Thermodynamic Potentials Why are thermodynamic potentials useful Consider U=U(T,V) Complete knowledge of equilibrium properties of a simple thermodynamic System requires in addition P=P(T,V) equation of state U=U(T,V)
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Transcript Thermodynamic Potentials Why are thermodynamic potentials useful Consider U=U(T,V) Complete knowledge of equilibrium properties of a simple thermodynamic System requires in addition P=P(T,V) equation of state U=U(T,V)
Thermodynamic Potentials
Why are thermodynamic potentials useful
Consider U=U(T,V)
Complete knowledge of equilibrium properties of a simple thermodynamic
System requires in addition
P=P(T,V)
equation of state
U=U(T,V) and P=P(T,V)
complete knowledge of equilibrium properties
U(T,V) is not a thermodynamic potential
However
We are going to show: U=U(S,V)
complete knowledge of equilibrium properties
U(S,V): thermodynamic potential
The thermodynamic potential U=U(S,V)
Consider first law in differential notation
dU dQ dW
dQ TdS
dQ
expressed by
inexact differentials
2nd law
exact differentials
dW P dV
dW
dU TdS PdV
Note: exact refers here to the
coordinate differentials dS and dV.
T dS and PdV are inexact
as we showed previously.
Legendre Transformations
dU TdS PdV
dU: differential of the function U=U(S,V)
natural coordinates
Legendre transformation
Special type of coordinate transformation
Partial derivatives of U(S,V) (vector field components)
Example:
dU TdS PdV
coordinates
Legendre transformation: One (or more) of the natural coordinates becomes
a vector field component
while the
associated coefficient becomes new coordinate.
Click for
graphic example
Back to our example
becomes a coordinate
dU TdS PdV
becomes a coefficient in front of dP
dU TdS dP V VdP
easy check:
Product
rule
dP V VdP VdP P dV VdP P dV
dU dP V TdS VdP
dU P V TdS VdP
=:H (enthalpy)
Enthalpy
H=H(S,P) is a thermodynamic potential
dH TdS VdP
dY Y dx d(Yx) xd (Y)
Geometrical interpretation of the
Legendre transformation
d(Y Yx) xd (Y)
- 1-dimensional example
: f
Note: natural variable of f is Y’
8
Y and X have to be expressed as Y=Y(Y’) and x=x(Y’)
2
T( x ) Y( x 0 ) Y( x 0 )(x x 0 )
6
0
Y e x Y
-2
Y-Y'x
Y
4
2
-4
0
-6
-2
T(0) Y(x 0 ) x 0 Y(x 0 )
-8
0.0
0.5
1.0
X
Legendre transformation:
1.5
2.0
0
1
2
3
4
5
6
7
Y'
mapping between the graph of the function and
the family of tangents of the graph
8
Legendre transformation
(T,V):
dU TdS PdV d(TS) SdT P dV
d( U TS) SdT P dV
from (S,V)
: F
to
(T,P):
Helmholtz free energy
dF SdT PdV SdT d(P V) VdP
d(F P V) SdT VdP
: G
Gibbs free energy
G F PV U TS PV H TS
equilibrium thermodynamics and potentials
thermodynamics potential
complete knowledge of equilibrium properties
Consider Helmholtz free energy F=F(T,V)
Differential reads:
F
S
T V
dF SdT PdV
Entropy
and
F
P
V
T
Equation of state
Response functions from 2nd derivatives
2F
S
C V T T 2
T
T V
V
2
P
F
B T V
V
V T
V 2
T
2
P V P F
and V BT
T V
V T T P T V
etc.
Maxwell relations
dF SdT PdV
differential of the function F=F(T,V)
dF is an exact differential
2F
2F
S
P
V T VT TV
T V
S
P
V T T V
In general: relations which follow from the exactness of the differentials
of thermodynamic potentials are called Maxwell relations
Properties of an ideal gas derived from the Helmholtz free energy
Helmholtz free energy F=U-TS
Reminder: U(T, V) ncv T nu0
S(T, V) nc V ln
T
F(T, V) ncV T1 ln
Tr
T
V
nR ln
S(Tr , Vr )
Tr
Vr
V
nRT ln
nu 0 Ts r
Vr
Equation of state:
F
P(T, V)
V
V T
T
V
nc
T
1
ln
nRT
ln
n
u
Ts
0 r
V
T
V
r
r
T
V
nRT
ln
Vr
V
nRT
V
T
F
T V
S(T,V) obtained from S
S(T, V) nc V ln
F
F
T
U(T,V) obtained from U=F+TS
T
V
T
V
nR ln
S(Tr , Vr )
Tr
Vr
U(T, V) ncv T nu0
Heat capacity at constant volume
Note: U derived from F
U
CV
ncV
T V
Isothermal bulk modulus
Equation of state derived from F
nRT nRT
P
P
B T V
V 2
V
V T
V
etc.
Systems in Contact with Reservoirs
Entropy statement of 2nd law: entropy always increased in an
adiabatically isolated system
What can we say about evolution of systems which are not adiabatically isolated
Consider system at constant temperature and pressure
adiabatic wall
changes from initial state with
G o U o TS o P Vo
System
T=const.
