Phys 362 Statistical and Thermal Physics: Homework 9 solved

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1 Entropy and free expansion

An isolated ideal gas expands freely from volume Vi to volume Vf .
a) Determine the change in entropy of the gas.
b) Does the change in entropy depend on whether the gas is monoatomic or diatomic?

2 Entropy and equations of state

For a particular gas the entropy is
S = S(E,V,N) = 3
2
N k ln !
E +
N2a
V

+ N k ln (V − N b) + f(N)
where a and b are constants and f(N) is some function of N only.

a) Determine an expression for T in terms of E,V,N and use this to determine E =
E(T,V,N).
b) Use the previous result to show that
S = S(T,V,N) = 3
2
N k ln T + N k ln (V − N b) + g(N)
where g(N) is a function of N only.
c) Determine an expression for P in terms of T,V,N.

3 Entropy changes in isothermal processes: ideal gas

An ideal gas (could be monoatomic, diatomic, etc.) with a fixed number of particles undergoes
an isothermal process in which its volume increases by a factor of three.

a) Determine an expression for the factor by which the pressure changes.
b) Determine an expression for the change in entropy of the system; use the expression for
the entropy of an ideal gas. Is this consistent with what the heat flow would predict for
this process?

4 Entropy changes in adiabatic processes: ideal gas

An ideal gas (could be monoatomic, diatomic, etc.) with a fixed number of particles undergoes
an adiabatic process in which its volume increases by a factor of three.

a) Determine an expression for the factor by which the pressure changes.
b) Determine an expression for the factor by which the temperature changes.

c) Determine an expression for the change in entropy of the system; use the expression for
the entropy of an ideal gas. Is this consistent with what the heat flow would predict for
this process?