Ph 111.01
December 15, 2008
PS 15a (39)
Problem 1 = Walker P. 1814.
Consider a system in which
2.60
mol of an ideal monatomic gas is
expanded at a constant pressure of 101 kPa from an initial volume of 2.15 L to a final volume of
4.09
L.
(a) Is heat added to or removed from the system during this process?
removed
no change in heat
added
Explain.
(b) Find the change in temperature for this process.
K
(c) Determine the amount of heat
added to or removed from the system
during this process.
J
Solution.
We will be using (at
least) three relations:
3
2
UQW
WPV
Un
R
T
PV
nRT
Δ=−
=Δ
=
=
(a)
From the fourth relation (the
ideal gas law) we know that the
temperature increases; and from the third relation we know that the internal energy
increases.
Then from the second law we know that the work is also positive.
So, finally from
the first line,
QU
W
=
Δ+
,
the heat added us positive.
(b)
We can use the ideal gas law to calculate initial and final temperatures.
P
V
1.01x10
5
Pa
0
2.15 L
4.09 L
2.60 moles of
ideal monatomic
gas
Q
W=P
Δ
V
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( )
(
)
(
)
3
11
1
3
22
2
101,000 Pa
0.00215 m
10.05 K
2.60 moles 8.31 J/(moleK)
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 Fall '08
 Staff
 Thermodynamics, Heat

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