L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction...
-
Upload
conrad-marshall -
Category
Documents
-
view
228 -
download
2
Transcript of L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction...
![Page 1: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/1.jpg)
L 18 Thermodynamics [3]L 18 Thermodynamics [3]
Heat transferHeat transfer• convection convection • conductionconduction• radiationradiation
• emitters of radiationemitters of radiation• seeing behind closed doorsseeing behind closed doors• Greenhouse effectGreenhouse effect
Heat CapacityHeat CapacityHow to boil waterHow to boil water
![Page 2: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/2.jpg)
Heat flow
• HEAT the energy that flows from one system to another because of temperature differences.
• But how does it flow? Three ways:• convection• conduction• radiation
![Page 3: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/3.jpg)
Convection• heat is carried from place to place by the
bulk movement of either liquids or gases• does not apply to solids• when water is boiled, hot liquid rises and
mixes with cooler liquid, thus the heat is transferred
• Hot air rises:• want heat into lower level of house (winter) • cooled air into upper levels (summer)
![Page 4: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/4.jpg)
Conduction• heat is transferred directly through a material,
with no bulk movement of stuff• only energy moves
iron is a particularlypoor conductor of heat
![Page 5: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/5.jpg)
heat conduction
HOT COLD
Heat Flow
Cross sectional
area A
L
Heat Flow rate depends on A / L
![Page 6: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/6.jpg)
Thermal Conductivity
• The effectiveness of a material in conducting heat is characterized by a parameter called the thermal conductivity
• there are good thermal conductors(metals) and poor ones (insulators)
Material Thermalconductivity
Copper 400
Silver 420Stainless steel 14
wood 0.15
glass 0.8
wool 0.04
Goose down 0.025
styrofoam 0.01
![Page 7: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/7.jpg)
Thermal Conductivities of Metals
Metal Thermal Conductivity
(W/mK)
Silver 406
Copper 385
Aluminum 205
Brass 109
Iron 80
Steel 50
![Page 8: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/8.jpg)
Grandma’s silver spoons
![Page 9: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/9.jpg)
radiation
• Radiation is the heat transfer by electromagnetic waves – thermal light waves - invisible to the eyes
• thermal radiation is a small part of the electromagnetic spectrum – waves are characterized by their frequency or wavelength
• different colors in the visible correspond to different wavelengths from red to blue
![Page 10: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/10.jpg)
electromagnetic spectrum
radio waves
microwaves,cell phones
visible x-rays
TV thermalradiation
![Page 11: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/11.jpg)
visible electromagnetic waves: LIGHT
visible lightthermal radiation UV radiation
produces sunburn
shorter wavelength more energy
![Page 12: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/12.jpg)
Thermal Radiation• The warmth you feel from
the sun is the sun’s thermal radiation
• It 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
![Page 13: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/13.jpg)
What produces thermal radiation?
• all objects whose temperature is above absolute zero emit thermal radiation
• The hotter the object, the more radiation it emits, the amount of radiation is ~ T4
• We all continuously emit thermal radiation• We also absorb it from objects and people
around us• If we just emitted radiation we would
eventually cool to absolute zero!
![Page 14: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/14.jpg)
Emission and Absorption are balanced
![Page 15: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/15.jpg)
Thermal radiation spectrum
• The intensity of radiation increases with temperature
• the color shifts toward the blue at higher temperatures
• The UV radiation from the sun is just beyond the violet (11,000 F)
![Page 16: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/16.jpg)
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 it
• as it gets hotter it glows red then orange then white
tungsten filament,can get very hot and not melt
evacuated glass bulb
![Page 17: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/17.jpg)
Radiation emitted by hot objects
• The hotter they are, the more they emit
• the efficiency with which an object emits thermal radiation is characterized be a parameter called its emissive e
• e is a number between 0 and 1
• a good emitter has an e close to 1
• a poor emitter has an e close to 0
![Page 18: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/18.jpg)
good emitters are good absorbers
• an object that is a good emitter is also a good absorber of thermal radiation
• a poor emitter is also a poor absorber
• generally dark, dull objects are the best emitters/absorbers
• shinny objects are poor emitters/absorbers
• If you do not want the edges of your pie to burn, you wrap it in aluminum foil
![Page 19: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/19.jpg)
good/bad emitters-Leslie’s cube
copper cubefilled with hot
water
this side ispainted black
infraredradiation sensor
![Page 20: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/20.jpg)
Practical considerations• wear light clothing
in summer light clothing absorbs less sunlight
• cover all body parts in winter warm body parts (like your head) emit radiation
![Page 21: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/21.jpg)
thermal radiation• all objects that are at a temperature above
absolute zero emit thermal radiation (waves)• the higher the temp, the more they emit• the color (wavelength) of the emitted waves
goes from redorangeyellow blue as the temperature increases
![Page 22: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/22.jpg)
seeing behind closed doors
Infrared sensorscan pick up temp-erature differencesof 0.05 degrees C.
we can “see” behind closed doors because of the heat signature left by warm objects on walls
![Page 23: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/23.jpg)
Which one is best?
A.silveredB. silvered and
un-evacuated
C. evacuatedD.un-silvered
andun-evacuated
![Page 24: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/24.jpg)
The Greenhouse effect
• http://earthguide.ucsd.edu/earthguide/diagrams/greenhouse/
C O 2
Sun’s visible light
infrared radiation is
trapped
![Page 25: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/25.jpg)
Greenhouse effect• the sun’s visible light can penetrate
through the atmosphere to the earth’s surface where it heats it
• the visible light energy is converted to thermal light energy
• the thermal radiation is reflected from CO2 in the atmosphere
![Page 26: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/26.jpg)
Greenhouse effect
• concentrations of CO2 have been increasing
rise in earth’s temperature
• same effect occurs in your car during the day.
![Page 27: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/27.jpg)
The ozone layer• ozone, O3 is a naturally
occurring trace element in the atmosphere
• It absorbs solar ultraviolet radiation, especially the harmful UV-B rays
• it is destroyed by Cfc’s (chlorofluorocarbons)
• loss affects us and environment
![Page 28: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/28.jpg)
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 it
• The answer would be different for a different material, say aluminum.
![Page 29: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/29.jpg)
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 Calories• for water it is 1 cal/g °C• heat Q = m • c • temp changehot plate
specific heatmass of sample
![Page 30: L 18 Thermodynamics [3] Heat transfer Heat transfer convectionconvection conductionconduction radiationradiation emitters of radiationemitters of radiation.](https://reader036.fdocuments.us/reader036/viewer/2022081504/5697c0151a28abf838ccdafc/html5/thumbnails/30.jpg)
Some heat capacities
Substance Specific heat in cal/g °C
water 1
Ethyl alcohol 0.58
Steel 0.11
Aluminum 0.215
lead 0.03