ME 311
THERMODYNAMICS
S. Masutani
FALL 2007
CHAPTER 3
The State Postulate & Properties of Simple Substances
It is clear that, to perform an energy (thermodynamic) analysis of a system, we need information
on the properties of the system and relationships between properties.
Recall that:
1.
the state (at equilibrium) is described by the values of properties;
2.
thermodynamic
properties are not independently variable, e.g., from chemistry, we know that
for an ideal gas,
PV = NkT
where P = pressure; V = volume; N = number of gas particles (atoms or molecules); k is the
Boltzmann constant; and T = absolute temperature.
Note that P and T are intensive
properties, while V and N are extensive properties.
In terms of all intensive properties, this can also be expressed as
Pv = RT
where v is the specific volume and R =
M
kN
M
R
A
=
(the universal gas constant
R
= 8.314
J/mole; k = 1.381 x 10
23
J/K; M is the molecular weight of the gas; and N
A
is the Avogadro
number = 6.022 x 10
23
mol
1
).
Observe that there are three properties (unknowns; e.g., P, v, and T) in this one equation
⇒
only
two of these can be varied independently in situations where the above Equation of State
holds.
3.
Thermodynamics deals with energy and thermodynamic properties are those which are, in
some way, related to energy.
It follows that:
The number of ways in which the energy of a given substance (or system) can be varied
independently is an indicator of the number of independent thermodynamic properties.
4.
Consider ways to transfer energy as work to a closed system:
i.
if compressible medium:
energy can be increased through PdV work;
ii.
if magnetic:
energy can be increased through magnetization work (
)
(
0
V
M
d
H
r
r
⋅
μ
).
∴
it can be shown that there will be at least one independently variable property for each
relevant
work mode.
Moreover, we can hold these properties fixed and vary energy
through transfer of energy as heat.
This gives us one more free variable.
1
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View Full DocumentME 311
THERMODYNAMICS
S. Masutani
FALL 2007
Look closer at this statement
:
∫
=
2
1
12
W
W
δ
where
FdX
W
=
Recall that:
F
≡
generalized force
dX
≡
generalized displacement
If
F is independent of the direction and rate of change of the process, then
the amount of
energy transferred to the system going from X
1
to (X
1
+ dX) is exactly equal to amount
transferred from the system as we move from (X
1
+ dX) to X
1
⇒
the work mode is reversible:
i.e.,
∫∫
−
=
2
1
1
2
FdX
FdX
(note:
nothing has been specified regarding the process
⇒
in general case, the process path
must be same)
•
Thus, if F is a property
of the thermodynamic state of the system, then the associated
work mode is reversible;
•
however, if F depends on the rate or direction of the process and not just on the
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 Fall '07
 Masutani
 Thermodynamics, S. Masutani, relevant reversible work, work mode

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