# Statistical Ph I I Statistical mechanics of interacting systems 35 3.1 System in contact with a...

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Statistial Physis II(PHYS*4240)Leture notes (Fall 2000)

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Eri PoissonDepartment of PhysisUniversity of Guelph

Contents

1 Review of thermodynamics 11.1 Thermodynamic variables 11.2 Equilibrium 11.3 Equation of state 21.4 Quasi-static transformations 21.5 First law of thermodynamics 3

1.5.1 Formulation 31.5.2 Work 31.5.3 Work depends on path 41.5.4 Heat 41.5.5 Heat capacities 41.5.6 Heat reservoir 5

1.6 Second law of thermodynamics 51.6.1 Two statements 51.6.2 Reversible, irreversible, and cyclic transformations 61.6.3 Clausius theorem 61.6.4 Entropy 71.6.5 Properties of the entropy 71.6.6 Example: system interacting with a heat reservoir 81.6.7 Third law of thermodynamics 8

1.7 Thermodynamic potentials 91.7.1 Energy 91.7.2 Enthalpy 91.7.3 Helmholtz free energy 101.7.4 Gibbs free energy 101.7.5 Landau potential 10

1.8 Maxwell relations 111.9 Scaling properties 111.10 Equilibrium conditions for isolated, composite systems 121.11 Equilibrium for interacting systems 131.12 Limitations of thermodynamics 151.13 Brief summary of thermodynamics 151.14 Problems 16

2 Statistical mechanics of isolated systems 192.1 Review of probabilities 19

2.1.1 Probabilities 192.1.2 Averages 192.1.3 Continuous variables 20

2.2 Macrostate and microstates 212.3 Statistical weight 22

2.3.1 Definition 22

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ii Contents

2.3.2 Example: single particle in a box 222.3.3 Example: N particles in a box 23

2.4 Statistical weight in classical mechanics 242.4.1 Single particle in one dimension 242.4.2 General systems 262.4.3 Example: N particles in a box 26

2.5 Fundamental postulates 272.6 Application: ideal gas 29

2.6.1 Entropy 292.6.2 Gibbs paradox 292.6.3 Thermodynamic quantities 30

2.7 Problems 31

3 Statistical mechanics of interacting systems 353.1 System in contact with a reservoir 35

3.1.1 Probability distributions 353.1.2 Entropy 37

3.2 Boltzmann distribution 383.2.1 Thermodynamic quantities 383.2.2 Energy distribution 403.2.3 Application: N simple harmonic oscillators 41

3.3 Gibbs distribution 443.3.1 Thermodynamic quantities 443.3.2 Fluctuations 45

3.4 Classical statistics 463.4.1 Partition function 473.4.2 The ideal gas again 473.4.3 Equipartition theorem 48

3.5 The meaning of probability 493.6 Brief summary of statistical mechanics 50

3.6.1 Boltzmann distribution 503.6.2 Gibbs distribution 51

3.7 Problems 52

4 Information theory 554.1 Missing information 55

4.1.1 Uniform probabilities 554.1.2 Assigned probabilities 56

4.2 Entropy 574.3 Boltzmann and Gibbs distributions 57

4.3.1 Maximum missing information 584.3.2 Lagrange multipliers 584.3.3 Probability distributions 594.3.4 Evaluation of the Lagrange multipliers 594.3.5 The first law 60

4.4 Conclusion 60

5 Paramagnetism 635.1 Magnetism 63

5.1.1 Magnetic moment 635.1.2 Potential energy 635.1.3 Magnetic moments of atoms and molecules 645.1.4 Forms of magnetism 65

5.2 General theory of paramagnetism 65

Contents iii

5.2.1 The model 665.2.2 Quantum and thermal averages 665.2.3 Energy eigenvalues 675.2.4 Partition function 675.2.5 Magnetization 68

5.3 Molecules with j = 12 685.4 Molecules with arbitrary j 695.5 Paramagnetic solid in isolation 70

5.5.1 Statistical weight and entropy 715.5.2 Temperature 715.5.3 Negative temperatures? 725.5.4 Correspondence with the canonical ensemble 73

5.6 Problems 73

6 Quantum statistics of ideal gases 756.1 Quantum statistics 75

6.1.1 Microstates 756.1.2 Bosons and fermions 776.1.3 Partition function 776.1.4 Grand partition function 776.1.5 Mean occupation numbers 786.1.6 Thermodynamic quantities 79

6.2 Bose-Einstein statistics 796.2.1 Grand partition function 796.2.2 Mean occupation numbers 80

6.3 Fermi-Dirac statistics 816.3.1 Grand partition function 816.3.2 Mean occupation numbers 81

6.4 Summary 826.5 Classical limit 83

6.5.1 Classical approximation 836.5.2 Evaluation of 846.5.3 Other thermodynamic quantities 856.5.4 Conclusion 86

6.6 Slightly degenerate ideal gases 866.6.1 Evaluation of 866.6.2 Energy and pressure 876.6.3 Higher-order corrections 88

