Thermodynamics
State of a thermodynamic system
 The state of a thermodynamic system is determined
by macroscopic variables like pressure (P), temperature (T), volume (V), internal energy
(U), number of particles (N), and other things like entropy (S) that we will talk about later.
These are all called “
state variables
”.
We can only choose three of these state variables to independently control.
Once we have
chosen three of them and fixed their values then all the others are fully determined and we
can’t change them without changing one of our three fixed variables.
Equations of state
let us calculate all other state variables from these three.
Each point on the graph represents a specific equilibrium state.
Notice: any function of state variables is also a state variable
H = U + PV
C
V
= dU/dT
with V held constant
C
P
= dH/dT
with P held constant
G = U  TS + PV
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The Thermodynamic model
Our thermodynamic model has two pieces:
1) Energy is conserved in the interactions between a thermodynamic system and its
surroundings.
You have already thought a lot about the energy transfers Q and W and the
resulting changes in the internal energy, U, of our system (remember that U = E
th
+ E
bond
so
!
U =
!
E
th
+
!
E
bond
).
1
st
Law of Thermodynamics: total energy in the universe does not change
!
U = Q + W
2) When a change is made, our system will settle into a new equilibrium state.
Information
about the equilibrium state is contained in a state variable called “entropy” (S).
2
nd
Law of Thermodynamics: total entropy in the universe may increase but never decreases
!
S
!
Q/T
The = sign refers to reversible processes (processes where our system is always in
equilibrium).
The > sign refers to irreversible processes (as our system comes to equil.)
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 Summer '09
 ?
 Thermodynamics, Energy, Entropy, equilibrium state

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