Heat Transfer and Energy AOS 101

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Heat Transfer and Energy AOS 101
Section 302 Ross A. Lazear February 20, 2007
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Review
What is heat?
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Review
What is heat?

• Heat is energy in the process of being
  transferred from one object to another because of
  the temperature difference between them.
• 2nd law of thermodynamics Heat is always
  transferred from warm to cold objects so as to
  warm the originally cooler object and cool the
  originally warmer object.
• Heat is measured in Joules.

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Review

• Two liters of boiling water has more heat
  (energy) than one liter of boiling water
• Heat will not flow between two objects of the
  same temperature
• The transfer of heat is always from a high
  temperature object to a lower temperature object.
  Heat transfer changes the internal energy of
  both systems involved, as in the first law of
  thermodynamics (conservation of energy).

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Specific Heat

• Specific heat Amount of heat energy required
  to raise the temperature of 1 g of a substance 1
  oC.
• An object with a high specific heat is an object
  that requires a large amount of heat energy in
  order to change its temperature (an example of
  this is water, with a specific heat of 1.00 cal
  g-1 oC-1)

• A good example of this is a sand beach

http//outside.away.com/images/travel_photo_galler
y/tropical_escapes_ss/image1.jpg
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Review
Heat can be transferred by
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Review
Heat can be transferred by

• CONDUCTION
• CONVECTION
• ADVECTION
• RADIATION

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Conduction

• Transfer of heat through a material substance,
  molecule by molecule
• If you were to put a metal rod over a flame,
  radiation from the flame will first transfer
  energy from the flame to the rod.

• Then, the molecules in direct contact with the
  flame will gain energy and vibrate faster
  (remember the definition of temperature!)
• The heated molecules will come in contact with
  nearby molecules, and gradually spread the heat
  through the material.

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Conduction
The measurement of how well (or how fast) a
material can transfer heat through conduction
depends on how the materials molecules are
structurally bounded together.
Substance Heat Conductivity (W/mK)
Still air at 20o C 0.023
Dry soil 0.25
Water at 20o C 0.6
Snow 0.63
Mud 2.1
Ice 2.1
Granite 2.7
Iron 80
Silver 427
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Conduction

• Air is a very poor conductor of heat, which is
  why styrofoam cups are used to hold hot coffee.
• In general, metals have very large
  conductivities. Thus, when they are heated,
  molecules will rapidly transfer heat through the
  metal.
• So, if air is such a poor conductor of heat, how
  does the air in the lower atmosphere heat up so
  quickly when the sun comes out in the morning?

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Convection

• Convection Heat transfer by the mass movement
  of a fluid in the vertical, occurring in liquids
  and gases (fluids).
• Warm air is less dense than cold air at the same
  atmospheric pressure (Ideal Gas Law!)
• Think of cold air as being heavier. Thus, when
  the Earths surface is heated by solar
  (shortwave) radiation, heat conduction occurs.
  The thin layer of air molecules at the Earths
  surface is heated through conduction. This must
  begin to rise, because warm air is less dense
  than cold air.
• Remember the Ideal Gas Law . . .

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Convection

• Pressure always decreases with increasing height
  in the atmosphere
• Recall, p rRT
• For a rising parcel of warm air, think of the
  parcel as a balloon.
• As the parcel rises, lower pressure will exert
  less of a force on the parcels wall. This
  will allow the parcel to expand, and the
  temperature must then decrease.
• Thus, a warm parcel will cool as it is lifted!
  This parcel is then displaced by other
  thermals. The process starts all over again,
  and a convective circulation arises.

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Convection

• The cooling rate of a DRY thermal (the rising
  portion of the convective circulation) can
  actually be calculated

If a thermal consists of dry air, it will always
cool at a rate of . . .
9.8 oC/km
If a thermal is moist, the rate of cooling due to
ascent varies based on the amount of water vapor
the thermal contains (which depends on
temperature).
http//www.flyaboveall.com
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Convection

• This is the same process that produces fair
  weather cumulus clouds and severe thunderstorms!

http//en.wikipedia.org/wiki/Cumulus_humilis
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Convection
Thanks to youtube, here are some great examples
of atmospheric convection
http//www.youtube.com/watch?v_cl0aw87LqA
http//www.youtube.com/watch?v1arvRoQBdWc
http//www.youtube.com/watch?vYXMVEpYRqyo
http//www.youtube.com/watch?vhV60nvuc7jc
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Advection

• The transfer of heat by wind

• The figure above demonstrates warm air
  advection.
• Warm air is being advected eastward by the
  surface wind.
• Over time, we expect the temperature to warm
  over the trees.

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Advection
Locate regions of warm advection, cold advection,
and neutral advection
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Radiation

• Transferred through wave energy
  Electromagnetic waves
• Shorter wavelengths carry more energy than
  longer wavelengths
• The wavelengths of the radiation emitted by an
  object depends on the temperature of that object
  (i.e., the sun mainly emits radiative energy in
  the visible spectrum, and the earth emits
  radiative energy in the infrared spectrum).

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Radiation

• A photon of ultra-violet radiation carries more
  energy than a photon of infrared radiation.
• The shortest wavelengths in the visible spectrum
  are purple, and the longest wavelengths are red.

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Radiation

• Emitted radiation can be
• Absorbed
• Increasing the internal energy of the gas
  molecules.
• Reflected
• Radiation is not absorbed or emitted from an
  object but it reaches the object and is sent
  back. The Albedo represents the reflectivity of
  an object and describes the percentage of light
  that is sent back.
• Scattered
• Scattered light is deflected in all directions,
  forward, backward, sideways. It is also called
  diffused light.
• Transmitted
• Radiation not absorbed, reflected, or scattered
  by a gas, the radiation passes through the gas
  unchanged.

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Summary