6.7 Highly degenerate Fermi gas. I: Zero temperature 896.7.1 Fermi gas at T = 0 896.7.2 Evaluation of the Fermi energy 906.7.3 Energy and pressure 906.7.4 Entropy 91

6.8 Highly degenerate Fermi gas. II: Low temperature 916.8.1 Fermi-Dirac integrals 916.8.2 Chemical potential 926.8.3 Energy and pressure 926.8.4 Heat capacity 93

6.9 Highly degenerate Fermi gas. III: Applications 936.9.1 Conduction electrons in metals 936.9.2 White dwarfs 946.9.3 Neutron stars 94

6.10 Highly degenerate Bose gas. I: Zero temperature 946.11 Highly degenerate Bose gas. II: Low temperatures 95

iv Contents

6.11.1 Bose-Einstein integrals 956.11.2 Number of particles in ground state 956.11.3 Energy and pressure 966.11.4 Heat capacity 97

6.12 Highly degenerate Bose gas. III: Bose-Einstein condensation 976.12.1 Superfluidity 976.12.2 Superconductivity 986.12.3 Holy Grail 98

6.13 Problems 98

7 Black-body radiation 1017.1 Photon statistics 1017.2 Energy eigenvalues 1027.3 Density of states 1037.4 Energy density 1037.5 Other thermodynamic quantities 1047.6 The cosmic microwave background radiation 105

7.6.1 Recombination in the early universe 1057.6.2 Dipole anisotropy 1067.6.3 Black-body radiation as seen by a moving observer 1067.6.4 Proper motion of the galaxy 1077.6.5 Quadrupole anisotropy 107

7.7 Problems 107

Chapter 1

Review of thermodynamics

1.1 Thermodynamic variables

The purpose of thermodynamics is to describe the properties of various macroscopicsystems at, or near, equilibrium. This is done with the help of several state variables,such as the internal energy E, the volume V , the pressure P , the number of particlesN , the temperature T , the entropy S, the chemical potential , and others.

These state variables depend only on what the state of the system is, and not onhow the system was brought to that state. The variables are not all independent, andthermodynamics provides general relations among them. Some of these relationsare not specific to particular systems, but are valid for all macroscopic systems.

1.2 Equilibrium

A system is in equilibrium if its physical properties do not change with time. Thestate variables must therefore be constant in time. More precisely, a system is inequilibrium if there are no

macroscopic motions (mechanical equilibrium)

macroscopic energy fluxes (thermal equilibrium)

unbalanced phase transitions or chemical reactions (chemical equilibrium)

occurring within the system.We will see in Sec. 10 that the requirement for mechanical equilibrium is that

P must be constant in time and uniform throughout the system. This is easilyunderstood: a pressure gradient produces an unbalanced macroscopic force which,in turn, produces bulk motion within the system; such a situation does not representequilibrium.

The requirement for thermal equilibrium is that T must be constant in time anduniform throughout the system. This also is easily understood: If one part of thesystem is hotter than the other, energy (in the form of heat) will flow from thatpart to the other; again such a situation does not represent equilibrium.

Finally, the requirement for chemical equilibrium is that must be constantin time and uniform throughout the system. The chemical potential is not a veryfamiliar quantity, and this requirement can perhaps not be understood easily. Butat least it is clear that unbalanced chemical reactions (we take this to include nuclearreactions as well) cannot occur in a system at equilibrium. Consider for examplethe reaction of -decay:

n p + e + .

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2 Review of thermodynamics

If this reaction is unbalanced (the reversed reaction can occur at sufficiently highdensities), the number of neutrons in the system will keep on decreasing, while thenumber of protons, electrons, and anti-neutrinos will keep on increasing. Such asituation does not represent equilibrium.

A system which is at once in mechanical, thermal, and chemical equilibrium issaid to be in thermodynamic equilibrium.

1.3 Equation of state

The equation of state of a thermodynamic system expresses the fact that not all ofits state variables are independent.

Consider a system described by two state variables X and Y , apart from thetemperature T . (For example, an ideal gas is described by the variables X V ,Y P .) We suppose that the system is initially in equilibrium at temperature T .

We ask the question: Can X and Y be varied independently, so that the sys-tem remains in equilibrium after the transformation? The answer, which must bedetermined empirically, is: No! For the system to remain in equilibrium, a givenvariation of X must be accompanied by a specific variation in Y . If Y is not variedby this amount, the system goes out of equilibrium.

There must therefore exist an equation of state,

f(X,Y, T ) = 0 , (1.1)

relating X, Y , and T for all equilibrium configurations. For non-equilibrium states,f 6= 0. [For example, the equation of state of an ideal gas is f(V, P, T ) = PV NkT = 0.]

The equation of state cannot be derived within the framework of thermodynam-ics. (This is one of the goals of statistical mechanics.) It must be provided as input,and determined empirically.

Suppose the system goes from one equilibrium configuration (X,Y, T ) to a neigh-bouring one (X + X, Y + Y, T ). The temperature is kept constant for simplicity;it could also be varied. We can use the equation of state to calculate