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Ph 111.01
December 5, 2008
PS 13c (35)
Problem 1 = Walker P. 1622.
When people sleep, their
metabolic rate is about 2.6
10
4
C/(s
·
kg). How many Calories
does a
63
kg person metabolize while sleeping
7.0
hours?
Cal
Q=RMt
Solution.
The rate constant given implies the equation
below.
()
( )
4
0 Cal/(skg) 63 kg
7 3600 s
413 Cal
x
x
−
=
2.6 1
QR
M
t
==
Problem 2 = Walker P. 1636.
A blacksmith drops a
0.45
kg iron
horseshoe into a bucket containing 25 kg of water.
m
h
m
w
T
w
T
h
(a) If the initial temperature of the horseshoe is
475
°C, and the initial
temperature of the water is
45
°C, what is the equilibrium temperature of the
system? Assume no heat is exchanged with the surroundings.
°C
(b) Suppose the
0.45
kg iron horseshoe had been a
2.0
kg lead horseshoe
instead. Would the equilibrium temperature in this case be greater than, less
than, or the same as in part (a)?
Select
Solution.
(a)
We use the “conservation of heat” equation,
( )
h
h
www
h
ww
hh
mcT mcT
mc
mc T
+=
+
giving for the final temperature
T
(
)
(
)
(
)
(
)
(
(
)
(
)
(
)
)
w
w
w
hw
w
0.45 kg
448 J/(kgC
475
C
25 kg
4168 J/(kgC
45
C
0.45 kg
448 J/(kgC
25 kg
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 Fall '08
 Staff

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