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1
Thermodynamics
Entropy
•
A thermodynamic property (like h, v,
etc.)
•
Entropy per unit mass is an intensive
property and has units of kJ/kg K
•
dS =(
δ
Q/T)
int rev
Can integrate this to get
change in entropy.
Note as with
energy we concern ourselves with
change and not the actual entropy.
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Thermodynamics
Entropy
•
Can be viewed as a measure of disorder
or disorganization.
•
Generating entropy is generating
disorder.
•
Efficient people lead low entropy lives.
They are organized.
They have a place for
everything (minimum uncertainty).
It
takes minimum energy to find something.
3
Thermodynamics
Entropy
•
Friction, unrestrained expansion of
gases, uncontrolled electron
exchange (chemical reactions) result in
entropy generation.
•
In the real world entropy is always
increasing. (Except for some of those
biological things we mentioned before.)
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Thermodynamics
Entropy
•
In statistical thermo,
S = k lnp
(Boltzmann relationship)
Where k = Boltzmann constant =
1.3806 x10
23
J/K
And p = thermodynamic probability, the
total number of possible microscopic
states of the system.
Consider an
expanding gas vs. a perfect crystal at 0 K.
5
Thermodynamics
Entropy
So what do we do with this entropy?
•
Evaluate the availability to do work
based on the source and sink
conditions we have
•
Evaluate the economics of process
changes
•
Simulate real world processes for
analysis (e.g. turbines, compressors)
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Thermodynamics
Entropy
Specifically in this class, we will use
entropy as a property.
We will take the
operation of a turbine, compressor or the
process in a diesel or Otto cycle and
assume the device behaves in an
isentropic fashion (constant entropy
across the device) much like we took a
throttling valve and assumed its process
was isenthalpic (constant enthalpy).
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This note was uploaded on 09/05/2011 for the course EML 3100 taught by Professor Sherif during the Summer '08 term at University of Florida.
 Summer '08
 Sherif

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