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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.

Short Answer

Expert verified
  1. The Bose-Einstein distribution is given as nBE=e/kT1+e/kT.
  2. The chemical potential is =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 T=7.089105鈥尘3K3/2/molT3/2.
  6. For spin, half particle number density is given as N=2VvQe/kT1e/kT8. The viral coefficient is given as T=NA82h22mkT3/2and the value of the virial coefficient is given as T=5.457105鈥尘3K3/2/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.

nBE=1e/kT1 鈥︹ (1)

Here, nBE is the Bose-Einstein Distribution, is the energy of bosons, 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 e/kTof equation (1).

nBE=e/kT1e/kT

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

nBE=e/kT1+e/kT

04

Part a. Step 4. Conclusion.

Thus, the Bose-Einstein distribution is given as nBE=e/kT1+e/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=0gnBEd 鈥︹ (2)

Here, Nis the number density, gis the energy density and nBEis Bose-Einstein Distribution.

Write the expression for Bose-Einstein Distribution.

nBE=e/kT1+e/kT

Here, nBEis the Bose-Einstein Distribution, is the energy of bosons, is the chemical potential, kis the Boltzmann constant and Tis the temperature.

Write the expression for density of states.

g=g0=22mh23/2V

Here, role="math" localid="1651740877830" gis 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鈥檚 constant, Vis the volume of gas, and is the energy of particles.

Write the expression for quantum volume.

vQ=h22mkT3/2

Here, vQ is the quantum volume.

07

Part b. Step 3. Calculation.

Substitute e/kT1+e/kTfor nBEin equation (2).

N=0ge/kT1+e/kTd

Substitute g0for gin the above equation.

N=g00e/kT1+e/kTd

N=g00e/kT+e2/kTd 鈥︹ (3)

Substitute xfor kTand dxfor 12kTxdin the first term of equation (3).

g00e/kTd=g0e/kT0e/kTd=2g0kT3/2e/kT0x2ex2dx

Substitute 4for 0x2ex2dxin the above equation.

g00e/kTd=2g0kT3/2e/kT4=2g0kT3/2e/kT

Substitute xfor kTand dxfor 12kTxdin the second term of equation (3).

g00e2/kTd=g0e2/kT0e2/kTd=2g0kT3/2e2/kT0x2e2x2dx

Substitute 48for 0x2e2x2dxin the above equation.

g00e2/kTd=2g0kT3/2e2/kT48=28g0kT3/2e2/kT

Substitute 2g0kT3/2e/kTfor g00e/kTdand 28g0kT3/2e2/kTfor g00e2/kTdin equation (3).

N=2g0kT3/2e/kT+28g0kT3/2e2/kT=2g0kT3/2e/kT1+e/kT8

Substitute 22mh23/2Vfor g0in the above equation.

N=222mh23/2VkT3/2e/kT1+e/kT8=2mkTh23/2Ve/kT1+e/kT8

Substitute vQfor h22mkT3/2in the above equation.

N=VvQe/kT1+e/kT8e/kT=VNvQ1+e/kT8

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

e/kT=VNvQ1+NvQ8

Take logarithm on both sides and solve for.

kT=lnVNvQ1+NvQ8=kTlnVNvQ1+NvQ8=kTlnVKvQkTln1+NvQ8V

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

=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 =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.

=PV 鈥︹ (4)

Here, is 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.

=kTlnZ 鈥︹. (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 鈥︹ (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 鈥︹ (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=11e/kT 鈥︹ (8)

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

Write the expression for quantum volume.

vQ=h22mkT3/2

Here, vQ is the quantum volume.

Write the expression for the pressure of the gas.

