Energy levels and density of states in one, two and three dimensions, electrical and thermal conductivities, Wiedemann–Franz law - Question Bank

1. The thermal conductivity of a semiconductor at room temperature is mainly due to:
A) Phonons
B) Electrons
C) Holes
D) Both electrons and phonons, often comparable
2. The electrical conductivity of a semiconductor is highly dependent on temperature because:
A) The number of charge carriers (electrons and holes) increases exponentially with temperature.
B) The mobility of charge carriers increases exponentially with temperature.
C) The band gap increases with temperature.
D) The scattering decreases with temperature.
3. In 3D, if the number of electrons per unit volume is n, the Fermi energy E_F is related to n by n ∝ E_F^(3/2). If n doubles, E_F changes by a factor of:
A) 2^(2/3)
B) 2^(3/2)
C) 2
D) √2
4. For a 2D free electron gas, the density of states is constant. This means that the number of available states between energy E and E+dE is:
A) Independent of E
B) Proportional to E
C) Proportional to √E
D) Proportional to 1/√E
5. In a 1D system, the density of states near the bottom of a band (low energy) is:
A) Low and increases with energy
B) High and decreases with energy
C) Constant
D) Zero
6. If the mean free path of electrons in a metal increases, what happens to its electrical conductivity?
A) Increases
B) Decreases
C) Stays the same
D) Becomes zero
7. The Wiedemann–Franz law is a consequence of the fact that:
A) Electrons are the primary carriers of both heat and charge in metals.
B) Phonons carry both heat and charge.
C) The material is perfectly crystalline.
D) The temperature is very high.
8. At room temperature, the thermal conductivity of a metal is primarily due to:
A) Electrons
B) Phonons
C) Radiation
D) Defects
9. An increase in impurity concentration in a metal typically leads to:
A) A decrease in electrical conductivity.
B) An increase in electrical conductivity.
C) No change in electrical conductivity.
D) An increase in thermal conductivity.
10. The density of states in 3D for free electrons is g(E) = C E^(1/2). If the maximum energy is E_max, the total number of electrons in a volume V is proportional to:
A) E_max^(3/2)
B) E_max^(1/2)
C) E_max
D) Constant
11. In a 2D system with E ∝ k², the density of states g(E) is:
A) Constant
B) Proportional to E
C) Proportional to √E
D) Proportional to 1/√E
12. Consider a 1D system where E = Ak. The density of states g(E) is proportional to:
A) Constant
B) E^(-1)
C) E
D) E^(1/2)
13. Why is the Wiedemann–Franz law more accurate for metals than for semiconductors?
A) In metals, electrons are the dominant carriers for both heat and charge; in semiconductors, phonons also play a significant role in heat transport.
B) Semiconductors have a higher density of states.
C) Metals have a smaller band gap.
D) The relaxation time is longer in semiconductors.
14. Which of the following is a consequence of the free electron model for density of states?
A) The density of states increases with energy in 3D.
B) The density of states decreases with energy in 1D.
C) The density of states is constant in 3D.
D) The density of states is proportional to E in 2D.
15. The Lorenz number L = κ / (σT). If the Wiedemann–Franz law holds, L should be:
A) A constant, independent of material and temperature.
B) Proportional to temperature.
C) Inversely proportional to temperature.
D) Proportional to the square of temperature.
16. What happens to the thermal conductivity of a pure metal as temperature increases?
A) It generally decreases slightly.
B) It increases significantly.
C) It stays constant.
D) It becomes negligible.
17. What happens to the electrical conductivity of a pure metal as temperature increases?
A) It decreases.
B) It increases.
C) It stays the same.
D) It becomes zero.
18. The density of states in 3D is given by g(E) = C * E^(1/2). If we double the volume of the system, how does the density of states change?
A) It doubles.
B) It remains the same.
C) It halves.
D) It increases by a factor of 4.
