Transcript C H A P T E R 13 The Transfer of Heat

C H A P T E R 13
The Transfer of Heat
The Transfer of Heat
Q: Name three methods of heat transfer?
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.
Boiling Water
Convection currents are set up when a pan of water is heated.
During a volcanic eruption, smoke at the top of the
plume rises thousands of meters 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?
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.
Most metals are excellent thermal conductors, while wood,
glass, and most plastics are common thermal insulators.
The free electrons in metals are responsible for the excellent
thermal conductivity of metals.
Conduction Of Heat
Through A Material
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°)
Thermal Conductivitiesaa of Selected Materials
Substance
Thermal Conductivity, k [J/(s · m ·
C°)]
Metals
Aluminum
Brass
Copper
Iron
240
110
390
79
Lead
Silver
Steel (stainless)
35
420
14
Gases
Air
Hydrogen (H2)
0.0256
0.180
Styrofoam
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
Black Body
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.
Summer Clothing
Q: People are uncomfortable wearing dark clothes during
the summer. Why?
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.
A White sifaka Lemur
To warm up in the morning, they turn their dark bellies
toward the sun.
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.
This is known as Stefan’s law and is expressed as follows,
where σ is the Stefan-Boltzmann constant, σ = 5.67  10-8
W/m2.K4.
Q
4
 P  eAT .
t
The factor e is called the emissivity, which is a number between
0 and 1. Perfect radiators have a value of 1 for e. A is the surface
area and T is the temperature of the radiator in Kelvin.
The net power is the power the object emits minus the power
it absorbs. Applying the Stefan-Boltzmann law as in Example
6 leads to the following expression for Pnet when the
temperature of the object is T and the temperature of the
environment is T0:
13.4 Applications
1. Rating thermal insulation by R values.
2. Regulating the temperature of an orbiting satellite.
3. A thermos bottle.
4. A halogen cooktop stove.
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.
Owens Corning
Owens Corning
Rafters and Collar Beams
Recommended Insulation Products:
R-21
Fiber Glass Insulation
5 1/2" Thick
R-19
Fiber Glass Insulation
6 1/4" Thick
R-38C
2 x 12 Cathedral Ceiling Fiber Glass Insulation
10 1/4" Thick
R-30C
2 x 10 Cathedral Ceiling Fiber Glass Insulation
8 1/4" Thick
Highly reflective metal foil covering this satellite minimizes heat
transfer by radiation.
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.