convection conduction radiation - uab college of arts and...
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Chapter 13 Transfer of Heat
Three mechanisms for transfer heat from hotter object to
cooler object:
• Convection
• Conduction
• Radiation
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Convection = process by which heat is carried from place to
place by the movement of a fluid (liquid or gas)
Convection currents =
actual movement of fluid
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Physics of heating a pot of water by convection
convection currents are set up when water is heated
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surrounding cooler and denser air pushes heated and less
dense air upwards; resulting updraft called a “thermal”
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Additional examples of (natural) convection
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Example of forced convection
cool air duct
warm air duct
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conduction
Conduction = process by which heat is transferred directly
through a material with no bulk motion of material;
energy (heat) is transferred through collisions of more
energetic atoms with their less energetic neighbors
in metals, a pool of electrons is also available to transport
energy throughout the metal and correspondingly metals
transfer heat very well
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thermal conductors = materials that conduct heat well
such as metals (silver, copper, aluminum,
steel) and diamond
thermal insulators = materials that do not conduct heat well
such as wood, glass, & most plastics
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What factors will determine the amount of heat Q
conducted from the warmer body to the cooler body?
• temperature difference DT
• time t
• cross-sectional area A
• length L
= thermal conductivity k
• thermal conducting property
of material
A
( 1/L)
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CONDUCTION OF HEAT THROUGH A MATERIAL
heat Q conducted during a time t through a bar of length
L and cross-sectional area A is
L
tTkAQ
Δ
SI Units of thermal conductivity: J/(s·m·oC)
thermal
conductivity
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excellent thermal
insulator
excellent thermal
conductors
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Materials such as styrofoam,
goose, and wool contain small,
dead-air spaces.
Net effect is very poor thermal
conductor (excellent thermal
insulator).
Dead-air spaces inhibit heat
transfer by convection currents,
and air itself has very low
thermal conductivity.
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QUESTION: Find the amount of heat per second
conducted through a glass picture window (1.5 m x 2.0 m)
that is 4.0-mm thick when the outside temperature is 5oC
(41 F) and the room is at 22.2oC (72 F)?
SOLUTION:
2.0 m
1.5 m
4.0 mm
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Assuming that the Al and Cu rods shown below have
identical cross-sectional areas A and lengths L, which
arrangement is the greater heat conducted?
(a) Heat conducted is the same in both arrangements.
(b) Arrangement A
(c) Arrangement B
(d) Not possible to determine without more information.
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QAl
QCu
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QAl QCu
Tm
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QUESTION: One wall of a house consists of plywood
backed by insulation. The thermal conductivities of the
insulation and plywood are, respectively, 0.030 and 0.080
J/(s·m·oC), and the area of the wall is 35 m2.
Find the amount of heat
conducted through the wall
in one hour.
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SOLUTION
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radiation = process in which
energy is transferred by means of
electromagnetic waves
A material that is a good absorber
is also a good emitter.
A material that absorbs
100% of the received
radiation is called a
perfect blackbody.
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THE STEFAN-BOLTZMANN LAW OF RADIATION
radiant energy Q, emitted in a time t by an object that has a
Kelvin temperature T, a surface area A, and an emissivity e, is
given byAtTeQ 4
emissivity e = dimensionless number between zero and one
= ratio of what an object radiates to what the
object would radiate if it were a perfect emitter
Stefan-Boltzmann constant
481067.5 K
2msJ
e = 1.0 for blackbody radiator
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QUESTION:
The supergiant star Betelgeuse has a surface
temperature of about 2900 K and emits a power of
approximately 4x1030 W (1 W = 1 J/s).
Assuming that Betelgeuse is a perfect emitter and
spherical, find its radius.
SOLUTION:
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QUESTION: One way that heat is transferred from place
to place inside the human body is by the flow of blood.
Which one of the following heat transfer processes best
describes this flow of blood?
(a) forced convection
(b) conduction
(c) radiation
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Label mechanism for transfer of heat to the appropriate arrow?
AB
C
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ratings of thermal insulation = R-values
R
TA
kL
TA
L
TkAtQ
D
D
D
//
R value = L/k
ADDITIONAL APPLICATIONS OF HEAT TRANSFER
larger R values reduce heat per unit time Q/t flowing
through material and, therefore, mean better insulation
increase R by increase thickness (length) of insulation or
using a material (combination of materials in series) with
smaller thermal conductivities k
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Thermos bottle minimizes heat transfer
via conduction, convection, and radiation.
Vacuum space
reflects radiation
no air convection
poor conductor
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Farmers spray strawberry
plants with water to put a coat
of ice (thermal insulator) and
insulate them against sub-
freezing temperatures.
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Chapter 14.4 Diffusion
diffusion = process in which molecules move from a region
of higher concentration to one of lower concentration
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Why Diffusion is Relatively Slow
A gas molecule has a translational rms speed of hundreds of
meters per second at room temperature. At such speed, a
molecule could travel across an ordinary room in just a fraction
of a second. Yet, it often takes several seconds, and sometimes
minutes, for the fragrance of a perfume to reach the other side
of the room. Why does it take so long?
zigzag path result of
millions of collisions =
Brownian motion
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A Transdermal Patch
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Diffusion
Conduction
Similarity between Diffusion and Conduction
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L
tCDAm
D
FICK’S LAW OF DIFFUSION
mass m of solute that diffuses in a time t through a solvent
contained in a channel of length L and cross sectional area A
concentration gradient
between ends, DCdiffusion
constant, D
SI Units for the Diffusion Constant: m2/s
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Water Given Off by Plant Leaves
Large amounts of water can be given
off by plants. Inside the leaf, water
passes from the liquid phase to the
vapor phase at the walls of the
mesophyll cells.
The diffusion constant for water is
2.4x10-5m2/s. A stomatal pore has a
cross sectional area of about
8.0x10-11m2 and a length of about
2.5x10-5m. The concentration on the
interior side of the pore is roughly
0.022 kg/m3, while that on the outside
is approximately 0.011 kg/m3.
Determine the mass of water that
passes through the stomatal pore in
one hour.
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SOLUTION: