Definitions
Many of the definitions below are also used in the thermodynamics of chemical reactions .
General basic quantities
Quantity (Common Name/s)
(Common) Symbol/s
SI Units
Dimension
Number of molecules
N
dimensionless
dimensionless
Number of moles
n
mol
[N]
Temperature
T
K
[Θ]
Heat Energy
Q, q
J
[M][L]2 [T]−2
Latent heat
QL
J
[M][L]2 [T]−2
General derived quantities
Quantity (Common Name/s)
(Common) Symbol/s
Defining Equation
SI Units
Dimension
Thermodynamic beta , Inverse temperature
β
β
=
1
/
k
B
T
{\displaystyle \beta =1/k_{B}T\,\!}
J−1
[T]2 [M]−1 [L]−2
Thermodynamic temperature
τ
τ
=
k
B
T
{\displaystyle \tau =k_{B}T\,\!}
τ
=
k
B
(
∂
U
/
∂
S
)
N
{\displaystyle \tau =k_{B}\left(\partial U/\partial S\right)_{N}\,\!}
1
/
τ
=
1
/
k
B
(
∂
S
/
∂
U
)
N
{\displaystyle 1/\tau =1/k_{B}\left(\partial S/\partial U\right)_{N}\,\!}
J
[M] [L]2 [T]−2
Entropy
S
S
=
−
k
B
∑
i
p
i
ln
p
i
{\displaystyle S=-k_{B}\sum _{i}p_{i}\ln p_{i))
S
=
−
(
∂
F
/
∂
T
)
V
{\displaystyle S=-\left(\partial F/\partial T\right)_{V}\,\!}
,
S
=
−
(
∂
G
/
∂
T
)
N
,
P
{\displaystyle S=-\left(\partial G/\partial T\right)_{N,P}\,\!}
J K−1
[M][L]2 [T]−2 [Θ]−1
Pressure
P
P
=
−
(
∂
F
/
∂
V
)
T
,
N
{\displaystyle P=-\left(\partial F/\partial V\right)_{T,N}\,\!}
P
=
−
(
∂
U
/
∂
V
)
S
,
N
{\displaystyle P=-\left(\partial U/\partial V\right)_{S,N}\,\!}
Pa
M L−1 T−2
Internal Energy
U
U
=
∑
i
E
i
{\displaystyle U=\sum _{i}E_{i}\!}
J
[M][L]2 [T]−2
Enthalpy
H
H
=
U
+
p
V
{\displaystyle H=U+pV\,\!}
J
[M][L]2 [T]−2
Partition Function
Z
dimensionless
dimensionless
Gibbs free energy
G
G
=
H
−
T
S
{\displaystyle G=H-TS\,\!}
J
[M][L]2 [T]−2
Chemical potential (of
component i in a mixture)
μi
μ
i
=
(
∂
U
/
∂
N
i
)
N
j
≠
i
,
S
,
V
{\displaystyle \mu _{i}=\left(\partial U/\partial N_{i}\right)_{N_{j\neq i},S,V}\,\!}
μ
i
=
(
∂
F
/
∂
N
i
)
T
,
V
{\displaystyle \mu _{i}=\left(\partial F/\partial N_{i}\right)_{T,V}\,\!}
, where F is not proportional to N because μi depends on pressure.
μ
i
=
(
∂
G
/
∂
N
i
)
T
,
P
{\displaystyle \mu _{i}=\left(\partial G/\partial N_{i}\right)_{T,P}\,\!}
, where G is proportional to N (as long as the molar ratio composition of the system remains the same) because μi depends only on temperature and pressure and composition.
