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Consider a system consisting of a single hydrogen atom/ion, which has two possible states: unoccupied (i.e., no electron present) and occupied (i.e., one electron present, in the ground state). Calculate the ratio of the probabilities of these two states, to obtain the Saha equation, already derived in Section 5.6 Treat the electrons as a monotonic ideal gas, for the purpose of determining μ. Neglect the fact that an electron has two independent spin states.

Short Answer

Expert verified

The ratio of probabilities of unoccupied state to occupied state is kTPeνQe-1kT.

Step by step solution

01

Step 1. Formula Gibb's Factor

Formula for Gibb's Factor is given as:

G(s)=e-(ε-μ)kT

G(s)=e-(ε-μ)kT

where, μis chemical potential, εis energy occupied, kis Boltzmann's constant, Tis temperature.

02

Step 2. Ratio of Probabilities

Substitute ε=0andμ=0for unoccupied state.

G(s)=e-(0-0)kT=e0=1

Substitute ε=-1for occupied state.

role="math" localid="1647153963926" G~(s)=e-(-1-μ)kT=e(1+μ)kT

The ratio of probability of unoccupied state to occupied state is:

role="math" localid="1647154152958" G(s)G~(s)=1e(1+μ)kT

03

Step 3. Substitution in chemical potential formula

Formula for chemical potential is given as:

μ=-kT×lnVZinNνQ

Substitute VN=kTPewhere Peis partial pressure of electron gas.

role="math" localid="1647154444368" μ=-kT×lnkTZinPeνQ

For mono atomic gas, substitute Zin=1

-μkT=lnkTPeνQe-μkT=kTPeνQ
04

Step 4. Final calculation

Ratio of probability can be written as,

G(s)G~(s)=1e(1+μ)kT=e-1kT×e-μkT

Substitute e-μkT=kTPeνQin above equation,

role="math" localid="1647155185719" G(s)G~(s)=e-1kT×kTPeνQ=kTPeνQe-1kT

Hence, the ratio of probabilities of unoccupied state to occupied state is kTPeνQe-1kT .

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Most popular questions from this chapter

Consider two single-particle states, A and B, in a system of fermions, where ϵA=μ-xand ϵB=μ+x; that is, level A lies below μ by the same amount that level B lies above μ. Prove that the probability of level B being occupied is the same as the probability of level A being unoccupied. In other words, the Fermi-Dirac distribution is "symmetrical" about the point where ϵ=μ.

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The sun is the only star whose size we can easily measure directly; astronomers therefore estimate the sizes of other stars using Stefan's law.

(a) The spectrum of Sirius A, plotted as a function of energy, peaks at a photon energy of2.4eV, while Sirius A is approximately 24times as luminous as the sun. How does the radius of Sirius A compare to the sun's radius?

(b) Sirius B, the companion of Sirius A (see Figure 7.12), is only role="math" localid="1647765883396" 3%as luminous as the sun. Its spectrum, plotted as a function of energy, peaks at about7eV. How does its radius compare to that of the sun?

(c) The spectrum of the star Betelgeuse, plotted as a function of energy, peaks at a photon energy of 0.8eV, while Betelgeuse is approximately10,000times as luminous as the sun. How does the radius of Betelgeuse compare to the sun's radius? Why is Betelgeuse called a "red supergiant"?

Consider a system consisting of a single impurity atom/ion in a semiconductor. Suppose that the impurity atom has one "extra" electron compared to the neighboring atoms, as would a phosphorus atom occupying a lattice site in a silicon crystal. The extra electron is then easily removed, leaving behind a positively charged ion. The ionized electron is called a conduction electron because it is free to move through the material; the impurity atom is called a donor, because it can "donate" a conduction electron. This system is analogous to the hydrogen atom considered in the previous two problems except that the ionization energy is much less, mainly due to the screening of the ionic charge by the dielectric behavior of the medium.

(a) Write down a formula for the probability of a single donor atom being ionized. Do not neglect the fact that the electron, if present, can have two independent spin states. Express your formula in terms of the temperature, the ionization energy I, and the chemical potential of the "gas" of ionized electrons.

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