L 18 Thermodynamics [3] Heat transfer convection conduction radiation emitters of radiation seeing...
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Transcript of L 18 Thermodynamics [3] Heat transfer convection conduction radiation emitters of radiation seeing...
L 18 Thermodynamics [3]
• Heat transfer• convection • conduction• radiation
• emitters of radiation• seeing behind closed doors• Greenhouse effect• global warming
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)
Conduction• heat is transferred directly through a material,
with no bulk movement of stuff• only energy moves
iron is a particularlypoor conductor of heat
heat conduction
HOT COLD
Heat Flow
Cross sectional
area A
L
Heat Flow rate depends on A / L
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
Thermal Conductivities of Metals
Metal Thermal Conductivity
(W/mK)
Silver 406
Copper 385
Aluminum 205
Brass 109
Iron 80
Steel 50
Grandma’s silver spoons
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
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
electromagnetic spectrum
radio waves
microwaves,cell phones
visible x-rays
TV thermalradiation
visible electromagnetic waves: LIGHT
visible lightthermal radiation UV radiation
produces sunburn
shorter wavelength more energy
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
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!
Emission and Absorption are balanced
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)
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
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 emissivity 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
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,
they are good reflectors of radiation• If you do not want the edges of your pie to
burn, you wrap it in aluminum foil
good/bad emitters-Leslie’s cube
copper cubefilled with hot
water
this side ispainted black
infraredradiation sensor
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
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
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
Which one is best?
A.silveredB. silvered and
un-evacuated
C. evacuatedD.un-silvered
andun-evacuated
Why are the poles colder than the equatorial regions?
• More of the Sun’s energy per unit area falls on the equatorial regions compared to the polar regions
• the earth reflects about 30% of incident solar energy• without the atmosphere the earth would be 30C cooler!
SU
N
The Greenhouse effect
• http://earthguide.ucsd.edu/earthguide/diagrams/greenhouse/
C O 2
Sun’s visible light
infrared radiation is
trapped
Effect of CO2
• 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
Greenhouse effect• concentrations of CO2 have been increasing
rise in earth’s temperature• same effect occurs in your car during the day.
James Hansen
• NASA Goddard Institute for Space Studies,
Columbia University, NY
• Born 1941 Denison, IA• PhD 1967 Univ. Iowa• MS 1965 Univ. Iowa• BA 1963 Univ. Iowa
http://www.columbia.edu/~jeh1/dots_feb2007.ppt
Temperature change 1880-2003
1960-2020
Global warming issues• 88,800,000 sites on Google• http://
www.hillsdale.edu/news/imprimis/archive/issue.asp?year=2007&month=08
• http://www.epa.gov/climatechange/• http://ipcc-wg1.ucar.edu/wg1/wg1-report.html• Is the buildup of Greenhouse gases due to human activity
(anthropogenic)?• (NRC 2001) Because of the large and still uncertain level
of natural variability inherent in the climate record and the uncertainties in the time histories of the various forcing agents (and particularly aerosols), a causal linkage between the buildup of greenhouse gases in the atmosphere and the observed climate changes during the 20th century cannot be unequivocally established.
• The IPCC (International Panel on Climate Change) 2/2/07: “global warming is “very likely” caused by man.
• buy cars with high mpg ratings and use fluorescent lights
The ozone layer- a related but different issue
• 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