μ
i
/
τ
=
−
1
/
k
B
(
∂
S
/
∂
N
i
)
U
,
V
{\displaystyle \mu _{i}/\tau =-1/k_{B}\left(\partial S/\partial N_{i}\right)_{U,V}\,\!}
J
[M][L]2 [T]−2
Helmholtz free energy
A, F
F
=
U
−
T
S
{\displaystyle F=U-TS\,\!}
J
[M][L]2 [T]−2
Landau potential , Landau Free Energy, Grand potential
Ω , ΦG
Ω
=
U
−
T
S
−
μ
N
{\displaystyle \Omega =U-TS-\mu N\,\!}
J
[M][L]2 [T]−2
Massieu Potential, Helmholtz free entropy
Φ
Φ
=
S
−
U
/
T
{\displaystyle \Phi =S-U/T\,\!}
J K−1
[M][L]2 [T]−2 [Θ]−1
Planck potential, Gibbs free entropy
Ξ
Ξ
=
Φ
−
p
V
/
T
{\displaystyle \Xi =\Phi -pV/T\,\!}
J K−1
[M][L]2 [T]−2 [Θ]−1
Thermal properties of matter
Quantity (common name/s)
(Common) symbol/s
Defining equation
SI units
Dimension
General heat/thermal capacity
C
C
=
∂
Q
/
∂
T
{\displaystyle C=\partial Q/\partial T\,\!}
J K −1
[M][L]2 [T]−2 [Θ]−1
Heat capacity (isobaric)
Cp
C
p
=
∂
H
/
∂
T
{\displaystyle C_{p}=\partial H/\partial T\,\!}
J K −1
[M][L]2 [T]−2 [Θ]−1
Specific heat capacity (isobaric)
Cmp
C
m
p
=
∂
2
Q
/
∂
m
∂
T
{\displaystyle C_{mp}=\partial ^{2}Q/\partial m\partial T\,\!}
J kg−1 K−1
[L]2 [T]−2 [Θ]−1
Molar specific heat capacity (isobaric)
Cnp
C
n
p
=
∂
2
Q
/
∂
n
∂
T
{\displaystyle C_{np}=\partial ^{2}Q/\partial n\partial T\,\!}
J K −1 mol−1
[M][L]2 [T]−2 [Θ]−1 [N]−1
Heat capacity (isochoric/volumetric)
CV
C
V
=
∂
U
/
∂
T
{\displaystyle C_{V}=\partial U/\partial T\,\!}
J K −1
[M][L]2 [T]−2 [Θ]−1
Specific heat capacity (isochoric)
CmV
C
m
V
=
∂
2
Q
/
∂
m
∂
T
{\displaystyle C_{mV}=\partial ^{2}Q/\partial m\partial T\,\!}
J kg−1 K−1
[L]2 [T]−2 [Θ]−1
Molar specific heat capacity (isochoric)
CnV
C
n
V
=
∂
2
Q
/
∂
n
∂
T
{\displaystyle C_{nV}=\partial ^{2}Q/\partial n\partial T\,\!}
J K −1 mol−1
[M][L]2 [T]−2 [Θ]−1 [N]−1
Specific latent heat
L
L
=
∂
Q
/
∂
m
{\displaystyle L=\partial Q/\partial m\,\!}
J kg−1
[L]2 [T]−2
Ratio of isobaric to isochoric heat capacity, heat capacity ratio , adiabatic index, Laplace coefficient
γ
γ
=
C
p
/
C
V
=
c
p
/
c
V
=
C
m
p
/
C
m
V
{\displaystyle \gamma =C_{p}/C_{V}=c_{p}/c_{V}=C_{mp}/C_{mV}\,\!}
dimensionless
dimensionless
Thermal transfer
Quantity (common name/s)
(Common) symbol/s
Defining equation
SI units
Dimension
Temperature gradient
No standard symbol
∇
T
{\displaystyle \nabla T\,\!}
K m−1
[Θ][L]−1
Thermal conduction rate, thermal current, thermal/heat flux , thermal power transfer
P
P
=
d
Q
/
d
t
{\displaystyle P=\mathrm {d} Q/\mathrm {d} t\,\!}
W = J s−1
[M] [L]2 [T]−3
Thermal intensity
I
I
=
d
P
/
d
A
{\displaystyle I=\mathrm {d} P/\mathrm {d} A}
W m−2
[M] [T]−3
Thermal/heat flux density (vector analogue of thermal intensity above)
q
Q
=
∬
q
⋅
d
S
d
t
{\displaystyle Q=\iint \mathbf {q} \cdot \mathrm {d} \mathbf {S} \mathrm {d} t\,\!}
W m−2
[M] [T]−3
Equations
The equations in this article are classified by subject.
Thermodynamic processes
Physical situation
Equations
Isentropic process (adiabatic and reversible)
Q
=
0
,
Δ
U
=
W
{\displaystyle Q=0,\quad \Delta U=W\,\!}
For an ideal gas
p
1
V
1
γ
=
p
2
V
2
γ
{\displaystyle p_{1}V_{1}^{\gamma }=p_{2}V_{2}^{\gamma }\,\!}
T
1
V
1
γ
−
1
=
T
2
V
2
γ
−
1
{\displaystyle T_{1}V_{1}^{\gamma -1}=T_{2}V_{2}^{\gamma -1}\,\!}
p
1
1
−
γ
T
1
γ
=
p
2
1
−
γ
T
2
γ
{\displaystyle p_{1}^{1-\gamma }T_{1}^{\gamma }=p_{2}^{1-\gamma }T_{2}^{\gamma }\,\!}
Isothermal process
Δ
U
=
0
,
W
=
Q
{\displaystyle \Delta U=0,\quad W=Q\,\!}
For an ideal gas
W
=
k
T
N
ln
(
V
2
/
V
1
)
{\displaystyle W=kTN\ln(V_{2}/V_{1})\,\!}
W
=
n
R
T
ln
(
V
2
/
V
1
)
{\displaystyle W=nRT\ln(V_{2}/V_{1})\,\!}
Isobaric process
p 1 = p 2 , p = constant
W
=
p
Δ
V
,
Q
=
Δ
U
+
p
δ
V
{\displaystyle W=p\Delta V,\quad Q=\Delta U+p\delta V\,\!}
Isochoric process
V 1 = V 2 , V = constant
W
=
0
,
Q
=
Δ
U
{\displaystyle W=0,\quad Q=\Delta U\,\!}
Free expansion
Δ
U
=
0
{\displaystyle \Delta U=0\,\!}
Work done by an expanding gas
Process
W
=
∫
V
1
V
2
p
d
V
{\displaystyle W=\int _{V_{1))^{V_{2))p\mathrm {d} V\,\!}
Net Work Done in Cyclic Processes
W
=
∮
c
y
c
l
e
p
d
V
=
∮
c
y
c
l
e
Δ
Q
{\displaystyle W=\oint _{\mathrm {cycle} }p\mathrm {d} V=\oint _{\mathrm {cycle} }\Delta Q\,\!}
Kinetic theory
Ideal gas
Entropy
S
=
k
B
ln
Ω
{\displaystyle S=k_{\mathrm {B} }\ln \Omega }
, where k B is the Boltzmann constant , and Ω denotes the volume of macrostate in the phase space or otherwise called thermodynamic probability.
d
S
=
δ
Q
T
{\displaystyle dS={\frac {\delta Q}{T))}
, for reversible processes only
Statistical physics
Below are useful results from the Maxwell–Boltzmann distribution for an ideal gas, and the implications of the Entropy quantity. The distribution is valid for atoms or molecules constituting ideal gases.
Physical situation
Nomenclature
Equations
Maxwell–Boltzmann distribution
v = velocity of atom/molecule,
m = mass of each molecule (all molecules are identical in kinetic theory),
γ (p ) = Lorentz factor as function of momentum (see below)
Ratio of thermal to rest mass-energy of each molecule:
θ
=
k
B
T
/
m
c
2
{\displaystyle \theta =k_{B}T/mc^{2}\,\!}
K 2 is the Modified Bessel function of the second kind.
