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For a system of particles at room temperature, how large must -be before the Fermi-Dirac, Bose-Einstein, and Boltzmann distributions agree within 1%? Is this condition ever violated for the gases in our atmosphere? Explain.

Short Answer

Expert verified

As far as gases in the atmosphere are concerned, the condition was never violated.

Step by step solution

01

Given Information 

The Fermi-Dirac, Bose-Einstein and Boltzmann distribution lies within 1%of1.

02

Explanation

The quantum volume expression is:

vQ=h2mkT3

h=Planck's constant,

m=mass of gas molecule,

k=gas constant

T=temperature

vQ=quantum volume.

The pressure of gas molecule will be denoted by,

P0=kTZinme/TTv

Zint=partition function,

=chemical potential

P0=pressure at fixed temperature.

Substitute 2h2mkT3for vQin above expression,

Rearrange the terms,

-/kT=lnkTZintP02mkTh3..(1)

03

Explanation

The Bose-Einstein distribution's expression is:

nBE=1e(x)kT1

=energy at any state and

nBE=Bose-Einstein distribution.

The Fermi-Dirac distribution's expression is:

nFD=1e(k-)kT+1

nFDBose-Einstein distribution.

Equation (I) divided by equation (II) is:

nBEnFD=e(x)dT+1e(x)XT1=1+e(c)kT1e(c)kT1+2e()/kT

Here, ()/kT1so, the above approximation is valid.

04

Explanation

Calculation:

The value of e-(-)/kTShould be less than 1/200for the ratio to be within 1%of 1 ; it means the value (-)/kT>ln200=5.3.

The value of cannot be negative.

So, -/kTmust be greater than 5.3

Substitute localid="1651131552143" 1.3810-23J/K=k

300K=T

4.651026kg=m

localid="1651131671206" 6.631034J.s

50=Zint

105Pa=Pin equation (I).

/kT=ln1.381023J/K(300K)(50)105Pa24.651026kg1.381023J/K(300K)6.631034Js3=ln3108=ln3+8ln10=19.52

This indicates that the condition is valid since -mu/kTis greater than 5.3.

No violation of the condition occurred as far as atmospheric gases are concerned.

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Most popular questions from this chapter

Sometimes it is useful to know the free energy of a photon gas.

(a) Calculate the (Helmholtz) free energy directly from the definition

(Express the answer in terms of T' and V.)

(b) Check the formula S=-(F/T)Vfor this system.

(c) Differentiate F with respect to V to obtain the pressure of a photon gas. Check that your result agrees with that of the previous problem.

(d) A more interesting way to calculate F is to apply the formula F=-kTlnZ separately to each mode (that is, each effective oscillator), then sum over all modes. Carry out this calculation, to obtain

F=8V(kT)4(hc)30x2ln1-e-xdx

Integrate by parts, and check that your answer agrees with part (a).

Consider a gas of noninteracting spin-0 bosons at high temperatures, when TTc. (Note that 鈥渉igh鈥 in this sense can still mean below 1 K.)

  1. Show that, in this limit, the Bose-Einstein function can be written approximately as
    nBE=e()/kT[1+e/kT+].
  2. Keeping only the terms shown above, plug this result into equation 7.122 to derive the first quantum correction to the chemical potential for gas of bosons.
  3. Use the properties of the grand free energy (Problems 5.23 and 7.7) to show that the pressure of any system is given by In P=(kT/V), where Zis the grand partition function. Argue that, for gas of noninteracting particles, In Zcan be computed as the sum over all modes (or single-particle states) of In Zi, where Zi; is the grand partition function for the ithmode.
  4. Continuing with the result of part (c), write the sum over modes as an integral over energy, using the density of states. Evaluate this integral explicitly for gas of noninteracting bosons in the high-temperature limit, using the result of part (b) for the chemical potential and expanding the logarithm as appropriate. When the smoke clears, you should find
    p=NkTV(1NvQ42V),
    again neglecting higher-order terms. Thus, quantum statistics results in a lowering of the pressure of a boson gas, as one might expect.
  5. Write the result of part (d) in the form of the virial expansion introduced in Problem 1.17, and read off the second virial coefficient, B(T). Plot the predicted B(T)for a hypothetical gas of noninteracting helium-4 atoms.
  6. Repeat this entire problem for gas of spin-1/2 fermions. (Very few modifications are necessary.) Discuss the results, and plot the predicted virial coefficient for a hypothetical gas of noninteracting helium-3 atoms.

Show that when a system is in thermal and diffusive equilibrium with a reservoir, the average number of particles in the system is

N=kTZZ

where the partial derivative is taken at fixed temperature and volume. Show also that the mean square number of particles is

N2=(kT)2Z2Z2

Use these results to show that the standard deviation of Nis

N=kTN/,

in analogy with Problem6.18Finally, apply this formula to an ideal gas, to obtain a simple expression forNin terms ofNDiscuss your result briefly.

Use the results of this section to estimate the contribution of conduction electrons to the heat capacity of one mole of copper at room temperature. How does this contribution compare to that of lattice vibrations, assuming that these are not frozen out? (The electronic contribution has been measured at low temperatures, and turns out to be about40% more than predicted by the free electron model used here.)

Starting from equation 7.83, derive a formula for the density of states of a photon gas (or any other gas of ultra relativistic particles having two polarisation states). Sketch this function.

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