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Q.7.11

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An Introduction to Thermal Physics
Found in: Page 269
An Introduction to Thermal Physics

An Introduction to Thermal Physics

Book edition 1st
Author(s) Daniel V. Schroeder
Pages 356 pages
ISBN 9780201380279

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Short Answer

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

(a) 1 eV less than μ

(b) 0.01 eV less than μ

(c) equal to μ

(d) 0.01 eV greater than μ

(e) 1 eV greater than μ

According to the Fermi-Dirac distribution, the probability of a state being occupied is given below:

nFD=1e(ε-μ)kT+1

Here, nFD is the Fermi-Dirac distribution, ε is the energy, μ is the chemical potential, k is the Boltzmann constant, and T is the absolute temperature.

Formula to energy for the occupied state is given below:

ε-μ=x

Here x is the energy of the state.

See the step by step solution

Step by Step Solution

(a) Calculate the energy for the occupied state 1 eV less than than μ as follows:

ε-μ=x

Substitute -1 eV for x in the equation ε-μ=x.

ε=μ-1 eVε-μ=-1 eV

The room temperature in kelvins is,

T=27oC=(27+273)K=300 K

Calculate the probability of a state being occupied state 1 eVless than μ as follows:

nFD=1e(ε-μ)kT+1

Substitute -1 eV for (ε-μ), 8.617×10-5 eV/K for k, and 300 K for T in the above equation.

nFD=1e-1eV(8.617×10-3eV/K)(300K)+1=0.9999nFD1.0

Therefore, the probability of a state being occupied state 1 eV less than μ is nFD1.0

(b) Calculate the energy for the occupied state 0.01 eV less than μ as follows:

ε-μ=x

Substitute -0.01 eV for x in the equation ε-μ=x.

ε=μ-0.01 eVε-μ=-0.01 eV

The room temperature in kelvins is,

T=27oC=(27+273)K=300 K

Calculate the probability of a state being occupied state 0.01 eV less than μ as follows:

nFD=1e(ε-μ)kT+1

Substitute -0.01 eV for ε-μ, 8.617×10-5 eV/K for k, and 300 K for T in the above equation.

nFD=1e-0.01 eV(8.617×10-5 eV/K)(300 K)+1nFD==0.5955

Therefore, the probability of a state being occupied state 0.01 eV less than μ is nFD=0.5955

(c) Calculate the energy for the occupied state energy is equal to μ as follows:

ε-μ=x

Substitute 0 eV for x in the equation ε-μ=x.

ε=μ-0

The room temperature in kelvins is,

T=27oC=(27+273)K=300 K

Calculate the probability of a state being occupied state equal to μ as follows:

nFD=1eε-μkT+1

Substitute 0 eV for ε-μ, 8.617×10-5 eV/K for k, and 300 K for T in the above equation.

role="math" localid="1647055630164" nFD=1e0 eV(8.617×10-5 eV/K)(300 K)+1nFD=0.5

Therefore, the probability of a state being occupied state equal to μ is nFD=0.5

(d) Calculate the energy for the occupied state 0.01 eV greater than μ as follows:

ε-μ=x

Substitute 0.01 eV for x in the equation role="math" localid="1647056653513" ε-μ=x

role="math" localid="1647056689711" ε=μ+0.01 eVε-μ=0.01 eV

The room temperature in kelvins is,

role="math" localid="1647056731594" T=27oC=(27+273) K=300 K

Calculate the probability of a state being occupied state 0.01 eV less than μ as follows:

role="math" nFD=1eε-μkT+1

Substitute 0.01 eV for role="math" localid="1647056852727" ε-μ, 8.617×10-5 eV/K for k, and 300 K for Tin the above equation.

role="math" localid="1647056921933" nFD=1e0.01 eV(8.617×10-5 eV/K)(300 K)+1nFD=0.4045

Therefore, the probability of a state being occupied state 0.01 eVgreater than μ is nFD=0.4045

(e) Calculate the energy for the occupied state 1 eV greater than μ as follows:

ε-μ=x

Substitute 1 eV for x in the equation ε-μ=x

ε=μ+1 eVε-μ=1 eV

The room temperature in kelvins is,

T=27oC=(27+273) K=300 K

Calculate the probability of a state being occupied state role="math" localid="1647056827269" 1 eV less than μ as follows:

nFD=1eε-μkT+1

Substitute 1 eV for ε-μ, 8.617×10-5 eV/K for k, and 300 K for Tin the above equation.

role="math" localid="1647057000902" nFD=1e1 eV(8.617×10-5 eV/K)(300 K)+1nFD=1.5852 ×10-17

Therefore, the probability of a state being occupied state 1 eV greater than μ is nFD=1.5852 ×10-17

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