# Ch1. Statistical Basis of Thermodynamics

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Ch1. Statistical Basis of Thermodynamics
1.1 The macroscopic state and the microscopic state Macrostate: a macrostate of a physical system is specified by macroscopic variables (N,V,E). Microstate: a microstate of a system is specified by the positions, velocities, and internal coordinates of all the molecules in the system. For a quantum system, Y(r1,r2,….,rN), specifies a microstate.

Microstate Number W(N,V,E)
For a given macrostate (N,V,E), there are a large number of possible microstates that can make the values of macroscopic variables. The actual number of all possible miscrostate is a function of macrostate variables. Consider a system of N identical particles confined to a space of volume V. N~1023. In thermodynamic limit: N¥, V¥, but n=N/V finite. Macrostate variables (N, V, E) Volume: V Total energy:

Macrostate variables Volume: V Total energy:
ni – the number of particles with energy ei ei - energy of the individual particles Microstate: all independent solutions of Schrodinger equation of the system. N-particle Schrodinger equation,

Physical siginificance of W(N,V,E)
For a given macrostate (N,V,E) of a physical system, the absolute value of entropy is given by Where k=1.38x10-23 J/K – Boltzman constant Consider two system A1 and A2 being separately in equilibrium. When allow two systems exchanging heat by thermal contact, the whole system has E(0)=E1+E2=const. macrostate (N,V, E(0))

Problem 1.2 Assume that the entropy S and the statistical number W of a physical system are related through an arbitrary function S=f(W). Show that the additive characters of S and the multiplicative character of W necessarily required that the function f(W) to be the form of f(W) = k ln(W) Solution: Consider two spatially separated systems A and B A B

1.3 Future contact between statistics and thermodynamics
Consider energy change between two sub-systems A1 and A2, both systems can change their volumes while keeping the total volume the constant. Energy change Volume variable No mass change A1 (N1,V1,E1) A2 (N2,V2,E2) E(0) = E1+E2=const V(0) = V1+V2=const N(0) = N1+N2=const

1.3 Future contact between statistics and thermodynamics –cont.
(N1,V1,E1) A2 (N2,V2,E2) Initial states System A1: (N1,V1, E1), S1(N1,V1,E1)=k lnW1(N1,V1,E1) System A2: (N2,V2, E2), S2(N2,V2,E2)=k lnW2(N1,V1,E1) Thermal contact process E(0) = E1+E2=const, E1, E2 changeable V(0) = V1+V2=const, V1, V2 changeable N(0) = N1+N2=const, N1, N2 changeable W(0) (N1,V1,E1; N2,V2,E2)= W1(N1,V1,E1)+W2(N2,V2,E2)

1.3 Future contact between statistics and thermodynamics –cont.
Thermal equilibrium state (N1*,V1*,E1*) m1=m2 P1=P2 T1=T2

Summary-how to derive thermodynamics from a statistical beginning?
1) Start from the macrostate (N,V,E) of the given system; 2) Determine the number of all possible microstate accessible to the system, W(N,V,E). 3) Calculate the entropy of the system in that macrostate 4) Determine system’s parameters, T,P, m 5) Determine the other parameters in thermodynamics Helmhohz free energy: A= E-T S Gibbs free energy: G = A + PV = mN Enthalpy: H = E + PV

Determine heat capacity
6) Determine heat capacity Cv and Cp;

1.4 Classical ideal gas Model: L
N particles of nonatomic molecules Free, nonrelativistic particles Confined in a cubic box of side L (V=L3) L Wavefunction and energy of each particle

1.4 Classical ideal gas-cont.
Hamiltonian of each particle Separation of variables Boundary conditions: Y(x) vanishes on the boundary,

1.4 Classical ideal gas-cont.
Boundary conditions: Y(x) vanishes on the boundary

Microstate of one particle
Boundary conditions: Y(x) vanishes on the boundary One microstate is a combination of (nx,ny,,nz)

The number of microstate of one particle W(1,e,V)
The number of distinct microstates for a particle with energy e is the number of independent solutions of (nx,ny,nz), satisfying nx ny nz The number W(1,e,V) is the volume in the shell of a 3 sphere. The volume of in (nx,ny,nz) space id 1.

Microstates of N particles
The total energy is One microstate with a given energy E is a solution of (n1,n2,……n3N) of 3N-dimension sphere with radius sqrt(E*)

The number of microstate of N particles W(N,E,V)
The volume of 3N-sphere with radius R=sqrt(E*) (Appendix C) The number W(N,E,V) is the volume in the shell of a 3N-sphere. n1 n2 n3

Entropy and thermodynamic properties of an ideal gas
Determine temperature Determine specific heat

State equation of an ideal gas
Determine pressure Specific heat ratio

1.5 The entropy of mixing ideal gases
Consider the mixing of two ideal gases 1 and 2, which are initially at the same temperature T. The temperature of the mixing would keep as the same. mixing N1,V,T N2,V,T N1,V1,T N2,V2,T Before mixing After mixing

P1-11 Four moles of nitrogen and one mole of oxygen at P=1 atm and T=300K are mixed together to form air at the same pressure and temperature. Calculate the entropy of the mixing per mole of the air formed.