Transcript Heat Transfer and Energy

Heat Transfer and Energy
101 – Section 302
Ross A. Lazear
February 20, 2007
AOS
Review
What is heat?
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.”
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).
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_gallery/tropical_escapes_ss/image1.jpg
Review
Heat can be transferred by:
Review
Heat can be transferred by:
 CONDUCTION
 CONVECTION
 ADVECTION
 RADIATION
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.
Conduction
The measurement of how well (or
how fast) a material can transfer heat
through conduction depends on how
the material’s 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
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?
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 Earth’s surface is heated by solar
(shortwave) radiation, heat conduction occurs. The thin layer of air molecules at the
Earth’s surface is heated through conduction. This must begin to rise, because warm air is
less dense than cold air.
• Remember the Ideal Gas Law . . .
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 parcel’s “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.
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
Convection
• This is the same process that produces fair weather cumulus
clouds and severe thunderstorms!
http://en.wikipedia.org/wiki/Cumulus_humilis
Convection
Thanks to youtube, here are some great examples of atmospheric convection:
http://www.youtube.com/watch?v=_cl0aw87LqA
http://www.youtube.com/watch?v=1arvRoQBdWc
http://www.youtube.com/watch?v=YXMVEpYRqyo
http://www.youtube.com/watch?v=hV60nvuc7jc
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.
Advection
Locate regions of warm advection, cold advection, and neutral advection
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).
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.
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.
Summary