19. In a 2D electron gas, the density of states is constant. If the number of states per unit area up to energy E is N_s(E), then N_s(E) is proportional to:
A) E
B) √E
C) 1/√E
D) 1/E
20. Consider a 1D chain of N atoms, each contributing one free electron. If the length of the chain is L, the density of states per unit length in energy E is proportional to:
A) E^(-1/2)
B) E^(1/2)
C) Constant
D) E
21. At very high temperatures, the Wiedemann–Franz law may deviate because:
A) Phonon scattering of electrons increases significantly.
B) The number of free carriers decreases.
C) The electron effective mass increases.
D) The Lorenz number becomes temperature-independent.
22. The Wiedemann–Franz law implies that materials that are good electrical conductors are also good thermal conductors. Which of the following is an exception?
A) Semiconductors at certain temperatures
B) All metals
C) Alloys
D) Superconductors below critical temperature
23. Thermal conductivity (κ) in metals is significantly contributed by electrons. A higher electron mobility implies:
A) Higher thermal conductivity.
B) Lower thermal conductivity.
C) No change in thermal conductivity.
D) Higher electrical resistance.
24. Electrical conductivity (σ) is inversely proportional to electrical resistivity (ρ). The formula for conductivity is often given by σ = ne²τ/m*, where 'n' is the carrier concentration, 'e' is the electron charge, 'τ' is the relaxation time, and 'm*' is the effective mass. What does 'τ' represent?
A) The average time between scattering events.
B) The time it takes for an electron to cross the material.
C) The period of oscillation of an electron.
D) The lifetime of a free electron.
25. The Fermi energy (E_F) is the highest occupied energy level at absolute zero temperature. How does the density of states near E_F influence properties?
A) A higher DOS at E_F leads to higher electronic contributions to specific heat and conductivity.
B) A higher DOS at E_F leads to lower electronic contributions.
C) DOS at E_F has no impact on electronic properties.
D) DOS at E_F only affects optical properties.
26. In 2D, if the energy E is proportional to k², how does the density of states g(E) behave?
A) It is constant.
B) It is proportional to √E.
C) It is proportional to 1/√E.
D) It is proportional to E.
27. For a free electron model in 1D, the dispersion relation is E(k) = ħ²k²/2m. The allowed values of k are quantized with spacing Δk = 2π/L, where L is the length of the system. The density of states per unit length is proportional to:
A) E^(-1/2)
B) E^(1/2)
C) Constant
D) E
28. The density of states function g(E) tells us:
A) How many quantum states are available within a given energy interval.
B) The probability of finding an electron at a specific energy.
C) The total energy of all electrons in the system.
D) The band structure of the material.
29. In insulators, thermal conductivity is primarily due to:
A) Phonons
B) Free electrons
C) Holes
D) Excitons
30. If the temperature of a metal is increased, its electrical conductivity generally:
A) Decreases due to increased lattice scattering.
B) Increases due to more free electrons.
C) Remains constant.
D) Increases due to reduced electron effective mass.
31. The Wiedemann–Franz law is expected to break down at very low temperatures because:
A) Phonon contribution to thermal conductivity becomes significant.
B) Electron contribution to thermal conductivity decreases.
C) Electrical conductivity becomes temperature-independent.
D) The Lorenz number is no longer constant.
32. Thermal conductivity in metals is significantly affected by:
A) The mean free path of electrons.
B) The band gap of the material.
C) The number of holes.
D) The dielectric constant.
33. Which of the following leads to a decrease in electrical conductivity?
A) Increased scattering of charge carriers by lattice vibrations.
B) Increased number of free charge carriers.
C) Decreased effective mass of charge carriers.
D) Increased mean free path of charge carriers.
34. In 3D, if the energy E is related to the wavevector k by E = ħ²k²/2m, and we consider a volume V, the density of states g(E) is proportional to:
A) E^(1/2)
B) E^(-1/2)
C) E
D) Constant
35. For a 2D square lattice with lattice constant 'a', and free electrons, the density of states per unit area is approximately:
A) Constant, independent of energy
B) Proportional to √E
C) Proportional to 1/√E
D) Proportional to E
36. In a 1D system, if the energy E is related to the wavevector k by E = ħ²k²/2m, what is the form of g(E)?
A) g(E) = (L/π) * (m/2ħ²E)^(1/2)
B) g(E) = (L/π) * (2m/ħ²)^(1/2)
C) g(E) = (L/π) * (2ħ²/m)^(1/2)
D) g(E) = (L/π) * (ħ²/2mE)^(1/2)
37. The Wiedemann–Franz law is generally a good approximation for metals at temperatures where which mechanism dominates heat transport?
A) Electron transport
B) Phonon transport
C) Radiation
D) Convection
38. The proportionality constant in the Wiedemann–Franz law is known as the Lorenz number (L). What is its approximate value at room temperature for many metals?
A) 2.44 × 10⁻⁸ W Ω K⁻²
B) 1.0 × 10⁻⁸ W Ω K⁻²
C) 5.0 × 10⁻⁸ W Ω K⁻²
D) 0.5 × 10⁻⁸ W Ω K⁻²
39. Mathematically, the Wiedemann–Franz law states that the ratio of thermal conductivity (κ) to electrical conductivity (σ) is proportional to what?
A) Temperature (T)
B) Volume (V)
C) Pressure (P)
D) Energy (E)
40. The Wiedemann–Franz law establishes a relationship between which two transport properties of metals?
A) Electrical conductivity and thermal conductivity.
B) Electrical resistivity and specific heat.
C) Thermal expansion and electrical resistivity.
D) Magnetic susceptibility and thermal conductivity.
41. In metals, which charge carriers are primarily responsible for both electrical and thermal conductivity?
A) Free electrons
B) Phonons (lattice vibrations)
C) Holes
D) Ions
42. What is thermal conductivity (κ) a measure of?
A) The ability of a material to conduct heat.
B) The ability of a material to store thermal energy.
C) The rate at which a material radiates heat.
D) The temperature at which a material melts.
43. Which of the following factors significantly influences the electrical conductivity of a metal?
A) Number of free charge carriers and their mobility.
B) The crystal structure of the insulating material.
C) The strength of the applied magnetic field.
D) The dielectric constant of the surrounding medium.
44. What is electrical conductivity (σ) a measure of?
A) The ability of a material to conduct electric current.
B) The resistance of a material to heat flow.
C) The tendency of a material to become polarized.
D) The magnetic susceptibility of a material.
45. In a three-dimensional system of free electrons, what is the relationship between the density of states g(E) and energy E?
A) g(E) ∝ E^(1/2)
B) g(E) ∝ E^(-1/2)
C) g(E) ∝ E
D) g(E) ∝ E^(-1)
46. For a two-dimensional free electron gas, how does the density of states vary with energy E?
A) g(E) is independent of E
B) g(E) is proportional to E^(-1/2)
C) g(E) is proportional to E^(1/2)
D) g(E) is proportional to E
47. In a one-dimensional system, how does the density of states typically vary with energy E, assuming a simple free electron model?
A) g(E) is proportional to E^(-1/2)
B) g(E) is proportional to E^(1/2)
C) g(E) is constant
D) g(E) is proportional to E
48. Which of the following best describes the density of states (DOS) function, g(E)?
A) The number of available energy states per unit volume per unit energy.
B) The probability of an electron occupying a given energy state.
C) The total number of electrons in a solid at a specific energy.
D) The energy difference between consecutive allowed quantum states.
49. What is the fundamental concept behind energy levels in solids?
A) Electrons occupy discrete, quantized energy states.
B) Electrons can possess any energy value within a continuous band.
C) Energy levels are determined solely by the solid's temperature.
D) Energy levels are directly proportional to the number of atoms in the solid.
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