6.1. Thermodynamics¶
6.1.1. Fundamental Thermodynamic Relation¶
The first law of thermodynamics is , where
is the heat
supplied to the system and
is the work done by the system.
The second law of thermodynamics is
. By substituting
into the first law, and expressing the work
using pressure and
volume, we obtain:
This can then be generalized to:
where is the chemical potential and
the number of particles in the
system.
6.1.2. Thermodynamic Potentials¶
We start by writing the internal energy (derived in the previous section)
as a function of entropy , volume
and a number of particles
. Now we
want to express it as a function of temperature
, pressure
or a chemical
potential
, without losing any information, i.e. we still want to just
differentiate to obtain other quantities. In order to do that, we have to use
the Legendre transform. Including
, there are 8 possible combinations of
Legendre transforms that one can do (three of them are applying it to just one
variable, three of them to two variables, one to all three variables):
Of these, is the internal energy,
is the Helmholtz free energy,
is
the enthalpy,
is the Gibbs free energy,
is the grand potential
(sometimes also called a Landau potential). The unnamed potentials are simply
labeled
,
and
. The
is sometimes called a null function.
From the differentials, we can then read off the derivatives (and what other variables are constant), here are all the combinations:
A large system is defined as: if the number of particles is made
times as large,
,
, and
all become
times larger. In other
words, the internal energy of a large system is a homogeneous function of
,
, and
of order one:
Now we can apply the Euler’s theorem (see Homogeneous Functions (Euler’s Theorem)):
And from the definitions of all the potentials we can calculate their forms for large systems:
Other first derivatives¶
Other commonly used first derivatives are the heat capacity at constant volume:
and the heat capacity at constant pressure
Note that these first derivatives are differentiating the thermodynamic potential that is not expressed in its canonical variables (the only canonical first derivatives are already enumerated in the previous section). In both cases the quantity can be expressed as a second derivative of a potential in its canonical variables. As shown below, the canonical second derivatives can also be enumerated.
Second derivatives¶
Here are the most commonly used second derivatives. The particle density:
The speed of sound:
where is the density and
is the specific
volume.
The isothermal speed of sound:
The adiabatic bulk modulus:
Adiabatic coefficient of compressibility:
The isothermal bulk modulus:
Isothermal coefficient of compressibility:
The Grüneisen parameter:
The coefficient of thermal expansion
Note: there are three possible second derivatives of the Gibbs free energy
with respect to
and
:
Every other second derivative of other thermodynamic potentials can be
expressed using these three derivatives (i.e., using ,
and
). For example:
6.1.3. Examples¶
Ideal Gas¶
The internal energy as a function of ,
and
is equal to:
(6.1.3.1)¶
where is the heat capacity at a constant volume (
for
monoatomic gases,
for diatomic gases),
is the
Boltzman constant and
is a constant that may vary for different gases,
but it is independent of the thermodynamic state of the gas.
At this level, the above expression is simply given. We would have to use statistical physics in order to calculate any of the thermodynamic potentials.
Now we calculate the free energy . First we must calculate the
temperature
:
(6.1.3.2)¶
In order to calculate the the free energy, we must use (6.1.3.2) to
eliminate :
(6.1.3.3)¶
and then express as a function of
,
and
only:
(6.1.3.4)¶
This calculation shows that one can also express the internal energy as a
function of ,
and
as
. This
is a valid expression, but unlike
, this is not a thermodynamic
potential, because we lost some information. In particular, if we use
to find
:
we can see, that we recovered the correct formula for except an
arbitrary function
of
and
. Compared to (6.1.3.1) we
can see that it must be
, but this information got lost. For this reason, only
as well as
, that we just calculated, are
thermodynamic potentials and both contain equivalent information. But
is not and it does not contain full information.
To convert back to
, we first calculate the entropy
:
which is the same equation as (6.1.3.3). From this, we express , we
get (6.1.3.2). Finally, we can calculate the internal energy and
substitute
for
using (6.1.3.2):
This is the same equation as (6.1.3.1). This shows that all thermodynamic potentials contain the same information and can be converted to one another using the Legendre transformation.
Note: in equations like , we can use any expressions for
and
(e.g. we can use
or
, etc.) in the
intermediate steps, but at the end, we must express the final formula using
,
and
only.
To calculate the Gibbs energy, we need to calculate pressure first. We can use
any of the potentials ,
,
or
to do so. Since the equation
of state is typicaly expressed as
, then the free energy
is the natural choice:
and we get the ideal gas law . The Gibbs energy is
equal to:
(6.1.3.5)¶
For the enthalpy, we first need:
we need to use this to express the volume :
now we can calculate :
(6.1.3.6)¶
The enthalpy in terms of temperature can be calculated as:
The specific heat capacity at a constant volume can be calculated as:
This provides proof that the in (6.1.3.1) is indeed the specific
heat capacity at a constant volume.
The specific heat capacity at a constant pressure can be calculated as:
Using this relation we can then express (6.1.3.5):
and (6.1.3.6) as:
In order to calculate the grand potential, we first need to find the chemical potential:
and express using
:
Now we can calculate :
(6.1.3.7)¶
To compute , we can do:
where we used:
As we can see, the last equation cannot be used to compute as a function of
, and also the
potential evaluates to zero, so it cannot be used as a
thermodynamic potential, because we have cannot convert it back to the other
potentials.