Heat Reservoir R
to final state with
G f U f TS f P Vf
G G f G 0 U TS PV
remain constant
From G U TS PV
Entropy change of :
S
U PV G
T
Aim: Find the total entropy change Stot S SR and apply 2nd law
Entropy change SR of the reservoir:
S R
dQ R
1
QR
d
Q
R
T
T
T
L
L
Heat reservoir: T=const.
Stot S SR
U PV G Q R
T
T
Q G Q R
T
T
With 1st law:
U Q W Q PV
Heat QR that, e.g., leaves the reservoir flows into the system
S tot
G
T
Q = -QR
Entropy statement of 2nd law: Stot 0
G
0
T
for an adiabatically isolated system
G 0 (T=const, P=const.)
Gibbs free energy never increases in a process at fixed pressure
in a system in contact with a heat reservoir.
Gibbs free energy will decrease if it can, since in doing so
it causes the total entropy to increase.
System with V=const. in contact with a heat reservoir
Special case, very important for problems in solid state physics
F U TS
S
U F
Q F
T
T
Q = -QR
Stot S SR
Q F Q R
T
T
F
T
F 0 (T=const, V=const.)
Intuitive interpretation of Helmholtz free energy
What is “free” about the free energy
Consider a system at constant T
S SR 0
S f S 0 SR 0
System
T S f S 0 T S R 0
T=const.
T S f S 0 QR 0
Heat Reservoir R
Q = -QR
T S f S 0 Q
with 1st law
U f U 0 Q W
T S f S 0 U f U 0 W
TS f U f TS 0 U 0 W
W F
Wmax F
The maximum amount of work one can obtain from a system at
constant temperature is given by
the decrease in the Helmholtz free energy
If we keep V=const for a PVT system
F 0
F f F0
W 0 F
as we have seen before
Summary: Thermodynamic potentials for PVT systems
Potential
Internal energy
U(S,V)
Enthalpy
H(S,P)
H=U+PV
Helmholtz free
energy
F(T,V)
F=U -TS
Gibbs free
energy
G(T,P)
G=U –TS+PV
differential
dU=TdS-PdV
dH=TdS+VdP
dF=-SdT-PdV
dG=-SdT+VdP
Vector field
components
U
F
T
, P U T H , V H S F , P
S V
V T
V S
T V
P S
S
P
Maxwell
relations
Properties
T
P
V S
S V
1st law:
U Q W
T V
P S S P
Isobaric
process
H Q
S
P
V T T V
G
G
S
, V
P T
T P
S
V
P T T S
T=const,V=const T=const,P=const
F 0
G 0
Open Systems and Chemical Potentials
Open system
Particle exchange with the surrounding allowed
Particle reservoir
T=const.
Heat Reservoir R
Thermodynamic potentials depend on variable particle number N
Example: U=U(S,V,N)
U(2 S,2 V,2 N) = 2 U(S,V,N)
In general:
U( S, V, N) U( S, V, N)
(homogeneous function of first order)
U
(S) U
(V) U
(N)
U(S, V, N)
(
S
)
(
V
)
(
N
)
V,N
S,N
S,V
S
V
holds and in particular for =1
U
U
U
S
V
N U(S, V, N)
S V, N
V S, N
N S,V
N
U
U
U
S
V
N U(S, V, N)
S V, N
V S, N
N S,V
keep N constant as in closed systems
U
T
S V, N
U
P
V S , N
U
:
N S,V
U(S, V, N) TS P V N
dU TdS P dV dN
U
U
U
dU
dS
dV
dN
S
V
N
V, N
S, N
S,V
Chemical potential
Intuitive meaning of the chemical potential μ
First law:
dU dQ dW
with
dQ TdS
dU TdS dW
mechanical work PdV +
work μdN required to change
# of particles by dN
How do the other potentials change when particle exchange is allowed
Helmholtz free energy F=U-TS
dU TdS P dV dN
dF dU d(TS) dU TdS SdT
dF SdT P dV dN
F
Gibbs free energy G=U -TS+PV
dG dF d(P V) dF PdV VdP
dF SdT P dV dN
dG SdT VdP dN
Properties of μ
U
F
G
N S,V N T,V N T,P
With
G U TS PV
G
N
and
both extensive
U TS P V N
(T, P) intensive (independent of N)
Equilibrium Conditions
Adiabatically isolating
rigid wall
System1:
T1,P1, 1
From
System2:
T2,P2, 2
dQ
dU TdS P dV dN
dS1
differentials of entropy change
dS2
dU1 P1
dV1 1 dN1
T1 T1
T1
dU 2 P2
dV2 2 dN 2
T2
T2
T2
Total entropy change
S S1 S2 0
2nd law
In equilibrium
dS dS1 dS2 0
With conservation of
-total internal energy U1 U2 const.
dU1 dU2
-total volume
V1 V2 const.
dV1 dV2
-total # of particles
N1 N2 const.
dN1 dN2
1 1
P P
dS dU1 1 2 dV1 1 2 dN1 0
T1 T2
T1 T2
T1 T2
1 1
P P
S dU1 1 2 dV1 1 2 dN1 0
T1 T2
T1 T2
T1 T2
0
0
small changes dU1, dV1, dN1
0
Equilibrium conditions
T1 = T2
Remark: T1 = T2 , P1 = P2
and
, P1 = P2
(T, P)
,1 = 2
1 = 2
1 = 2 no new information for system in a single phase
but
Important information if system separated into several phases (see next chapter)