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Suppose you have three particles, and three distinct one-particle stateΨaX,ΨbX,andΨcxare available. How many different three-particle states can be constructed (a) if they are distinguishable particles, (b) if they are identical bosons, (c) if they are identical fermions? (The particles need not be in different states -ΨaX1,ΨaX2Ψax3would be one possibility, if the particles are distinguishable.)

Short Answer

Expert verified

(a)27

(b)10

(c)1

Step by step solution

01

(a)if they are distinguishable

Each particle has 3 possible states: 3 × 3 × 3 = 27.

02

(b) if they are identical bosons

All in same state: aaa, bbb, ccc⇒3.

2 in one state: aab, aac, bba, bbc, cca, ccb⇒6 (each symmetrized).

3 different states: abc (symmetrized)⇒1.

Total: 10.

If they are identical fermions

Only abc (antisymmetrized) ⟹ 1.

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Most popular questions from this chapter

Show that most of the energies determined by Equation 5.64are doubly degenerate. What are the exceptional cases? Hint: Try it for N=1,2,3,4.... , to see how it goes. What are the possible values of cos(ka)in each case?

Suppose you had three particles, one in stateψa(x), one in stateψb(x), and one in stateψc(x). Assuming ψa,ψb, andψc are orthonormal, construct the three-particle states (analogous to Equations 5.15,5.16, and 5.17) representing

(a) distinguishable particles,

(b) identical bosons, and

(c) identical fermions.

Keep in mind that (b) must be completely symmetric, under interchange of any pair of particles, and (c) must be completely antisymmetric, in the same sense. Comment: There's a cute trick for constructing completely antisymmetric wave functions: Form the Slater determinant, whose first row isψa(x1),ψb(x1),ψc(x1) , etc., whese second row isψa(x2),ψb(x2),ψc(x2) , etc., and so on (this device works for any number of particles).

Imagine two noninteracting particles, each of mass m, in the infinite square well. If one is in the stateψn(Equation 2.28 ), and the other in state ψ1(l≠n), calculate localid="1658214464999" (x1-x2)2, assuming (a) they are distinguishable particles, (b) they are identical bosons, and (c) they are identical fermions.

(a) Construct the completely anti symmetric wave function ψ(xA,xB,xC)for three identical fermions, one in the state ψ5, one in the state ψ7,and one in the state ψ17

(b)Construct the completely symmetric wave function ψ(xA,xB,xC)for three identical bosons (i) if all are in state ψ11(ii) if two are in state ψ19and another one is role="math" localid="1658224351718" ψ1c) one in the state ψ5, one in the state ψ7,and one in the stateψ17

We can extend the theory of a free electron gas (Section 5.3.1) to the relativistic domain by replacing the classical kinetic energy, E=p2/2m,,with the relativistic formula, E=p2c2+m2c4-mc2. Momentum is related to the wave vector in the usual way: p=hk. In particular, in the extreme relativistic limit, E≈pc=hck.

(a) Replace h2k2n Equation 5.55 by the ultra-relativistic expression, hck, and calculateEtotin this regime.

dE=h2k22mVÏ€2k2dk (5.55).

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(c) At extremely high density, inverse beta decaye-+p+→n+v,converts virtually all of the protons and electrons into neutrons (liberating neutrinos, which carry off energy, in the process). Eventually neutron degeneracy pressure stabilizes the collapse, just as electron degeneracy does for the white dwarf (see Problem 5.35). Calculate the radius of a neutron star with the mass of the sun. Also calculate the (neutron) Fermi energy, and compare it to the rest energy of a neutron. Is it reasonable to treat a neutron star non relativistic ally?

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