Week 2 (Thermodynamic Properties l - Property Tables) Flashcards
Thermodynamic Properties 1 - Property Tables
Recap from week 1
Specific values are properties of the…
substance
Recap from week 1
Original values are properties of the…
system / subsystem itself
It is hard to define some properties, so all we really do…
measure things like mass, length and time by comparing them to some excepted standards (e.g. a kilogram, a meter, a
second).
Other “derived” quantities then fall in to place. Eg force comes from Newton’s 2nd law and is defined in terms of mass and acceleration (and not the other way round.
The above ideas of properties of matter being interrelated is one which we will use to our
advantage in our study of thermodynamics
How?
It allows us to evaluate properties of
substances and systems without always having to measure them directly.
Simple fluids (3 conditions)
idealised liquids and gases but experiments have shown that real fluids
behave as simple fluids
provided:
no chemical reactions occur
that electromagnetic effects
are not present
that the volume is large compared to the mean free path of the molecules making up the fluid.
Conditions which hold in the vast majority of real
engineering situations
Enthalpy Equation
H = E + PV
Entropy
the ideas of reversibility
of disorder in systems and of the “availability” of a systems thermal energy for
conversion into mechanical work.
None of these phrases shed any real light on what
entropy really is (any more than “mass is the quantity of matter in an object”).
We learned above, however, that being able to define a property in words is often not as useful as being able to relate it to other properties. To understand and use entropy we must first accept the simple fact that entropy is just another thermodynamic property. It is quantifiable, and hence measurable (although measuring it is not easy or straightforward) and hence its value for a given system can be evaluated, either directly or indirectly from its relationship to other properties.
This fact, coupled with one unique characteristic of entropy makes it an incredibly
useful tool in thermodynamics.
Entropy Postulate
sums up the uniquely observable characteristic of entropy
“There exists an extensive property, entropy, such that for an ISOLATED SYSTEM, entropy
can only increase in any real process.”
Note that this is no more “unacceptable” than the First Law of Thermodynamics (itself
sometimes referred to as “the energy postulate”). Both are based purely on empirical
observation rather than any underlying physical Law. They are accepted because they have
yet to be proven wrong
There are two important corollaries of the entropy postulate
Firstly, if a system is allowed to interact with its surroundings and is then isolated, after a
period of “settling down” during which the system reaches equilibrium, the entropy of the
resulting system will have attained its maximum value.
Secondly, an ideal process for an isolated system, for which entropy remains constant, is
called a “reversible” process, since it could spontaneously reverse itself without entropy
needing to decrease, i.e. without defying the entropy postulate
One important characteristic of the so called simple fluid, is the two property rule:
This states that for equilibrium states of a closed system containing a simple fluid, if two
properties are known and fixed, then the others will have unique values and can be evaluated.
For instance, referring to the six properties we will be concerned with here, if pressure and
temperature are know for a particular system containing a simple fluid, say water in the
gaseous phase, then the other four properties can either be looked up in Tables of Thermodynamic Properties compiled from experiments, or calculated using “equations of
state”, themselves often simply “curve fits” to experimental data.
The simple example of an “equation of state” is the ideal gas equation, which holds for many real gases at relatively low pressures.
Ideal Gas equation:
P v = R T
The other three quantities of interest, specific internal energy, enthalpy and entropy will
also have unique values for this particular system but additional, more complex equations
would be needed to evaluate them.
In what conditions might the ideal gas equation not apply
at high pressures, and other more accurate equations of state may not be available.
In these cases we might have recourse to Thermodynamics Property Tables or Data Bases
as an alternative method of applying the 2 property rule to obtain properties.
