Thermal & Fluids Engineering I. Spring 2006.
Homework #2 Solution.
WebCT post date: 02/02/2006
217
Calculate the work, in joules, that is done in the quasiequilibrium process from state 1 to state 2
shown in the figure.
Approach:
The work done is the area under the curve of
P
versus
V
. Use geometry to calculate this area.
Assumptions:
1. The process is quasiequilibrium.
Solution:
The work for a quasiequilibrium process is
W
=
PdV
∫
The area under the curve from state 1 to state 2 is represented as
W
= (100kPa)(21)cm
3
+ (50kPa)(32)cm
3
= (150 kPa
⋅
cm
3
)
1000Pa
1kPa
3
1
100
3
3
m
cm
0.15 J
W
=
Answer
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Thermal & Fluids Engineering I. Spring 2006. Homework #2 Solution.
Page 2
225
In the figure below, a piston is resting on a set of stops. The cylinder contains CO
2
initially at –30
°
C and 45 kPa. The mass of the piston is 1.2 kg and its diameter is 0.06 m. Assuming atmospheric
pressure is 101 kPa, how much heat must be added to just lift the piston off the stops.
Approach:
A force balance on the piston can be used to
calculate the gas pressure at piston liftoff.
The ideal gas law is used to determine the
final temperature. As a last step, apply the
first law to find heat added.
Assumptions:
1. Specific heat is constant.
2. Carbon dioxide behaves like an ideal gas
under these conditions.
Solution:
The piston will just lift off when the pressure inside equals the pressure outside plus the weight of the
piston per unit area.
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 Spring '06
 Tascuic
 Fluids Engineering I.

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