P=kTVlnZ 鈥︹ (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 n2sinin equation (7).

lnZtot=0/2d0/2sind0n2lnZndn

lnZtot=20n2lnZndn 鈥︹ (9)

Take logarithm of equation (8).

lnZn=ln1e/kT

Expand the logarithm of the above equation.

lnZn=e/kT+12e2/kT

Substitute e/kT+12e2/kTfor lnZnin equation (9).

lnZn=20n2e/kT+12e2/kTdn=20n2e/kTdn+120n2e2/kTdn=2e/kT0n2e/kTdn+120n2e2/kTdn

Substitute xfor kT, h28mL2kTfor xand dxfor dkTin the above equation.

lnZtot=28mL2kTh23/2e/kT0x2ex2dx+12e/kT0x2e2x2dx

Substitute 4for 0x2ex2dxand 48for 0x2e2x2dxin the above equation.

lnZtot=28mL2kTh23/2e/kT4+e/kT88

Substitute vQfor h22mkT3/2and V for L3 in the above equation.

lnZtot=VvQe/kT1+e/kT128

Substitute VNvQ1NvQ8for e/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+TV/n 鈥︹ (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 Tis a viral coefficient.

Write the expression for pressure of the gas.

P=NkTV1NvQ42V 鈥︹ (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.

T=NAvQ42

Substitute h22mkT3/2for vQ in the above equation.

T=NA42h22mkT3/2

Substitute 6.0221023鈥尘ol1for NA, 6.6261034鈥塉sfor h, 6.641027鈥塳驳form and 1.381023鈥塉/Kfor kin the above equation.

T=6.0221023鈥尘ol1426.6261034鈥塉s223.146.641027鈥塳驳1.381023鈥塉/KT3/2=7.089105鈥尘3K3/2/molT3/2

Plot Tversus TK.

20

Part e. Step 4. Conclusion.

Thus, the value of the virial coefficient for helium is given as T=7.089105鈥尘3K3/2/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=VvQe/kT1e/kT8 鈥︹ (12)

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

Write the expression for the grand partition function.

Z=1+e/kT

Write the expression for the viral coefficient.

T=NAvQ42 鈥︹ (13)

Here, T 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=2VvQe/kT1e/kT8

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

T=NAvQ82

Substitute h22mkT3/2for vQ in the above equation.

T=NA82h22mkT3/2

Substitute 6.0221023鈥尘ol1for NA, 6.6261034鈥塉sfor h, 31.661027鈥塳驳form and 1.381023鈥塉/Kfor kin the above equation.

T=6.0221023鈥尘ol1826.6261034鈥塉s223.1431.661027鈥塳驳1.381023鈥塉/KT3/2=5.457105鈥尘3K3/2/molT3/2

Plot Tversus TK.

24

Part f. Step 4. Conclusion.

Thus, for a spin, half particle number density is given as N=2VvQe/kT1e/kT8. The viral coefficient is given as T=NA82h22mkT3/2and the value of virial coefficient is given as T=5.457105鈥尘3K3/2/molT3/2.

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Most popular questions from this chapter

Change variables in equation 7.83 to =hc/ and thus derive a formula for the photon spectrum as a function of wavelength. Plot this spectrum, and find a numerical formula for the wavelength where the spectrum peaks, in terms of hc/kT. Explain why the peak does not occur at hc/(2.82kT).

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(d) Because gis a constant for this system, it is possible to carry out the integral 7.53 for the number of particles analytically. Do so, and solve for as a function of N. Show that the resulting formula has the expected qualitative behavior.

(e) Show that in the high-temperature limit, kTF, the chemical potential of this system is the same as that of an ordinary ideal gas.

For a system of fermions at room temperature, compute the probability of a single-particle state being occupied if its energy is

(a) 1eVless than

(b) 0.01eVless than

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Evaluate the integrand in equation 7.112as a power series in x, keeping terms through x4鈥 Then carry out the integral to find a more accurate expression for the energy in the high-temperature limit. Differentiate this expression to obtain the heat capacity, and use the result to estimate the percent deviation of Cvfrom3NkatT=TDandT=2TD.

Figure 7.37 shows the heat capacity of a Bose gas as a function of temperature. In this problem you will calculate the shape of this unusual graph.

(a) Write down an expression for the total energy of a gas of Nbosons confined to a volume V, in terms of an integral (analogous to equation 7.122).

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(c) Explain why the heat capacity must approach 32Nkin the high- Tlimit.

(d) For T>Tcyou can evaluate the integral using the values of calculated in Problem 7.69. Do this to obtain the energy as a function of temperature, then numerically differentiate the result to obtain the heat capacity. Plot the heat capacity, and check that your graph agrees with Figure 7.37.

Figure 7.37. Heat capacity of an ideal Bose gas in a three-dimensional box.

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