L 18 Thermodynamics [3]

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L 18 Thermodynamics 3

• Heat transfer
• convection
• conduction
• radiation
• emitters of radiation
• seeing behind closed doors
• Greenhouse effect
• Heat Capacity
• How to boil water

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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

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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)

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Conduction

• heat is transferred directly through a material,
  with no bulk movement of stuff
• only energy moves

iron is a particularly poor conductor of heat
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heat conduction
Cross sectional area A
L
HOT
COLD
Heat Flow
Heat Flow rate depends on A / L
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Thermal Conductivity
Material Thermalconductivity
Copper 400
Silver 420
Steel (stain.) 14
wood 0.15
glass 0.8
wool 0.04
Goose down 0.025
styrofoam 0.01

• 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)

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Grandmas silver spoons
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? radiation
Heat as moving light

• 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

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electromagnetic spectrum
thermal radiation
microwaves, cell phones
TV
radio waves
x-rays
visible
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visible electromagnetic waves LIGHT
shorter wavelength ? more energy
visible light
thermal radiation
UV radiation produces sunburn
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Thermal Radiation

• The warmth you feel from the sun is the suns
  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

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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!

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Emission and Absorption are balanced
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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

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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 cant 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
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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

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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

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good/bad emitters-Leslies cube
infrared radiation sensor
copper cube filled with hot water
this side is painted black
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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

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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 red?orange?yellow? blue as the temperature
  increases

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seeing behind closed doors
we can see behind closed doors because of the
heat signature left by warm objects on walls
Infrared sensors can pick up temp- erature
differences of 0.05 degrees C.
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The Greenhouse effect
Suns visible light
infrared radiation is trapped
C O2
EARTH

• http//earthguide.ucsd.edu/earthguide/diagrams/gre
  enhouse/

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Greenhouse effect

• the suns visible light can penetrate through the
  atmosphere to the earths 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

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Greenhouse effect

• concentrations of CO2 have been increasing
• ? rise in earths temperature
• same effect occurs in your car during the day.

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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 Cfcs (chlorofluorocarbons)
• loss affects us and environment

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back to some practical topics
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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.

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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 change

hot plate
mass of sample
specific heat
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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