Non-relativistic speeds
P
(
v
)
=
4
π
(
m
2
π
k
B
T
)
3
/
2
v
2
e
−
m
v
2
/
2
k
B
T
{\displaystyle P\left(v\right)=4\pi \left({\frac {m}{2\pi k_{B}T))\right)^{3/2}v^{2}e^{-mv^{2}/2k_{B}T}\,\!}
Relativistic speeds (Maxwell-Jüttner distribution)
f
(
p
)
=
1
4
π
m
3
c
3
θ
K
2
(
1
/
θ
)
e
−
γ
(
p
)
/
θ
{\displaystyle f(p)={\frac {1}{4\pi m^{3}c^{3}\theta K_{2}(1/\theta )))e^{-\gamma (p)/\theta ))
Entropy Logarithm of the density of states
Pi = probability of system in microstate i
Ω = total number of microstates
S
=
−
k
B
∑
i
P
i
ln
P
i
=
k
B
ln
Ω
{\displaystyle S=-k_{B}\sum _{i}P_{i}\ln P_{i}=k_{\mathrm {B} }\ln \Omega \,\!}
where:
P
i
=
1
/
Ω
{\displaystyle P_{i}=1/\Omega \,\!}
Entropy change
Δ
S
=
∫
Q
1
Q
2
d
Q
T
{\displaystyle \Delta S=\int _{Q_{1))^{Q_{2)){\frac {\mathrm {d} Q}{T))\,\!}
Δ
S
=
k
B
N
ln
V
2
V
1
+
N
C
V
ln
T
2
T
1
{\displaystyle \Delta S=k_{B}N\ln {\frac {V_{2)){V_{1))}+NC_{V}\ln {\frac {T_{2)){T_{1))}\,\!}
Entropic force
F
S
=
−
T
∇
S
{\displaystyle \mathbf {F} _{\mathrm {S} }=-T\nabla S\,\!}
Equipartition theorem
df = degree of freedom
Average kinetic energy per degree of freedom
⟨
E
k
⟩
=
1
2
k
T
{\displaystyle \langle E_{\mathrm {k} }\rangle ={\frac {1}{2))kT\,\!}
Internal energy
U
=
d
f
⟨
E
k
⟩
=
d
f
2
k
T
{\displaystyle U=d_{f}\langle E_{\mathrm {k} }\rangle ={\frac {d_{f)){2))kT\,\!}
Corollaries of the non-relativistic Maxwell–Boltzmann distribution are below.
Physical situation
Nomenclature
Equations
Mean speed
⟨
v
⟩
=
8
k
B
T
π
m
{\displaystyle \langle v\rangle ={\sqrt {\frac {8k_{B}T}{\pi m))}\,\!}
Root mean square speed
v
r
m
s
=
⟨
v
2
⟩
=
3
k
B
T
m
{\displaystyle v_{\mathrm {rms} }={\sqrt {\langle v^{2}\rangle ))={\sqrt {\frac {3k_{B}T}{m))}\,\!}
Modal speed
v
m
o
d
e
=
2
k
B
T
m
{\displaystyle v_{\mathrm {mode} }={\sqrt {\frac {2k_{B}T}{m))}\,\!}
Mean free path
σ = Effective cross-section
n = Volume density of number of target particles
ℓ = Mean free path
ℓ
=
1
/
2
n
σ
{\displaystyle \ell =1/{\sqrt {2))n\sigma \,\!}
Quasi-static and reversible processes
For quasi-static and reversible processes, the first law of thermodynamics is:
d
U
=
δ
Q
−
δ
W
{\displaystyle dU=\delta Q-\delta W}
where δQ is the heat supplied to the system and δW is the work done by the system.
