Heat Transfer

1
Heat Transfer

• How does the energy move from a hotter to a
  colder object?
• Three mechanisms
• Conduction
• Convection
• Radiation

2
Heat Transfer

• There are three ways in which heat can be
  transferred from one object to another
• Conduction when two objects are in physical
  contact.

k thermal conductivity Q heat transferred A
cross sectional area t duration of
heat transfer L length
DT temperature difference between two ends
In a hot oven the air and the metal rack are at
the same temperature, but which one feels hotter
and why?
3
Thermal Conductivities, Table 16-3

• Metals have high thermal conductivity, most
  electrical insulators also have low thermal
  conductivity.
• Air is a great insulator, except that large air
  spaces allow heat flow by convection.

4
Convection and Radiation

• Convection when heat is carried by a moving
  fluid
• Example heat house with radiator
• Gulf stream transports Heat from Caribbean to
  Europe
• Radiation when electromagnetic waves
  (radiation) carry heat from one object to
  another.
• Example heat you feel when you are near a fire
• Example Heat from the sun
• Formation of frost (ice) at night, T(air) gt 0ºC

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Heat transfer
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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.
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Conduction
8
Conduction Of Heat Through A Material
9
Conduction
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 ?T is the
temperature difference between the two ends.
SI Unit of Thermal Conductivity J/(s m C)
10
High conductivity
High conductivity
High conductivity
11
Styrofoam
12
Why is the mother shielding her cub?
13
Owens Corning
14
How do fish in a lake survive?
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.
15
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.
16
Volcanic Eruption
During a volcanic eruption, smoke at the top of
the plume rises thousands of meters because of
convection.
17
Convection
Explains why breezes come from the ocean in the
day and from the land at night
18
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.
19
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.
20
Radiation

• Energy carried by electromagnetic waves
• Light, microwaves, radio waves, x-rays
• Wavelength is related to vibration frequency

21
Radiation
22
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.
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Suntans
Suntans are produced by radiation, ultraviolet
rays.
24
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.
25
A White sifaka Lemur
To warm up in the morning, they turn their dark
bellies toward the sun.
26
Why is the mother shielding her cub?
Havent we seen this before?
Ratio of the surface area of a cub to its volume
is much larger than for its mother.
To cool food, we cut it into smaller pieces, why?
27
The StefanBOLTZMANN 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 Stefans
law and is expressed as follows, where s is the
Stefan-Boltzmann constant, s 5.67 ? 10-8
W/m2.K4.
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.
28
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.
29
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.
30
Metal foil
Highly reflective metal foil covering this
satellite minimizes heat transfer by radiation.
31
Conduction

• There are three ways in which heat can be
  transferred from one object to another
• Conduction when two objects are in physical
  contact.

k thermal conductivity Q heat transferred A
cross sectional area t duration of
heat transfer L length
DT temperature difference between two ends
In a hot oven the air and the metal rack are at
the same temperature, but which one feels hotter
and why?
32
Thermal Conductivities, Table 16-3

• Metals have high thermal conductivity, most
  electrical insulators also have low thermal
  conductivity.
• Air is a great insulator, except that large air
  spaces allow heat flow by convection.

33
Convection and Radiation

• Convection when heat is carried by a moving
  fluid
• Example heat house with radiator
• Gulf stream transports Heat from Caribbean to
  Europe
• Radiation when electromagnetic waves
  (radiation) carry heat from one object to
  another.
• Example heat you feel when you are near a fire
• Example Heat from the sun
• Formation of frost (ice) at night, T(air) gt 0ºC

34
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.
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Microscopic reasons why a conductor is a conductor
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In contrast to conductors, insulators are
materials which impede the free flow of electrons
from atom to atom and molecule to molecule. If
charge is transferred to an insulator at a given
location, the excess charge will remain at the
initial location of charging.
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In a conductor, electric current can flow freely,
in an insulator it cannot. Metals such as copper
typify conductors, while most non-metallic solids
are said to be good insulators, having extremely
high resistance to the flow of charge through
them. "Conductor" implies that the outer
electrons of the atoms are loosely bound and free
to move through the material. Most atoms hold on
to their electrons tightly and are insulators. In
copper, the valence electrons are essentially
free and strongly repel each other. Any external
influence which moves one of them will cause a
repulsion of other electrons which propagates,
"domino fashion" through the conductor. Simply
stated, most metals are good electrical
conductors, most nonmetals are not. Metals are
also generally good heat conductors while
nonmetals are not.
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This website has a very good introduction.
However, it is in form of a flash movie, so I
cannot put it on a PowerPoint presentation. I
believe it can be done, but I do not know how.
http//www.thetech.org/exhibits/online/topics/10a.
html
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Black Body Radiation

• Any object heated to a temperature T (on an
  absolute scale) radiates Electromagnetic Energy
  (light) with total power
• P e s A T4
• 0ltelt1 emissivity property of material
• s 5.67 10 8 W/(m2 K4)
• A surface area of object
• Early triumph of quantum theory (M. Planck) to
  predict this equation, including the value of s.
• Peak wavelength occurs at l (5.110-3 m K ) /
  T (Chap 30)
• If the surroundings have temperature TS, then the
  net power radiated is
• P e s A T4 - TS4
• Dark, dry, night, TS 3 K, Black body radiation
  cools the surface faster than conduction can
  transport heat from the ground or air. Frost can
  form even if air temperature gt 0C.