Engineering Physics Multiple Choice Questions on “Free Electron Theory”.
1. What does the conductivity of metals depend upon?
a) The nature of the material
b) Number of free electrons
c) Resistance of the metal
d) Number of electrons
Answer: b
Clarification: The conducting property of a solid is not a function of a total number of electrons in the metal, but it is due to the number of valance electrons called free electrons.
2. The free electrons collide with the lattice elastically.
a) True
b) False
Answer: a
Clarification: The free electrons move randomly in all directions. The free electrons collide with each other and also with the lattice Elastically, without loss in energy.
3. What happens to the free electrons when an electric field is applied?
a) They move randomly and collide with each other
b) They move in the direction of the field
c) They remain stable
d) They move in the direction opposite to that of the field
Answer: d
Clarification: The free electrons move in the direction opposite to that of field direction. Since they are assumed to be a perfect gas as they obey classical kinetic theory of gases and the electron velocities in the metal obey the Maxwell-Boltzmann statistics.
4. Thermal conductivity is due to photons.
a) True
b) False
Answer: a
Clarification: Thermal conductivity is due to both photons and free electrons and not just photons.
5. Which of the following theories cannot be explained by classical theory?
a) Electron theory
b) Lorentz theory
c) Photo-electric effect
d) Classical free electron theory
Answer: c
Clarification: Classical theory states that all free electrons will absorb energy. This theory cannot explain the photo electric effect.
6. Which of the following theories can be adopted to rectify the drawbacks of classical theory?
a) Compton theory
b) Quantum theory
c) Band theory
d) Electron theory
Answer: b
Clarification: In classical theory, the properties of metals, such as electrical and thermal conductivities are well explained on the assumption that the electrons in the metal freely moves like the particles of a gas. Hence it can be used to rectify the drawbacks of classical theory.
7. What is the level that acts as a reference which separated the vacant and filled states at 0K?
a) Excited level
b) Ground level
c) Valance orbit
d) Fermi energy level
Answer: d
Clarification: Fermi energy level is the maximum energy level up to which the electrons can be filled at 0K. Thus it acts as reference level which separated the vacant and filled states at 0K.
8. A uniform silver wire has a resistivity of 1.54×10-18 ohm/m at room temperature. For an electric field along the wire of 1 volt/cm. Compute the mobility, assuming that there are 5.8×1028 conduction electrons/m3.
a) 1.54 m2/Vs
b) 6.9973m2/Vs
c) 6.9973×10-3 m2/Vs
d) 0.69973m/s
Answer: c
Clarification: Mobility of the electrons = 1/ƿne
Mobility = 6.9973×10-3 m2/Vs.
9. Calculate the drift velocity of the free electrons with mobility of 3.5×10-3 m2/Vs in copper for an electric field strength of 0.5 V/m.
a) 3.5 m/s
b) 1.75×103 m/s
c) 11.5 m/s
d) 1.75×10-3 m/s