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Coursewebpagesh7p://personal.rhul.ac.uk/UHAP/027/PH4211/
YouwillberedirectedherefromtheMOODLEpage
4211 Statistical Mechanics Week 1
4211StatisticalMechanicsBrianCowan
1
‘LudwigBoltzmann,whospentmuchofhislifestudyingsta8s8calmechanics,diedin1906,byhisownhand.PaulEhrenfest,carryingonthework,diedsimilarlyin1933.Nowitisourturntostudysta8s8calmechanics.
Perhapsitwillbewisetoapproachthesubjectcau8ously.’
DavidGoodstein—inStatesofMatter,1975,DoverN.Y.
4211 Statistical Mechanics Week 1
StatisticalMechanics
2
‘StaLsLcalmechanics / thermodynamics,beingwhatmany consider to be one of the most intellectuallydifficult subjects of all, is noted for its prevalence ofsuicidesandsuicidea7emptsbyasignificantnumberofitsfounders’
4211 Statistical Mechanics Week 1
StatisticalMechanics
3
!!Deepthinking–Depression!!
RobertMayer:1850–jumpedoutofa3rd-floorwindow
LudwigBoltzmann:1906–hangedhimself
PaulEhrenfest:1933–shothisdaughterandhimself
FritzHaber’sfamily:1915–wifeshotherself;1947,bothsonanddaughtershotthemselves
GilbertLewis:1946–tookcyanidea\ernotge]ngNobelprize
AlanTuring:1954–bitintoacyanide-lacedapple
PercyBridgman:1961–shothimself4211 Statistical Mechanics Week 1
StatisticalMechanics
4
StatisticalMechanics Thisisa‘paperlesscourse’Seecoursewebpagesfor:
• Coursedetails–syllabusetc.• LectureLmetable/calendar• Lecturenotes• Problemsheets• OtherusefulinformaLon• Pastexampapers
AndFANTASTICbook!!!!!!
4211 Statistical Mechanics Week 1 5
CourseOutline
• MethodologyofStaLsLcalMechanics• Non-IdealGases• PhaseTransiLons• FluctuaLonsandDynamics
4211 Statistical Mechanics Week 1 6
Chapter 1
The Methodology of Statistical Mechanics
1.1 Terminology and Methodology
1.1.1 Approaches to the subject
Thermodynamics is the study of the relationship between macroscopic properties of systems, such as temperature, volume, pressure, magnetisation, compressibility etc. etc.
Statistical Mechanics is concerned with understanding how the various macroscopic properties arise as a consequence of the microscopic nature of the system. In essence it makes macroscopic deductions from microscopic models.
4211 Statistical Mechanics Week 1 7
AlbertEinstein……. ‘Atheoryisthemoreimpressivethegreaterthesimplicityofitspremisesis,themoredifferentkindsofthingsitrelates,andthemoreextendedisitsareaofapplicability.
Thereforethedeepimpressionwhichclassicalthermodynamicsmadeuponme;itistheonlyphysicaltheoryofuniversalcontentconcerningwhichIamconvincedthat,withintheframeworkoftheapplicabilityofitsbasicconcepts,willneverbeoverthrown.’
AutobiographicalNotes,AlbertEinstein:Philosopher-ScienLst,Ed.P.A.Schilpp,OpenCourtPublishingCo.1949
4211 Statistical Mechanics Week 1 8
LevLandau…..‘Sta8s8calphysicsandthermodynamicstogetherformaunit.
Alltheconceptsandquan88esofthermodynamicsfollowmostnaturally,simplyandrigorouslyfromtheconceptsofsta8s8calphysics.
Althoughthegeneralstatementsofthermodynamicscanbeformulatednon-sta8s8cally,theirapplica8ontospecificcasesalwaysrequirestheuseofsta8s8calphysics.’
InthePrefacetoSta%s%calPhysics,vol5ofLandauandLifshitzCourseofTheoreLcalPhysics.
