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Compute the quantum volume for an N2molecule at room temperature, and argue that a gas of such molecules at atmospheric pressure can be

treated using Boltzmann statistics. At about what temperature would quantum statistics become relevant for this system (keeping the density constant and pretending that the gas does not liquefy)?

Short Answer

Expert verified

The quantum volume for an N2molecule at room temperature is 6.910-33m3. The temperature of system at which quantum statistics become relevant is 5.681K.

Step by step solution

01

Step 1. Formula quantum volume

Formula for quantum volume is:

Q=h2蟺尘办罢3

where,Tis temperature,role="math" localid="1647248715772" mis molecule mass,his Planck's constant,kis Boltzmann constant.

02

Step 2. Calculation quantum volume

Mass of N2 is calculated as:

role="math" localid="1647249031293" m=(28u)1.6710-27kg1u=46.4810-27kg

Substitute variables in quantum volume formula:

Q=6.6310-34Js2(46.4810-27kg)(1.3810-23J/K)(300K)3=6.6310-34347.9110-503=6.910-33m3

So, quantum volume ofN2molecule is6.910-33m3.

03

Step 3. Calculation ideal gas equation

Ideal gas equation is PV=NkT

So, VN=kTP

Substitute Pressure,P=1.013105N/m2, Temperature,T=300K.

role="math" localid="1647253225593" VN=(1.3810-23J/K)(300K)1.013105N/m2=410-26m3

04

Step 4. Calculation temperature

In quantum statistics, VNQ

kTPh2蟺尘办罢3T.T32h3(2蟺尘碍)32PkT52Ph3(2蟺尘)32k52T1kP2h6(2蟺尘)315

Substitute the values of variables in above equation:

T11.3810-23J/K(1.013105N/m2)2(6.6310-34Js)6(2(281.6710-27kg))315T=5.681K

So, temperature of the system is5.681K.

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Most popular questions from this chapter

The tungsten filament of an incandescent light bulb has a temperature of approximately 3000K. The emissivity of tungsten is approximately 13, and you may assume that it is independent of wavelength.

(a) If the bulb gives off a total of 100watts, what is the surface area of its filament in square millimetres?

(b) At what value of the photon energy does the peak in the bulb's spectrum occur? What is the wavelength corresponding to this photon energy?

(c) Sketch (or use a computer to plot) the spectrum of light given off by the filament. Indicate the region on the graph that corresponds to visible wavelengths, between400and700nm.

(d) Calculate the fraction of the bulb's energy that comes out as visible light. (Do the integral numerically on a calculator or computer.) Check your result qualitatively from the graph of part (c).

( e) To increase the efficiency of an incandescent bulb, would you want to raise or lower the temperature? (Some incandescent bulbs do attain slightly higher efficiency by using a different temperature.)

(f) Estimate the maximum possible efficiency (i.e., fraction of energy in the visible spectrum) of an incandescent bulb, and the corresponding filament temperature. Neglect the fact that tungsten melts at 3695K.

The argument given above for why CvTdoes not depend on the details of the energy levels available to the fermions, so it should also apply to the model considered in Problem 7.16: a gas of fermions trapped in such a way that the energy levels are evenly spaced and non-degenerate.

(a) Show that, in this model, the number of possible system states for a given value of q is equal to the number of distinct ways of writing q as a sum of positive integers. (For example, there are three system states for q = 3, corresponding to the sums 3, 2 + 1, and 1 + 1 + 1. Note that 2 + 1 and 1 + 2 are not counted separately.) This combinatorial function is called the number of unrestricted partitions of q, denoted p(q). For example, p(3) = 3.

(b) By enumerating the partitions explicitly, compute p(7) and p(8).

(c) Make a table of p(q) for values of q up to 100, by either looking up the values in a mathematical reference book, or using a software package that can compute them, or writing your own program to compute them. From this table, compute the entropy, temperature, and heat capacity of this system, using the same methods as in Section 3.3. Plot the heat capacity as a function of temperature, and note that it is approximately linear.

(d) Ramanujan and Hardy (two famous mathematicians) have shown that when q is large, the number of unrestricted partitions of q is given approximately by

p(q)e2q343q

Check the accuracy of this formula for q = 10 and for q = 100. Working in this approximation, calculate the entropy, temperature, and heat capacity of this system. Express the heat. capacity as a series in decreasing powers of kT/, assuming that this ratio is large and keeping the two largest terms. Compare to the numerical results you obtained in part (c). Why is the heat capacity of this system independent of N, unlike that of the three dimensional box of fermions discussed in the text?

Suppose that the concentration of infrared-absorbing gases in earth's atmosphere were to double, effectively creating a second "blanket" to warm the surface. Estimate the equilibrium surface temperature of the earth that would result from this catastrophe. (Hint: First show that the lower atmospheric blanket is warmer than the upper one by a factor of 21/4. The surface is warmer than the lower blanket by a smaller factor.)

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).

Starting from the formula for CV derived in Problem 7.70(b), calculate the entropy, Helmholtz free energy, and pressure of a Bose gas for T<Tc. Notice that the pressure is independent of volume; how can this be the case?

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