Chapter 11
Energy in Thermal Processes
!
U
=
Q
"
P
!
V
Vocabulary, 3 Kinds of Energy
•
Internal Energy
U = Energy of microscopic motion and inter
molucular forces
•
Work
W = F
!
x = P
!
V is work done by compression
(next chapter)
•
Heat
Q = Energy transfer from microscopic contact
next chapter
Q
=
mc
!
T
Temperature and Specific Heat
•
Add energy > T rises
•
c
H20
= 1.0 cal/(gºC)
•
1 calorie = 4.186 J
Property of material
Mass
Example 11.1
Bobby Joe drinks a 130 “calorie” can of soda. If the
efficiency for turning energy into work is 20%, how
many 4 meter floors must Bobby Joe ascend in order
to work off the soda and maintain her 55 kg mass?
N
floors
= 50.4
Example 11.2
Aluminum has a specific heat of .0924 cal/gºC. If 110
g of hot water at 90 ºC is added to an aluminum cup of
mass 50 g which is originally at a temperature of 23
ºC, what is the final temperature of the equilibrated
water/cup combo?
T = 87.3 ºC
Q
=
mL
Phase Changes and Latent Heat
•
T does not rise when phases change (at constant P)
•
Examples: solid > liquid (fusion), liquid > vapor
(vaporization)
•
Latent heat = energy required to change phases
Property of substance /transition
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View Full DocumentExample 11.3
1.0 liters of water is heated from 12 ºC to 100 ºC,
then boiled away.
a) How much energy is required to bring the water
to boiling?
b) How much extra energy is required to vaporize
the water?
c) If electricity costs $75 per MWhr, what was
the cost of heating and boiling the water?
a) Q = 8.8x10
4
cal = 3.68x10
5
J
b) Q = 5.4x10
5
cal = 2.26x10
6
J
c)
5.5 ¢
Example 11.4
Consider Bobby Joe from the previous example. If
the 80% of the 130 kcals from her soda went into
heat which was taken from her body from
radiation, how much water was perspired to
maintain her normal body temperature? (Assume a
latent heat of vaporization of 540 cal/g even
though T = 37 ºC)
= 193 g
A can of soda has ~ 325 g of H
2
0
Some fluid drips away
Three Kinds of Heat Transfer
•
Conduction
•
Shake your neighbor  pass it down
•
Examples: Heating a skillet, losing heat
through the walls
•
Convection
•
Move hot region to a different location
•
Examples: Hotwater heating for buildings
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 Spring '06
 Pratt
 Physics, Thermodynamics, Energy, Force, Work, Heat, Heat Transfer

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