ME 311
THERMODYNAMICS
S. Masutani
FALL 2007
CHAPTER 6
The preceding lectures attempted to provide insight on how entropy can be evaluated using
quantum–statistical (microscopic) concepts.
As mentioned previously, the difference in the
entropy of two thermodynamic states can also be determined from macroscopic properties that
can be measured directly.
The Gibbs equations are important relationships that are employed to determine entropy from
macroscopic properties.
Before we proceed with this discussion, we first need to develop
thermodynamic definitions of temperature and pressure.
RECALL:
1.
Entropy is a thermodynamic property of matter (at equilibrium
).
What this means:
a.
for, say, a SCS:
S = S(U,V)
for an incompressible liquid:
S = S(U)
for a SMS:
)
,
(
M
V
U
S
S
r
=
;
M
r
= magnetization vector
b.
Intensive entropy (entropy/unit mass) is defined as
M
S
s
≡
;
units of s are energy/(temperature x mass) = J/K-kg or BTU/R-lbm or cal/K-g
c.
The specific entropy of a liquid-vapor mixture is
g
f
s
s
s
χ
χ
+
−
=
)
1
(
Thermodynamic Definition of Temperature
Review
:
What is temperature?
•
Bring two masses in contact; if energy transfer as heat occurs between them
⇒
temperatures are different.
•
Energy passes from “hotter” body to “colder” body.
•
Temperature is an indicator of the direction of heat transfer.
•
Differences in temperature reflect a lack of equilibrium
⇒
may be able to form basic
definition of temperature through considerations of thermal equilibrium.
Thus:
Search for conditions under which no energy transfer as heat occurs when two masses, each at
equilibrium with themselves, are brought together.
Find a property that has same value for both
bodies under this condition of thermal equilibrium.
This leads us to the definition of
temperature.
1

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