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

  1. Show that, in this limit, the Bose-Einstein function can be written approximately as
    n¯BE=e(μ)/kT[1+eμ/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.

Short Answer

Expert verified
  1. The Bose-Einstein distribution is given as n¯BE=eεμ/kT1+eεμ/kT.
  2. The chemical potential is μ=kTlnVKvQkTNvQ8V.
  3. The pressure of a gas is given as P=kTVlnZand the logarithm of the grand partition function is computed as a sum over all modes.
  4. The pressure of a gas is P=NkTV1NvQ42V.
  5. The value of the virial coefficient for helium is given as βT=7.089×105 m3K3/2/molT3/2.
  6. For spin, half particle number density is given as N=2VvQeμ/kT1eμ/kT8. The viral coefficient is given as βT=NA82h22πmkT3/2and the value of the virial coefficient is given as βT=5.457×105 m3K3/2/molT3/2.

Step by step solution

01

Part a. Step 1. Given.

There are non-interacting spin-0 bosons at high temperatures.

02

Part a. Step 2. Formula used.

Write the expression for Bose-Einstein Distribution.

n¯BE=1eεμ/kT1 …… (1)

Here, n¯BE is the Bose-Einstein Distribution, ε is the energy of bosons, μ is the chemical potential, k is the Boltzmann constant and Tis the temperature.

03

Part a. Step 3. Calculation.

Multiply and divide the RHS by eεμ/kTof equation (1).

n¯BE=eεμ/kT1eεμ/kT

At high temperatures, the factor eεμ/kTis less than 1, so using binomial expansion.

n¯BE=eεμ/kT1+eεμ/kT

04

Part a. Step 4. Conclusion.

Thus, the Bose-Einstein distribution is given as n¯BE=eεμ/kT1+eεμ/kT.

05

Part b. Step 1. Given.

There are non-interacting spin-0 bosons at high temperatures.

06

Part b. Step 2. Formula used.

Write the expression for Bose-Einstein Distribution.

N=0gεn¯BEdε …… (2)

Here, Nis the number density, gεis the energy density and n¯BEis Bose-Einstein Distribution.

Write the expression for Bose-Einstein Distribution.

n¯BE=eεμ/kT1+eεμ/kT

Here, n¯BEis the Bose-Einstein Distribution, εis the energy of bosons, μis the chemical potential, kis the Boltzmann constant and Tis the temperature.

Write the expression for density of states.

gε=g0ε=2π2πmh23/2Vε

Here, role="math" localid="1651740877830" gεis the density of states, role="math" localid="1651740907072" g0is the constant of the density of states, mis the mass of particles, role="math" localid="1651740949903" his the Planck’s constant, Vis the volume of gas, and εis the energy of particles.

Write the expression for quantum volume.

vQ=h22πmkT3/2

Here, vQ is the quantum volume.

07

Part b. Step 3. Calculation.

Substitute eεμ/kT1+eεμ/kTfor n¯BEin equation (2).

N=0gεeεμ/kT1+eεμ/kTdε

Substitute g0εfor gεin the above equation.

N=g00eεμ/kT1+eεμ/kTεdε

N=g00eεμ/kT+e2εμ/kTεdε …… (3)

Substitute xfor εkTand dxfor 12kTxdεin the first term of equation (3).

g00eεμ/kTεdε=g0eμ/kT0eε/kTεdε=2g0kT3/2eμ/kT0x2ex2dx

Substitute π4for 0x2ex2dxin the above equation.

g00eεμ/kTεdε=2g0kT3/2eμ/kTπ4=π2g0kT3/2eμ/kT

Substitute xfor εkTand dxfor 12kTxdεin the second term of equation (3).

g00e2εμ/kTεdε=g0e2μ/kT0e2ε/kTεdε=2g0kT3/2e2μ/kT0x2e2x2dx

Substitute π48for 0x2e2x2dxin the above equation.

g00e2εμ/kTεdε=2g0kT3/2e2μ/kTπ48=π28g0kT3/2e2μ/kT

Substitute π2g0kT3/2eμ/kTfor g00eεμ/kTεdεand π28g0kT3/2e2μ/kTfor g00e2εμ/kTεdεin equation (3).

