1Revision of equilibrium statistical physics
III Theoretical Physics of Soft Condensed Matter
1.1 Thermodynamics
A central concept in statistical physics is entropy.
Definition (Entropy). The entropy of a system is
S = −k
B
X
i
p
i
log p
i
,
where
k
B
is Boltzmann’s constant,
i
is a microstate — a complete specification
of the microscopics (e.g. the list of all particle coordinates and velocities) — and
p
i
is the probability of being in a certain microstate.
The axiom of Gibbs is that a system in thermal equilibrium maximizes
S
subject to applicable constraints.
Example.
In an isolated system, the number of particles
N
, the energy
E
and
the volume
V
are all fixed. Our microstates then range over all microstates
that have this prescribed number of particles, energy and volume only. After
restricting to such states, the only constraint is
X
i
p
i
= 1.
Gibbs says we should maximize
S
. Writing
λ
for the Lagrange multiplier
maintaining this constraint, we require
∂
∂p
i
S − λ
X
i
p
i
!
= 0.
So we find that
−k
B
log p
i
+ 1 − λ = 0
for all i. Thus, we see that all p
i
are equal.
The above example does not give rise to the Boltzmann distribution, since
our system is completely isolated. In the Boltzmann distribution, instead of
fixing E, we fix the average value of E instead.
Example.
Consider a system of fixed
N, V
in contact with a heat bath. So
E
is no longer fixed, and fluctuates around some average
hEi
=
¯
E
. So we can
apply Gibbs’ principle again, where we now sum over all states of all
E
, with
the restrictions
X
p
i
E
i
=
¯
E,
X
p
i
= 1.
So our equation is
∂
∂p
i
S − λ
I
X
p
i
− λ
E
X
p
i
E
i
= 0.
Differentiating this with respect to p
i
, we get
−k
B
(log p
i
+ 1) − λ
I
− λ
E
E
i
= 0.
So it follows that
p
i
=
1
Z
e
−βE
i
,
where Z =
P
i
e
−βE
i
and β = λ
E
/k
B
. This is the Boltzmann distribution.
What is this mysterious
β
? Recall that the Lagrange multiplier
λ
E
measures
how S reacts to a change in
¯
E. In other words,
∂S
∂E
= λ
E
= k
B
β.
Moreover, by definition of temperature, we have
∂S
∂E
V,N,...
=
1
T
.
So it follows that
β =
1
k
B
T
.
Recall that the first law of thermodynamics says
dE = T dS − P dV + µ dN + ··· .
This is a natural object to deal with when we have fixed
S, V, N
, etc. However,
often, it is temperature that is fixed, and it is more natural to consider the free
energy:
Definition
(Helmholtz free energy)
.
The Helmholtz free energy of a system at
fixed temperature, volume and particle number is defined by
F (T, V, N) = U − T S =
¯
E − T S = −k
B
T log Z.
This satisfies
dF = −S dT −P dV + µ dN + ··· ,
and is minimized at equilibrium for fixed T, V, N.