Thermodynamic potentials
The following energies are called the thermodynamic potentials ,
Name
Symbol
Formula
Natural variables
Internal energy
U
{\displaystyle U}
∫
(
T
d
S
−
p
d
V
+
∑
i
μ
i
d
N
i
)
{\displaystyle \int \left(T\,\mathrm {d} S-p\,\mathrm {d} V+\sum _{i}\mu _{i}\mathrm {d} N_{i}\right)}
S
,
V
,
{
N
i
}
{\displaystyle S,V,\{N_{i}\))
Helmholtz free energy
F
{\displaystyle F}
U
−
T
S
{\displaystyle U-TS}
T
,
V
,
{
N
i
}
{\displaystyle T,V,\{N_{i}\))
Enthalpy
H
{\displaystyle H}
U
+
p
V
{\displaystyle U+pV}
S
,
p
,
{
N
i
}
{\displaystyle S,p,\{N_{i}\))
Gibbs free energy
G
{\displaystyle G}
U
+
p
V
−
T
S
{\displaystyle U+pV-TS}
T
,
p
,
{
N
i
}
{\displaystyle T,p,\{N_{i}\))
Landau potential, or grand potential
Ω
{\displaystyle \Omega }
,
Φ
G
{\displaystyle \Phi _{\text{G))}
U
−
T
S
−
{\displaystyle U-TS-}
∑
i
{\displaystyle \sum _{i}\,}
μ
i
N
i
{\displaystyle \mu _{i}N_{i))
T
,
V
,
{
μ
i
}
{\displaystyle T,V,\{\mu _{i}\))
and the corresponding fundamental thermodynamic relations or "master equations"[2] are:
Potential
Differential
Internal energy
d
U
(
S
,
V
,
N
i
)
=
T
d
S
−
p
d
V
+
∑
i
μ
i
d
N
i
{\displaystyle dU\left(S,V,{N_{i))\right)=TdS-pdV+\sum _{i}\mu _{i}dN_{i))
Enthalpy
d
H
(
S
,
p
,
N
i
)
=
T
d
S
+
V
d
p
+
∑
i
μ
i
d
N
i
{\displaystyle dH\left(S,p,{N_{i))\right)=TdS+Vdp+\sum _{i}\mu _{i}dN_{i))
Helmholtz free energy
d
F
(
T
,
V
,
N
i
)
=
−
S
d
T
−
p
d
V
+
∑
i
μ
i
d
N
i
{\displaystyle dF\left(T,V,{N_{i))\right)=-SdT-pdV+\sum _{i}\mu _{i}dN_{i))
Gibbs free energy
d
G
(
T
,
p
,
N
i
)
=
−
S
d
T
+
V
d
p
+
∑
i
μ
i
d
N
i
{\displaystyle dG\left(T,p,{N_{i))\right)=-SdT+Vdp+\sum _{i}\mu _{i}dN_{i))
Maxwell's relations
The four most common Maxwell's relations are:
Physical situation
Nomenclature
Equations
Thermodynamic potentials as functions of their natural variables
U
(
S
,
V
)
{\displaystyle U(S,V)\,}
= Internal energy
H
(
S
,
P
)
{\displaystyle H(S,P)\,}
= Enthalpy
F
(
T
,
V
)
{\displaystyle F(T,V)\,}
= Helmholtz free energy
G
(
T
,
P
)
{\displaystyle G(T,P)\,}
= Gibbs free energy
(
∂
T
∂
V
)
S
=
−
(
∂
P
∂
S
)
V
=
∂
2
U
∂
S
∂
V
{\displaystyle \left({\frac {\partial T}{\partial V))\right)_{S}=-\left({\frac {\partial P}{\partial S))\right)_{V}={\frac {\partial ^{2}U}{\partial S\partial V))}
(
∂
T
∂
P
)
S
=
+
(
∂
V
∂
S
)
P
=
∂
2
H
∂
S
∂
P
{\displaystyle \left({\frac {\partial T}{\partial P))\right)_{S}=+\left({\frac {\partial V}{\partial S))\right)_{P}={\frac {\partial ^{2}H}{\partial S\partial P))}
+
(
∂
S
∂
V
)
T
=
(
∂
P
∂
T
)
V
=
−
∂
2
F
∂
T
∂
V
{\displaystyle +\left({\frac {\partial S}{\partial V))\right)_{T}=\left({\frac {\partial P}{\partial T))\right)_{V}=-{\frac {\partial ^{2}F}{\partial T\partial V))}
−
(
∂
S
∂
P
)
T
=
(
∂
V
∂
T
)
P
=
∂
2
G
∂
T
∂
P
{\displaystyle -\left({\frac {\partial S}{\partial P))\right)_{T}=\left({\frac {\partial V}{\partial T))\right)_{P}={\frac {\partial ^{2}G}{\partial T\partial P))}
More relations include the following.
