Statistical Interpretation of Entropy
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Statistical Interpretation of Entropy. Statistical mechanics bridges between. phenomenological thermodynamics. underlying microscopic behavior. -Statistical mechanics requires a separate lecture ( see textbook part II ). but.

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Statistical Interpretation of Entropy

Statistical mechanics bridges between

phenomenological thermodynamics

underlying microscopic behavior

-Statistical mechanics requires a separate lecture (see textbook part II)


abstract nature of entropy asks for an intuitive picture for state function S

Here: heuristic approach to statistical interpretation of entropy by

Ludwig Boltzman

Consider a gas in a box from an atomistic point of view

atoms (or molecules) are moving in a disordered manner within the box

having collisions with the walls



Probability to find a particular atom in V2 reads

Let's pick two atom:

Probability to find both of them at the same time in V2 reads



Probability to find all N atom at the same time in V2 reads

Let W1 be the “thermodynamic probability” to find system in the homogeneously occupied

state (more precisely:

# of possible microstates)

Thermodynamic probability to find system in a state with

all atoms in V2 reads:


Note: W1>>W2 for N large

Entropy S quantifies the thermodynamic probability W of a particular state

We know: S extensive






Thermodynamic probability WA+B to find combined system A+B in a state where

subsystem A is in a state of thermodynamic probability WA


subsystem B is in a state of thermodynamic probability WB reads

Which function does the job

so that

= particular state


Let’s determine C

We know entropy change with volume changeV0Vf for an ideal gas

in an adiabatically isolated box

(see lecture)

Now we apply


comparison with particular state

W: # of possible microstates

Principle of increase of entropy

Adiabatically insulated system

approaches state of maximum