy Darkness is often due to reflection of light back and forth many times partially absorbing with each reflection Good reflectors are poor absorbers Copyright 2008 Pearson Education Inc
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