(
∂
S
∂
U
)
V
,
N
=
1
T
{\displaystyle \left({\partial S \over \partial U}\right)_{V,N}={1 \over T))
(
∂
S
∂
V
)
N
,
U
=
p
T
{\displaystyle \left({\partial S \over \partial V}\right)_{N,U}={p \over T))
(
∂
S
∂
N
)
V
,
U
=
−
μ
T
{\displaystyle \left({\partial S \over \partial N}\right)_{V,U}=-{\mu \over T))
(
∂
T
∂
S
)
V
=
T
C
V
{\displaystyle \left({\partial T \over \partial S}\right)_{V}={T \over C_{V))}
(
∂
T
∂
S
)
P
=
T
C
P
{\displaystyle \left({\partial T \over \partial S}\right)_{P}={T \over C_{P))}
−
(
∂
p
∂
V
)
T
=
1
V
K
T
{\displaystyle -\left({\partial p \over \partial V}\right)_{T}={1 \over {VK_{T))))
Other differential equations are:
Name
H
U
G
Gibbs–Helmholtz equation
H
=
−
T
2
(
∂
(
G
/
T
)
∂
T
)
p
{\displaystyle H=-T^{2}\left({\frac {\partial \left(G/T\right)}{\partial T))\right)_{p))
U
=
−
T
2
(
∂
(
F
/
T
)
∂
T
)
V
{\displaystyle U=-T^{2}\left({\frac {\partial \left(F/T\right)}{\partial T))\right)_{V))
G
=
−
V
2
(
∂
(
F
/
V
)
∂
V
)
T
{\displaystyle G=-V^{2}\left({\frac {\partial \left(F/V\right)}{\partial V))\right)_{T))
(
∂
H
∂
p
)
T
=
V
−
T
(
∂
V
∂
T
)
P
{\displaystyle \left({\frac {\partial H}{\partial p))\right)_{T}=V-T\left({\frac {\partial V}{\partial T))\right)_{P))
(
∂
U
∂
V
)
T
=
T
(
∂
P
∂
T
)
V
−
P
{\displaystyle \left({\frac {\partial U}{\partial V))\right)_{T}=T\left({\frac {\partial P}{\partial T))\right)_{V}-P}
Quantum properties
U
=
N
k
B
T
2
(
∂
ln
Z
∂
T
)
V
{\displaystyle U=Nk_{B}T^{2}\left({\frac {\partial \ln Z}{\partial T))\right)_{V}~}
S
=
U
T
+
N
k
B
ln
Z
−
N
k
ln
N
+
N
k
{\displaystyle S={\frac {U}{T))+Nk_{B}\ln Z-Nk\ln N+Nk~}
Indistinguishable Particleswhere N is number of particles, h is Planck's constant , I is moment of inertia , and Z is the partition function , in various forms:
Degree of freedom
Partition function
Translation
Z
t
=
(
2
π
m
k
B
T
)
3
2
V
h
3
{\displaystyle Z_{t}={\frac {(2\pi mk_{B}T)^{\frac {3}{2))V}{h^{3))))
Vibration
Z
v
=
1
1
−
e
−
h
ω
2
π
k
B
T
{\displaystyle Z_{v}={\frac {1}{1-e^{\frac {-h\omega }{2\pi k_{B}T))))}
Rotation
Z
r
=
2
I
k
B
T
σ
(
h
2
π
)
2
{\displaystyle Z_{r}={\frac {2Ik_{B}T}{\sigma ({\frac {h}{2\pi )))^{2))))