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At the center of the sun, the temperature is approximately 107K and the concentration of electrons is approximately 1032 per cubic meter. Would it be (approximately) valid to treat these electrons as a "classical" ideal gas (using Boltzmann statistics), or as a degenerate Fermi gas (with T0 ), or neither?

Short Answer

Expert verified

The gas cannot be treated as degenerate at T=0, and ordinary classical ideal gas at TTF because the temperature T is not much less than or greater than TF.

Step by step solution

01

Step 1. Given Information 

We are given that the temperature at the centre of the sun is approximately 107Kand the concentration of electrons is approximately 1032per cubic meter.

02

Step 2. Finding fermi temperature

Calculating the fermi temperature for the electron gas at the center of the Sun to check if the given statement is correct or not.

The Fermi temperature for the electron gas at the center of the Sun is given by,

TF=EFk

The fermi energy of electrons can be expressed in terms of free electron density as follows,

F=h28me3NV23

03

Step 3. Finding fermi temperature

Putting the values, we get

TF=1kh28me3NV23TF=6.6310-34Js21.3810-23J/K(8)9.110-31kg31032m-323=9.1106K

The Fermi temperature is of the same order of magnitude as the temperature of the Sun 107K.

04

Step 4. About the Statement

Here, the temperature T is not much less than or greater than TF. Hence, the approximation is not very accurate.

Due to this reason, the gas cannot be treated as degenerate at T=0, and ordinary classical ideal gas at TTF.

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Most popular questions from this chapter

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?

Problem 7.69. If you have a computer system that can do numerical integrals, it's not particularly difficult to evaluate forT>Tc.

(a) As usual when solving a problem on a computer, it's best to start by putting everything in terms of dimensionless variables. So define t=T/Tc,c=/kTc,andx=/kTc. Express the integral that defines , equation 7.22, in terms of these variables. You should obtain the equation

2.315=0xdxe(x-c)/t-1

(b) According to Figure

the correct value of cwhen T=2Tcis approximately -0.8. Plug in these values and check that the equation above is approximately satisfied.

(c) Now vary , holding Tfixed, to find the precise value of for T=2Tc. Repeat for values of T/Tcranging from 1.2up to 3.0, in increments of 0.2. Plot a graph of as a function of temperature.

Consider the electromagnetic radiation inside a kiln, with a volume of V= I m3 and a temperature of 1500 K.

(a) What is the total energy of this radiation?

(b) Sketch the spectrum of the radiation as a function of photon energy.

(c) What fraction of all the energy is in the visible portion of the spectrum, with wavelengths between 400 nm and 700 nm?

Imagine that there exists a third type of particle, which can share a single-particle state with one other particle of the same type but no more. Thus the number of these particles in any state can be 0,1 or 2 . Derive the distribution function for the average occupancy of a state by particles of this type, and plot the occupancy as a function of the state's energy, for several different temperatures.

A black hole is a blackbody if ever there was one, so it should emit blackbody radiation, called Hawking radiation. A black hole of mass M has a total energy of Mc2, a surface area of 16G2M2/c4, and a temperature ofhc3/162kGM(as shown in Problem 3.7).

(a) Estimate the typical wavelength of the Hawking radiation emitted by a one-solar-mass (2 x 1030 kg) black hole. Compare your answer to the size of the black hole.

(b) Calculate the total power radiated by a one-solar-mass black hole.

(c) Imagine a black hole in empty space, where it emits radiation but absorbs nothing. As it loses energy, its mass must decrease; one could say it "evaporates." Derive a differential equation for the mass as a function of time, and solve this equation to obtain an expression for the lifetime of a black hole in terms of its initial mass.

(d) Calculate the lifetime of a one-solar-mass black hole, and compare to the estimated age of the known universe (1010 years).

(e) Suppose that a black hole that was created early in the history of the universe finishes evaporating today. What was its initial mass? In what part of the electromagnetic spectrum would most of its radiation have been emitted?

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