N=π2g0kT3/2eμ/kT+π28g0kT3/2e2μ/kT=π2g0kT3/2eμ/kT1+eμ/kT8

Substitute 2π2πmh23/2Vfor g0in the above equation.

N=π22π2πmh23/2VkT3/2eμ/kT1+eμ/kT8=2πmkTh23/2Veμ/kT1+eμ/kT8

Substitute vQfor h22πmkT3/2in the above equation.

N=VvQeμ/kT1+eμ/kT8eμ/kT=VNvQ1+eμ/kT8

Substitute NvQfor eμ/kTin bracket term of the above equation.

eμ/kT=VNvQ1+NvQ8

Take logarithm on both sides and solve forμ.

μkT=lnVNvQ1+NvQ8μ=kTlnVNvQ1+NvQ8=kTlnVKvQkTln1+NvQ8V

Expand the logarithm term as ln1+xxin the above equation.

μ=kTlnVKvQkTNvQ8V

The first term is classical chemical potential and the second term is the correction term.

08

Part b. Step 4. Conclusion.

Thus, the chemical potential is μ=kTlnVKvQkTNvQ8V.

09

Part c. Step 1. Given.

There are non-interacting spin-0 bosons at high temperatures.

10

Part c. Step 2. Formula used.

Write the expression for grand potential.

Φ=PV …… (4)

Here, Φis the grand potential, Pis the pressure of gas and Vis the volume of gas.

Write the expression for grand potential in form of a grand partition function.

Φ=kTlnZ ……. (5)

Here, kis the Boltzmann constant, Tis the temperature and Zis the partition function.

Consider a system of non-interacting particles in a box. Write the expression for the grand partition function.

Ztot=Z1Z2Z3.....=nZn …… (6)

11

Part c. Step 3. Calculation.

Equate equation (4) and equation (5).

PV=kTlnZP=kTVlnZ

Take the logarithm on both sides of equation (6).

lnZtot=lnZ1Z2Z3...=lnZ1+lnZ2+lnZ3+...=nlnZn

12

Part c. Step 4. Conclusion.

Thus, the pressure of a gas is given as P=kTVlnZand the logarithm of the grand partition function is computed as the sum over all modes.

13

Part d. Step 1. Given.

There are non-interacting spin-0 bosons at high temperatures.

14

Part d. Step 2. Formula used.

Write the expression for the logarithm of the grand partition function.

lnZtot=nlnZn …… (7)

Here, Ztotis the total sum of the grand partition function and Znis the grand partition function for nparticles.

Write the expression for the grand partition function.

Zn=11eεμ/kT …… (8)

Here, μ is the chemical potential, k is the Boltzmann constant and Tis the temperature.

Write the expression for quantum volume.

vQ=h22πmkT3/2

Here, vQ is the quantum volume.

Write the expression for the pressure of the gas.

P=kTVlnZ …… (9)

Here, Z is the grand partition function, P is the pressure of gas and Vis the volume of gas.

15

Part d. Step 3. Calculation.

Change the summation into integral and multiply it with the spherical co-ordinate factor n2sinθin equation (7).

lnZtot=0π/2dΦ0π/2sinθdθ0n2lnZndn

lnZtot=π20n2lnZndn …… (9)

Take logarithm of equation (8).

lnZn=ln1eεμ/kT

Expand the logarithm of the above equation.

lnZn=eεμ/kT+12e2εμ/kT

Substitute eεμ/kT+12e2εμ/kTfor lnZnin equation (9).

lnZn=π20n2eεμ/kT+12e2εμ/kTdn=π20n2eεμ/kTdn+120n2e2εμ/kTdn=π2eμ/kT0n2eε/kTdn+120n2e2ε/kTdn

Substitute xfor εkT, h28mL2kTfor xand dxfor dεkTin the above equation.

lnZtot=π28mL2kTh23/2eμ/kT0x2ex2dx+12eμ/kT0x2e2x2dx

Substitute π4for 0x2ex2dxand π48for 0x2e2x2dxin the above equation.

lnZtot=π28mL2kTh23/2eμ/kTπ4+eμ/kTπ88

Substitute vQfor h22πmkT3/2and V for L3 in the above equation.

lnZtot=VvQeμ/kT1+eμ/kT128

Substitute VNvQ1NvQ8for eμ/kTin the above equation.

role="math" localid="1651745720233" lnZtot=VvQ1+NvQV128NvQV1NvQ8V=N1NvQ8V1+NvQV128

Simplify the above term and neglect the higher terms.

lnZtot=N1NvQ42V

Substitute N1NvQ42Vfor lnZin equation (9).

