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C H A P T E R 13
The Transfer of Heat
Q: Name three methods of heat transfer?
A: Convection, Conduction, and Radiation.
13.1 Convection
Convection is the process in which heat is carried from place
to place by the bulk movement of a fluid.
Convection currents are set up
when a pan of water is heated.
During a volcanic eruption,
plume of smoke rises because of
convection.
Q: In the living room, the heating unit is placed in the floor but the
the refrigerator has a top-mounted cooling coil. Why?
A: Air warmed by the baseboard heating unit is pushed to the top of
the room by the cooler and denser air. Air cooled by the cooling coil
sinks to the bottom of the refrigerator.
13.2 Conduction
Conduction is the process whereby heat is transferred directly
through a material, any bulk motion of the material playing no role
in the transfer.
Those materials that conduct heat well are called thermal
conductors, while those that conduct heat poorly are known as
thermal insulators.
Metals are excellent thermal conductors.
Wood, glass, and most plastics are thermal insulators.
The free electrons in metals are responsible
for the excellent thermal conductivity of metals.
Conduction: Theory
Rate of heat transfer by conduction, Q/t through the length, L
across the cross-sectional area, A is given by the following
equation, where k is the thermal conductivity and ΔT is the
temperature difference between the two ends.
Q kAT

.
t
L
SI Unit of Thermal Conductivity: J/(s · m · C°)
Table 13.1: Thermal Conductivities of
Selected Materials
Thermal Conductivity, k [J/(s · m · C°)]
Metals
Other Materials
Aluminum
240
Brass
110
Copper
390
Iron
79
Lead
35
Silver
Steel
(stainless)
Asbestos
0.090
Body fat
0.20
Concrete
1.1
Diamond
2450
420
Glass
0.80
14
Goose down
0.025
Ice
2.2
Styrofoam
0.010
Gases
Air
0.0256
Water
0.60
Hydrogen
0.180
Wood (oak)
0.15
Nitrogen
0.0258
Oxygen
0.0265
Wool
0.040
Styrofoam
Styrofoam is an excellent
thermal insulator because it
contains many small, dead-air
spaces. These small spaces
inhibit heat transfer by
convection currents, and air
itself has a very low thermal
conductivity.
Fruit growers sometimes spray water to protect their
crops against freezing. After a subzero night, these
berries are visible in their insulating jackets of ice.
13.3 Radiation
Radiation is the process in which energy is transferred by means
of electromagnetic waves.
Heat transfer by radiation can take place through vacuum. This is
because electromagnetic waves are involved in radiation and
they can propagate through empty space.
Suntans
Suntans are produced by radiation, ultraviolet rays.
Black Body
The temperature of the lampblack
block rises faster than the silver block
because the black surface absorbs
radiant energy from the sun at a
greater rate.
Since absorption and emission are
balanced, a material that is a good
absorber, like lampblack, is also a
good emitter, and a material that is a
poor absorber, like polished silver, is
also a poor emitter.
A perfect blackbody, being a perfect
absorber, is also a perfect emitter.
Summer Clothing
Q: People are uncomfortable wearing dark clothes during
the summer. Why?
A: Dark clothes absorb a large fraction of the sun's radiation
and then reemit it in all directions. About one-half of the
emitted radiation is directed inward toward the body and
creates the sensation of warmth. Light-colored clothes, in
contrast, are cooler to wear, since they absorb and reemit
relatively little of the incident radiation.
The Stefan–BOLTZMANN
Law Of Radiation
The rate at which an object emits radiant energy is proportional to
the fourth power of its absolute temperature.
Q
 P  eAT 4 .
t
σ is the Stefan-Boltzmann constant: σ = 5.67  10-8 W/m2.K4.
e is called the emissivity: a number between 0 and 1.
For perfect radiators e = 1.
A is the surface area.
T is the temperature of the radiator in Kelvin.
Net Radiant Power
The net power is the power the object emits minus the power
it absorbs. Applying the Stefan-Boltzmann law leads to the
following expression for Pnet when the temperature of the
object is T and the temperature of the environment is T0:
Q
4
4
 Pnet  eA(T  T0 ).
t
Home Insulation: R Value
The term L/k in the denominator is called the R value of the
insulation.
In the US R-values are expressed in British units, ft2.h.F0/Btu.
For example, the 6.0 inch thick fiber-glass batting has an Rvalue of 19 ft2.h.F0/Btu.
Regulating the temperature of
an orbiting satellite
The highly reflective metal foil covering this
satellite (the Hubble Space Telescope) minimizes
radiation temperature changes.
Thermos Bottle
A thermos bottle minimizes energy transfer
due to convection, conduction, and radiation.
Stopper- minimize conduction.
Double-walled glass vessel with the space
between the walls is evacuated to minimize
energy losses due to conduction and
convection.
The silvered surfaces reflect most of the
radiant energy that would otherwise enter or
leave the liquid in the thermos.
Halogen Cooktop
In a halogen cooktop, quartz-iodine lamps emit a large amount of
electromagnetic energy that is absorbed directly by a pot or pan.