4211 Statistical Mechanics Week 1 9
ArnoldSommerfeld(notactuallyrelevant,but….)Sommerfeldwasaskedwhyhehadneverwri7enabookonthermodynamics–havingwri7enbooksonmostotherareasofphysics.Hereplied:
“Thermodynamicsisafunnysubject.Thefirst%meyougothroughit,youdon'tunderstanditatall.Thesecond%meyougothroughit,youthinkyouunderstandit,exceptforoneortwosmallpoints.Thethird%meyougothroughit,youknowyoudon'tunderstandit,butbythat%meyouaresousedtoit,itdoesn'tbotheryouanymore.”
Howeverlater,in1951,wriLngabookonthermodynamicsandstaLsLcalmechanics,hewaskilledfollowingatrafficaccident.Thebookwaspublishedposthumouslythefollowingyear.
4211 Statistical Mechanics Week 1 10
Relationship between Statistical Mechanics and Thermodynamics
PARADOX
Landau: disciplines are ‘similar’ – reductionism – Thermodynamics explained by Statistical Mechanics
Einstein: disciplines are ‘opposites’ – the domain of Thermodynamics is independent of the microscopics
Resolution – concept of emergence – Macroscopic behaviour is consistent with microscopic description, but robust against microscopic details.
4211 Statistical Mechanics Week 1 11
1.1.2 Description of states
Macrostate
Microstate
4211 Statistical Mechanics Week 1 12
1.1.3 Extensivity and the Thermodynamic Limit
• Extensive variables
• Intensive variables
Stat. Mech. deals with large systems – thermodynamic limit: N ⟶ ∞, V ⟶ ∞, while N/V remains constant.
Surface effects can be neglected – Short-range forces!
4211 Statistical Mechanics Week 1 13
1.2 The Fundamental Principles
1.2.1 The laws of thermodynamics
• Zeroth Law
• First Law
• Second Law
• Third Law
4211 Statistical Mechanics Week 1 14
Zeroth Law
If system A is in equilibrium with system B and system A is in equilibrium with system C
then system B is in equilibrium with system C.
• Equilibrium of 2 bodies: No change when put together
• The existence of equilibrium states
• The existence of temperature: subtle – but essentially simple arguments. See Pippard (or Adkins).
4211 Statistical Mechanics Week 1 15
First Law
The internal energy of a body can change by the flow of heat or by doing work.
4211 Statistical Mechanics Week 1
E Q WΔ = Δ +Δ
Beware different sign conventions for ΔW. (older books)
Physics of the First Law is obvious to us – but it is important in the historical context: heat is a form of energy.
16
Second Law
Clausius: Heat flows from hot to cold.
Kelvin: It is not possible to convert all heat energy to work.
Carathéodory: In the neighbourhood of any equilibrium state of a thermally isolated system there are states which are inaccessible.
Callen: There is an extensive quantity, which we call entropy, which never decreases in a physical process.
4211 Statistical Mechanics Week 1 17
Third Law
Nernst, Simon and others
The entropy of a body tends to zero as the temperature tends to absolute zero.
4211 Statistical Mechanics Week 1 18
1.2.2 Probabilistic interpretation of the First Law
4211 Statistical Mechanics Week 1
j jj
E P E=∑
d d dj j j jj j
E P E E P= +∑ ∑
d d d dj j j jj jP E W E P Q= =∑ ∑
Shifting energy levels Changing populations of states
Work Heat
19
1.2.3 Microscopic basis for entropy
Postulate – All microstates of an isolated system are equally likely. Isolated system specified by E, V, N.
4211 Statistical Mechanics Week 1
( ), ,E V NΩ is number of microstates in macrostate (E, V, N)
( ) ( ), , , ,P E V N E V N∝Ω
Ω is multiplicative, so ln Ω is additive – extensive variable
So define entropy S:
( ) ( ), , ln , ,S E V N k E V N= Ω
20
4211 Statistical Mechanics Week 1
Ludwig Boltzmann
1844 - 1902
21
1.3 Interactions – The conditions for Equilibrium
When systems interact their states will often change, the composite system evolving to a state of equilibrium. We shall investigate what determines the final state. We will see that quantities such as temperature emerge in the description of these equilibrium states.
4211 Statistical Mechanics Week 1 22
1.3.1 Thermal interaction – Temperature
E1 V1 N1 E2 V2 N2 E1 V1 N1 E2 V2 N2
fixed diathermal wall
4211 Statistical Mechanics Week 1 23
1.3.1 Thermal interaction – Temperature
E1 and E2 can vary subject to E1 + E2 = const = E0.
