Presentation on theme: "L 18 Thermodynamics  Heat transfer Heat Capacity convection"— Presentation transcript:
1 L 18 Thermodynamics  Heat transfer Heat Capacity convection conductionradiationemitters of radiationseeing behind closed doorsGreenhouse effectHeat CapacityHow to boil water
2 Heat flowHEAT the energy that flows from one system to another because of temperature differences.But how does it flow? Three ways:convectionconductionradiation
3 Convectionheat is carried from place to place by the bulk movement of either liquids or gasesdoes not apply to solidswhen water is boiled, hot liquid rises and mixes with cooler liquid, thus the heat is transferredHot air rises:want heat into lower level of house (winter)cooled air into upper levels (summer)
4 Conductionheat is transferred directly through a material, with no bulk movement of stuffonly energy movesiron is a particularlypoor conductor of heat
5 heat conduction HOT COLD Heat Flow Heat Flow rate depends on A / L Cross sectionalarea ALHOTCOLDHeat FlowHeat Flow rate depends on A / L
6 Thermal Conductivity Material Thermal conductivity Copper 400 Silver 420Stainless steel14wood0.15glass0.8wool0.04Goose down0.025styrofoam0.01The effectiveness of a material in conducting heat is characterized by a parameter called the thermal conductivitythere are good thermal conductors(metals) and poor ones (insulators)
7 Thermal Conductivities of Metals Thermal Conductivity(W/mK)Silver406Copper385Aluminum205Brass109Iron80Steel50
9 radiation Heat as moving light Radiation is the heat transfer by electromagnetic waves – thermal light waves - invisible to the eyesthermal radiation is a small part of the electromagnetic spectrum – waves are characterized by their frequency or wavelengthdifferent colors in the visible correspond to different wavelengths from red to blue
12 Thermal RadiationThe warmth you feel from the sun is the sun’s thermal radiationIt travels through the vacuum of space to reach earth, no material is necessary (takes 8 minutes)you can feel its effects even though you cannot see the radiation.you can feel the thermal radiation from a fireplace
13 What produces thermal radiation? all objects whose temperature is above absolute zero emit thermal radiationThe hotter the object, the more radiation it emits, the amount of radiation is ~ T4We all continuously emit thermal radiationWe also absorb it from objects and people around usIf we just emitted radiation we would eventually cool to absolute zero!
15 Thermal radiation spectrum The intensity of radiation increases with temperaturethe color shifts toward the blue at higher temperaturesThe UV radiation from the sun is just beyond the violet (11,000 F)
16 sources of thermal radiation the incandescent light bulb ( the ones that have a filament) are sources of both visible light and heat.when electricity flows through a wire it gets hot.it emits radiation even though you can’t see itas it gets hotter it glows red then orange then whitetungsten filament,can get very hotand not meltevacuated glass bulb
17 Radiation emitted by hot objects The hotter they are, the more they emitthe efficiency with which an object emits thermal radiation is characterized be a parameter called its emissive ee is a number between 0 and 1a good emitter has an e close to 1a poor emitter has an e close to 0
18 good emitters are good absorbers an object that is a good emitter is also a good absorber of thermal radiationa poor emitter is also a poor absorbergenerally dark, dull objects are the best emitters/absorbersshinny objects are poor emitters/absorbersIf you do not want the edges of your pie to burn, you wrap it in aluminum foil
19 good/bad emitters-Leslie’s cube infraredradiation sensorcopper cubefilled with hotwaterthis side ispainted black
20 Practical considerations wear light clothing in summer light clothing absorbs less sunlightcover all body parts in winter warm body parts (like your head) emit radiation
21 thermal radiationall objects that are at a temperature above absolute zero emit thermal radiation (waves)the higher the temp, the more they emitthe color (wavelength) of the emitted waves goes from redorangeyellow blue as the temperature increases
22 seeing behind closed doors we can “see” behind closed doors because of the heat signature left by warm objects on wallsInfrared sensorscan pick up temp-erature differencesof 0.05 degrees C.
23 Which one is best? silvered evacuated un-silvered and un-evacuated
24 infrared radiation is trapped The Greenhouse effectSun’s visible lightinfrared radiation is trappedC O2EARTH
25 Greenhouse effectthe sun’s visible light can penetrate through the atmosphere to the earth’s surface where it heats itthe visible light energy is converted to thermal light energythe thermal radiation is reflected from CO2 in the atmosphere
26 Greenhouse effect concentrations of CO2 have been increasing rise in earth’s temperaturesame effect occurs in your car during the day.
27 The ozone layerozone, O3 is a naturally occurring trace element in the atmosphereIt absorbs solar ultraviolet radiation, especially the harmful UV-B raysit is destroyed by Cfc’s (chlorofluorocarbons)loss affects us and environment
28 How do I boil water? How much heat does it take to boil water? Simpler question how much heat is required to raise the temperature of water by so many degrees?The answer depends on how much water you have and how hot you want to get itThe answer would be different for a different material, say aluminum.
29 Heat Capacity or specific heat The heat capacity is the amount of heat that is required to raise the temperature of 1 g of a substance by 1 degree C.it is measured in Caloriesfor water it is 1 cal/g °Cheat Q = m • c • temp changehot platemass of samplespecific heat
30 Specific heat in cal/g °C Some heat capacitiesSubstanceSpecific heat in cal/g °Cwater1Ethyl alcohol0.58Steel0.11Aluminum0.215lead0.03