P=NkTV1NvQ42V

16

Part d. Step 4. Conclusion.

Thus, the pressure of a gas is P=NkTV1NvQ42V.

17

Part e. Step 1. Given.

There are non-interacting spin-0 bosons at high temperature

18

Part e. Step 2. Formula used.

Write the expression of pressure in terms of virial expansion.

P=NkTV1+βTV/n …… (10)

Here, Pis the pressure of the gas, Vis the volume of gas, Nis the number of particles, kis the Boltzmann constant, Tis the temperature of the gas, nis the number of a mole in gas and βTis a viral coefficient.

Write the expression for pressure of the gas.

P=NkTV1NvQ42V …… (11)

Here, vQ is the quantum volume.

19

Part e. Step 3. Calculation.

Equate the equation (10) and equation (11) to get the value of the virial coefficient.

βT=NAvQ42

Substitute h22πmkT3/2for vQ in the above equation.

βT=NA42h22πmkT3/2

Substitute 6.022×1023 mol1for NA, 6.626×1034 Jsfor h, 6.64×1027 kgform and 1.38×1023 J/Kfor kin the above equation.

βT=6.022×1023 mol1426.626×1034 Js223.146.64×1027 kg1.38×1023 J/KT3/2=7.089×105 m3K3/2/molT3/2

Plot βTversus TK.

20

Part e. Step 4. Conclusion.

Thus, the value of the virial coefficient for helium is given as βT=7.089×105 m3K3/2/molT3/2.

21

Part f. Step 1. Given.

There are non-interacting spin-0 bosons at high temperatures.

22

Part f. Step 2. Formula used.

Write the expression of number density.

N=VvQeμ/kT1eμ/kT8 …… (12)

Here, N is the number density of particles, vQ is the quantum volume, V is the volume of gas, μ is the chemical potential, k is the Boltzmann constant and Tis the temperature.

Write the expression for the grand partition function.

Z=1+eεμ/kT

Write the expression for the viral coefficient.

βT=NAvQ42 …… (13)

Here, βT is the virial coefficient, NA is the Avogadro number, and vQis the quantum volume

23

Part f. Step 3. Calculation.

Multiply by the factor of 2 with equation (12) for spin half particles.

N=2VvQeμ/kT1eμ/kT8

Multiply the factor of 12with equation (13) for spin half particles.

βT=NAvQ82

Substitute h22πmkT3/2for vQ in the above equation.

βT=NA82h22πmkT3/2

Substitute 6.022×1023 mol1for NA, 6.626×1034 Jsfor h, 3×1.66×1027 kgform and 1.38×1023 J/Kfor kin the above equation.

βT=6.022×1023 mol1826.626×1034 Js223.143×1.66×1027 kg1.38×1023 J/KT3/2=5.457×105 m3K3/2/molT3/2

Plot βTversus TK.

24

Part f. Step 4. Conclusion.

Thus, for a spin, half particle number density is given as N=2VvQeμ/kT1eμ/kT8. The viral coefficient is given as βT=NA82h22πmkT3/2and the value of virial coefficient is given as βT=5.457×105 m3K3/2/molT3/2.

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Most popular questions from this chapter

An atomic nucleus can be crudely modeled as a gas of nucleons with a number density of 0.18fm-3(where 1fm=10-15m). Because nucleons come in two different types (protons and neutrons), each with spin 1/2, each spatial wavefunction can hold four nucleons. Calculate the Fermi energy of this system, in MeV. Also calculate the Fermi temperature, and comment on the result.

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A = probability of spontaneous decay per unit time.

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B'=BandAB=8πhf3c3

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