4211 Statistical Mechanics Week 1 24
1.3.1 Thermal interaction – Temperature
E1 and E2 can vary subject to E1 + E2 = const = E0.
4211 Statistical Mechanics Week 1
( ) ( )1 1 2 2E EΩ =Ω Ω
( ) ( )1 2 0E E EΩ =Ω Ω −
Maximise Ω subject to varying E.
1 22 1
d dd 0d d dE E E
Ω ΩΩ = Ω −Ω =
1 2
1 2
d d1 1d dE EΩ Ω=
Ω Ω
1 2 1 2dln dln d dord d d d
S SE E E EΩ Ω= =
25
1.3.2 Volume change – Pressure
E1 V1 N1 E2 V2 N2 E1 V1 N1 E2 V2 N2
movable diathermal wall
4211 Statistical Mechanics Week 1 26
1.3.3 Particle interchange – chemical potential.
E1 V1 N1 E2 V2 N2 E1 V1 N1 E2 V2 N2
fixed permeable diathermal wall
4211 Statistical Mechanics Week 1 27
1.3.4 Thermal interaction with the rest of the world – The Boltzmann factor
The Canonical Distribution Function
fixed diathermal wall
heat bath
system of interestenergy E
ΩB
Ω
total energy ET
4211 Statistical Mechanics Week 1 28
1.3.4 Thermal interaction with the rest of the world – The Boltzmann factor
The Canonical Distribution Function
4211 Statistical Mechanics Week 1
( ) E kTP E e−∝
The Boltzmann factor is a key result. Richard Feynman says:
“This fundamental law is the summit of statistical mechanics, and the entire subject is either a slide-down from the summit, as the principle is applied to various cases, or the climb-up to where the fundamental law is derived and the concepts of thermal equilibrium and temperature clarified”.
R. P. Feynman, Statistical Mechanics, Benjamin 1972.
29
1.3.5 Particle and energy exchange with the rest of the world – the Gibbs factor
The Grand Canonical Distribution Function
fixed diathermalpermeable wall
heat bath
system of interestenergy E
ΩB
Ω
total energy ET
particles N
total particles NT
4211 Statistical Mechanics Week 1 30
1.4 Thermodynamic averages
1.4.1 The partition function
4211 Statistical Mechanics Week 1
( ) E kTP E e−∝
( )j
E kT
E kT
j
eP EZ
Z e
−
−
=
=∑
Z is the “partition” function – amazingly important!
31
1.4.2 Generalised expression for Entropy
4211 Statistical Mechanics Week 1
system of interest
collection of identical systems
ln
ln .
j jj
j
S k P P
k P
= −
= −
∑
32
1.4.3 Free Energy
4211 Statistical Mechanics Week 1
( )( )
( ) ( ) ( ),
,, , , , ,, ,
ln ln
ln
ln
lnln
jj
E N V kTE N V kT
jj
jj
j
eP N V T Z N V T eZ N V T
EP Z
kT
ES k Z
kTES k ZT
E TS kT ZF E TS F kT Z
−−= =
⎛ ⎞= − +⎜ ⎟
⎝ ⎠
= +
= +
− = −= − = −
∑
33
1.4.4 Thermodynamic Variables
A host of thermodynamic variables can be obtained from the partition function. This is seen from the differential of the free energy.
4211 Statistical Mechanics Week 1
d d d dbut so that d d d dThen we can identify the partial derivatives:
E T S p V NF E TSF S T p V N
µ
µ
= − += −= − − +
34
1.4.4 Thermodynamic Variables
4211 Statistical Mechanics Week 1
, ,
, ,
, ,
2
,
d d d dlnso that ln
ln
ln
lnand then
V N V N
T N T N
T V T V
V N
F S T p V NF ZS kT k ZT T
F Zp kTV V
F ZkTN N
ZE F TS kTT
µ
µ
= − − +∂ ∂= − = +∂ ∂
∂ ∂= − =∂ ∂
∂ ∂= = −∂ ∂
∂= + =∂
★ So all follows from differentiating Z.
35