Download Heat Transfer Mechanisms

Heat Transfer Mechanisms
Conduction
Convection
Radiation
Homework
Conduction
Conduction is energy transfer due to an exchange of kinetic energy between molecules
–
Less energetic molecules gain energy by colliding with more energetic molecules
Some substances are good thermal conductors while others are poor
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Metals are good conductors because they contain large numbers of electrons that are free to
move and transport energy
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Materials such as asbestos, cork, and fiber glass are poor conductors
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Gases are poor thermal conductors because the molecules are relatively far apart
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In general, materials that are good electrical conductors are also good thermal conductors
Rate of Energy Transfer by Conduction
with its opposite faces at
and cross-sectional area
as shown below
Consider a slab of material of thickness
different temperatures
and , where
The rate of energy transfer by heat is
has units of watts.
where
, we can write the law of
and temperature difference
For a slab of infinitesimal thickness
conduction as
is the thermal conductivity of the material and
where
is the temperature gradient.
Example
(a) Calculate the rate at which body heat is conducted through the clothing of a skier in a steady-state
process, given the following data: the body surface area is 1.80 m and the clothing is 1.00 cm thick;
the skin surface temperature is 33.0 C and the outer surface of the clothing is at 1.00 C; the thermal
conductivity of the clothing is 0.040 W/m K. (b) How would the answer to (a) change if, after a fall,
the skier’s clothes became soaked with water of thermal conductivity 0.600 W/m K?
Convection
Convection is energy transfer by motion within a fluid
An example is the way a room is heated by a radiator
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The temperature of the air in contact with the radiator increases
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The warm air expands and becomes less dense
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Because the warm air is now lighter than the surrounding cooler air, buoyant forces cause it to
rise
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The surrounding cooler air then flows to take the place of the rising warm air
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The warm air sinks as it cools, setting up a continuous air current
Radiation
Radiation is energy transfer via the emission of electromagnetic energy
The rate at which an object emits energy via thermal radiation is known as Stefan’s law and can
be expressed as
is the power radiated by the object in watts
–
is called the Stefan-Boltzmann constant
is the surface area of the object
is a constant called the emissivity
–
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–
–
is the surface temperature of the body
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At the same time that it radiates, an object also absorbs electromagnetic radiation from its environment
The net rate of energy change for the object due to radiation is
0
/
.
-
*+,
is the temperature of the environment
/
where
Example 2
A sphere of radius 0.500 m, temperature 27.0 C, and emissivity 0.850 is located in an environment
of temperature 77.0 C. At what rate does the sphere (a) emit and (b) absorb thermal radiation? (c)
What is the sphere’s net rate of energy exchange?
Homework Set 7 - Due Fri. Jan. 23
Read Section 17.10
Answer Question 17.21
Do Problems 17.44, 17.48, 17.49 & 17.51