Lecture 37
pls37.1
Entropy and the 2
nd
Law of Thermodynamics
Media: 1.
Fan w/ hot, cold water and LN
2
2.
Figs. 1, 2
1.
S
and
∆
S
The entropy
S
is a function of the thermodynamic state of the system,
similar to the internal energy
U
and pressure
P
.
E.g., for an IMG, if we know
N
,
V
, and
T
, we know
T
Nk
U
B
2
3
=
and
T
k
V
N
P
B
=
Similarly,
=
C
N
V
N
U
Nk
S
B
2
3
ln
For a system at temperature
T
, if we add a small amount of heat
Q
∆
,
then the change
in entropy of the system is
T
Q
S
∆
=
∆
Remember,
Q
is not a state function, but the entropy
S
is.
Relationship to
PV
diagram (
Fig. 1
):
(i)
On an adiabat
0
=
∆
Q
Æ
0
=
∆
S
Æ
S
is
constant along an adiabat.
That is, an adiabat
is a constant entropy
curve.
(
C
is a constant that depends upon
the mass of the atoms in the gas)
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Lecture 37
pls37.2
(ii)
Because
S
is a state function, there is a unique value of
S
(for
the gas) at each point of the diagram (as is true for other state
functions
P
,
V
,
U
, and
T
.)
2.
Entropy and the Carnot Cycle
—Carnot cycle = isothermadiabat cycle
Fig. 2
:
paths 1, 3: isotherms
paths 2, 4: adiabats
(i)
Around the cycle
A
Æ
B
Æ
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 Fall '11
 KODERA
 Physics, Thermodynamics, Energy, Entropy, Heat, ∆s, Qin TH

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