CHEE2325 Thermodynamics Lecture 4 Summary

CHEE2325 – Lecture 4


Incompressible fluids:
For incompressible fluids, C P =CV .
-
^
∆ S=C ln
T2
T1 ( )
So if T 2=T 1, then the process is isentropic.




Ideal gases:
-
For ideal gases:
V^ 2
( )
T2


( )
T
dT dT
2
P
∆ S=∫ CV
^ + R ln ∆ S =∫ C P
^ −R ln ⁡ 2
T T 1
V^ 1 T 1
T P1
-
Or, if heat capacity is constant,

^
∆ S=C V ln
T2
T1
+ R ( ) ( )
ln
V^ 2 ^
^
V1
∆ S=C P ln
T2
T1 ( ) ( )
−R ln
P2
P1
- For an isentropic process of an ideal gas:

( )( ) ( )( )
T2 ^ 2 γ−1 T 2
V γ −1
P2 γ C
= = γ= P
T1 ^1
V T1 P1 CV
- We can define absolute entropy for an ideal gas, with 0 K as a reference
temperature.
T2
dT
∫ CP T
=∆ S
0

T1




Heat engines:
- Heat engines are devices that convert heat to work.
- Heat engines vary in design, but have the following features in common:
o They receive heat from a high temperature source.
o They convert part of this heat to work.
o They reject the remaining waste heat to a low temperature sink.
o They are cyclic devices, meaning they undergo a series of
thermodynamic processes and ultimately return to their original
thermodynamic state.
- Applying the first law of thermodynamics on the system:
W =QH −QC