SHORT QUESTIONS WITH ANSWERS
1.What is meant by a free electron?
The electron moves freely in all directions in the absence of electric field is called free electron (or) valance electron. These electrons collide with each other, the collisions are perfectly elastic collisions hence there is no loss of energy. Since the free electron is in random motion.
2. Define Drift velocity of electrons.
Drift velocity (Vd) is the average velocity acquired by an electron in a particular direction due to applied electric field.
3. Define mobility of electrons.
The mobility is defined as the drift velocity (Vd) acquired by the free electron per unit electric field (E)
4. Define mean free path.
The average distance travelled by an electron between two successive collisions is called mean free path.
5. Define relaxation time.
Relaxation time is the time taken by the electron to reach equilibrium position from its disturbed position in the presence of electric field.
6. Define electrical conductivity.
The electrical conductivity is defined as the quantity of electricity flowing per unit area per unit time at a constant potential gradient.
7,Define Thermal Conductivity.
Thermal conductivity of material is defined as the amount of heat flowing through an unit area per unit time of temperature gradient.
8. State Widemann-Franz law.
The ratio between the thermal conductivity (K) and electrical conductivity ( ) of a metal is directly proportional to the absolute temperature of the metal.
9.List out the three main theories developed to describe the structure of materials. (or) List the types of electron theory of metals.
Classical free electron theory
Quantum free electron theory
Zone (or) Band theory
10. What are the Sources of resistance in metals?
The resistance in metals is due to
Presence of impurities in the metals.
Temperature of the metal.
Number o free electrons.
11. What is the effect of temperature on metals
When temperature of the metal increases, the mobility of the electron decreases and hence the electrical conductivity decreases. The addition of impurities in the metal decreases the electrical conductivity.
12. What are the uses (or) success of classical free electron theory?
It is used to verify the Ohm’s law.
It is used to explain electrical conductivity and thermal conductivity of metals.
It is used to derive Widemann-Franz law.
3. It is used to explain the optical properties of metal.
13.What are the drawbacks of classical free electron theory? (or) State any four demerits of Classical free electron theory?
It is a macroscopic theory.
According to classical free electron theory, all the free electrons will absorb energy, but the quantum free electron theory states that only few electrons will absorb energy.
This theory cannot explain the Compton effect, Photo-electric effect, para-magnetism and ferromagnetism, etc.,
This theory cannot explain the electron conductivity of semiconductors and insulators.
Dual nature of light radiation cannot be explained.
The theoretical and experimental values of specific heat and electronic specific heat are not matched.
The Lorentz number obtained by classical theory does not have good agreement with experimental value and it is rectified by quantum theory.
14. What is Lorentz Number?
It is found that the classical value of Lorentz number is only one half of the experimental value (2.44 × 10–8 W K–2). The discrepancy of L value is the failure of the classical theory. This can be rectified by quantum theory.
15.What is the basic assumption of Zone theory or Band theory of solids?
According to quantum free electron theory, the electrons in a metal were assumed to be moving in a region of constant potential but it fails to explain, why some solids behave as conductors, some as insulators and some as semiconductors.
Therefore instead of considering an electron to move in a constant potential, the Zone theory of solids tells that the electrons are assumed to move in a field of periodic potential.
16.Distinguish between Electrical conductivity and Thermal conductivity.
The electical conductivity is defined as the quantity of electricity flowing per unit area per unit time at a constant potential gratient.
Electrical conductivity is purely depend on free electrons.
Conduction of electricity takes place from higher potential end to lower potential end.
Thermal conductivity is defined as the amount of heat flowing through an unit area per unit time of temperature gratient.
Thermal conductivity is due to both free electrons and photons.
Conduction of heat takes place from hot end to cold end.
17.What are the similarities between electrical and thermal conductivity of metals?
The electrical and thermal conductivities decrease with the increase in temperature and impurities.
The electrical and thermal conductivity is very high at low temperatures.
For non-metals the electrical and thermal conductivity is very less.
18. Distinguish between relaxation time and collision time.
19. Write microscopic form of Ohm’s law and state whether it is true for all temperature.
Microscopically we can write V = IR as J = E
Since the resistivity varies with respect to the temperature, the microscopic form of ohm’s law is not true for all the temperature.
What are the factors that affect the electrical resistivity of materials?
4. Magnetic field
5. Pressure and strain.
21. Define Fermi level, Fermi energy and this importance.
Fermi level : The Fermi level is the highest reference energy level of a particle at absolute zero.
Importance : It is the reference energy level which separates the filled energy level and vacant energy levels.
Fermi energy: It is the maximum energy of the quantum state corresponding to Fermi energy level at absolute zero.
Importance : Fermi energy determines the energy of the particle at any temperature.
22. Define Fermi Distribution function.
It is an expression for the distribution of electrons among the energy levels as a function of temperature and it is the probability of finding an electron in particular energy state of energy E is given by,
23.Define density of states and its importance.
Density of states is defined the as the number of energy states per unit volume in an energy interval of a metal. It is use to calculate the number of charge carriers per unit volume of any solid.1exp E E
Importance : It is used for the Fermi energy calculation at any temperature.
24.Define work function
It is define as the minimum energy required to remove an electron from the metal surface at 0K. in order to make it escape, an additional amount of energy equal to is required. i.e., .This difference in energy is called Work function.
25.What do you mean by carrier concentration in metal?
In metal carrier concentration is number of free electrons per unit volume in between the energy interval 0 to it is given by
Carrier concentration N D (E) F(E) dE
26. How classical free electron theory failed to account for specific heat of solid?
According to classical free electron theory, the experimental and theoretical value of specific heat of solid are not matched. Hence classical free electron theory is failed.