Heat Transfer by
Conduction
d
Q
T
P   kA
t
d
Question
Why do citrus farmers ice over their crop
before a frost?
Water is a poor thermal
conductor so the heat can’t
easily get in or out.
Similar idea: igloos
Example #1
How much heat was lost in 1 hr by
conduction through a concrete wall 2 m
high and 3.65 m long and 20 cm thick if
one side of the wall is at 20°C and the
outside is 5°C? (find Q, K= 0.8)
Q
T
P   kA
t
d
Example #1
Time=1 hr
Q
T
P   kA
d= 20cm = 0.20 m
t
d
2
Area = 2 *3.65 = 7.3m
T = 20- 5 = 15°C
K= 0.8 Watts/mC for concrete
P= 0.8*7.3*15/.2 = 438 J/sec
Q= Pt = 438 J/sec * 3600 sec = 1.58x106 J
Example #2a
An aluminum rod (k=238) and iron rod (k=80) are
joined end to end in good thermal contact.
The two rods have equal lengths and radii.
The free end of the aluminum is maintained at
100°C and the free end of the iron is
maintained at 0°C.
Determine the temperature of the interface of the
two rods.
Example #2a
100°C
T
0°C.
Heat flow from either side is same if temp constant
Q/t
= Q/ t
kAT/d
=
kAT/d
238*A* (100-T)/d
= 80*A*(T-0)/d
23800- 238T
= 80T
74.8 = T
Heat Transfer by Convection
Heat Transfer by Radiation

Q
4
4
Pnet   Ae Ts  T
t

  5.6696 108 W
2
mK
4
Question
If black is such a good absorber of heat, why
do some desert nomads use dark colored
robes?
Question
If you sit in front of a fire in a fireplace with
your eyes closed, you feel significant
warmth on your eyelids. If you now put
on a pair of eyeglasses
and repeat this activity,
your eyelids will not feel
warm. Why???
Example #1
Suppose your skin has an emissivity of 0.7
and that has an exposed skin area of 0.27
m2. How much energy will be radiated per
second from this area if the ambient room
temperature is 20°C.
Greenhouse Effect
(a) The greenhouse gases of the atmosphere, particularly water
vapor and carbon dioxide, are selective absorbers with absorption
properties similar to those of glass used in greenhouses. Visible light
is transmitted and heats the surface, while some of the infrared
radiation that is re-emitted is absorbed. (b) A greenhouse operates in
a similar way.