All exercise Short Questions of Nuclear Models of book Nuclear Physics for BS/MSc Physics students.
Q. No. 1 What is an electron gas?
Definition of electron gas. : a population of free electrons in a vacuum or in a metallic conductor.
Q. No. 2 What do you mean by Fermi gas?
A Fermi gas is an ensemble of fermions which do not interact with each other except for Pauli exclusion. The fermi energy is the energy of the most energetic fermion when you fill up the available eigen states with all the fermions you have.
Q. No. 3 What is free electron Fermi gas?
The maximum energy of the fermions at zero temperature is called the Fermi energy. The Fermi energy surface in reciprocal space is known as the Fermi surface. The nearly free electron model adapts the Fermi gas model to consider the crystal structure of metals and semiconductors.
Q. No.4 What is Fermi Dirac function?
The Fermi–Dirac distribution function, also called Fermi function, provides the probability of occupancy of energy levels by Fermions. Fermions are half-integer spin particles, which obey the Pauli exclusion principle. The Pauli exclusion principle postulates that only one Fermion can occupy a single quantum state.
Q. No. 5 What is boson gas?
An ideal Bose gas is a quantum-mechanical phase of matter, analogous to a classical ideal gas. It is composed of bosons, which have an integer value of spin, and obey Bose–Einstein statistics.
Q. No. 6 What is Fermi velocity?
The Fermi velocity is the velocity associated with the Fermi energy by solving. for , where m is the particle mass, giving.
Q. No. 7 What is the Fermi factor?
The number of electrons in the conduction band of an insulator or intrinsic semiconductor is governed chiefly by the Fermi factor. Fermi factor is given by the equation – exp(-Eg/ 2kT), where Eg is the energy gap.
Q. No. 8 What is degenerate Fermi gas?
Degenerate gas, in physics, a particular configuration, usually reached at high densities, of a gas composed of subatomic particles with half-integral intrinsic angular momentum (spin). … Such particles as electrons, protons, neutrons, and neutrinos are all fermions and obey Fermi-Dirac statistics.
Q. No. 9 How does Fermi Dirac distribution function varies with temperature?
Effect of temperature on Fermi–Dirac Distribution Function
This inturn means that no energy states which lie above the Fermi-level are occupied by electrons. … However as the temperature increases, the electrons gain more and more energy due to which they can even rise to the conduction band.
Q. No. 10 What do you mean by free electron gas?
n. (General Physics) any electron that is not attached to an ion, atom, or molecule and is free to move under the influence of an applied electric or magnetic field.
Q. No. 11 What is the difference between Fermi level and Fermi energy?
Fermi level as a state with 50% chance of being occupied by an electron for the given temperature of the solid and at absolute zero temperature occupancy is 100%.Fermi energy is the corresponding energy of Fermi level. Fermi energy isdifference of energies of highest and lowest occupied single state particle.
Q. No. 12 Why is Fermi level important?
The Fermi level plays an important role in the band theory of solids. … The Fermi level is referred to as the electron chemical potential in other contexts. In metals, theFermi energy gives us information about the velocities of the electrons which participate in ordinary electrical conduction.
Q. No. 13 What is the value of Fermi?
Fermi is a length measurement unit
The metric unit prefix femto means one quadrillionth, or one (1) with fourteen (14) nulls in front of it, i.e. 0.000,000,000,000,001 or 1×10-15. That makes one femtometer (and one fermi) equal to one quadrillionth of a meter.
Q. No. 14 What is Fermi wavelength?
The Fermi energy is commonly referred as the top most filled energy level at absolute zero K and the corresponding Fermi wavelength is the de Brogliewavelength of the electrons present near the Fermi energy level. … During this process the electrons usually get interacted with the average potential of the lattice.
Q. No. 15 What is Fermi sea?
The Fermi energy is the maximum energy occupied by an electron at 0K. By the Pauli exclusion principle, we know that the electrons will fill all available energy levels, and the top of that “Fermi sea” of electrons is called the Fermi energy orFermi level.
Q. No. 16 How does Fermi level change with temperature?
So, the Fermi energy does not change with temperature. The Fermi level is the chemical potential of a system of electrons in a solid, which depends ontemperature. The chemical potential is the energy required to add a particle to a system in thermodynamic equilibrium, which can change with temperature.F
Q. No. 17 What is non degenerate gas?
A degenerate gas is one where quantum mechanical effects on the electrons dominate the behavior: Pauli’s exclusion principle: No two electrons can have exactly the same quantum mechanical state meaning that no two can have identical velocities, spins, and positions.
Q. No. 18 What is ideal Fermi gas?
An ideal Fermi gas or free Fermi gas is a physical model assuming a collection of non-interacting fermions in a constant potential well. It is the quantum mechanical version of an ideal gas, for the case of fermionic particles.
Q. No. 19 What is Fermi level in physics?
Fermi level is the highest energy state occupied by electrons in a material at absolute zero temperature. As the temperature is increased, electrons start to exist in higher energy states too. … Fermi level is also defined as the work done to add an electron to the system.
Q. No. 20 Do electrons have zero energy at 0k?
Certainly, the metals would stay conducting at zero temperature. External electric field tries to increase the energy of the electron and there is enough unoccupied states above the Fermi level. … 0k does not mean zero energy.
Q. No. 21 What is liquid drop?
Liquid–drop model, in nuclear physics, a description of atomic nuclei in which the nucleons (neutrons and protons) behave like the molecules in a drop of liquid.
Q. No. 22 What is liquid drop model of nucleus?
Liquid Drop Model of Nucleus. … The liquid drop model of the nucleus takes into account the fact that the nuclear forces on the nucleons on the surface are different from those on nucleons in the interior of the nucleus. The interior nucleons are completely surrounded by other attracting nucleons.
Q. No. 23 Who proposed liquid drop model?
It was first proposed by George Gamow and then developed by Niels Bohr and John Archibald Wheeler. The nucleus is made of nucleons (protons and neutrons), which are held together by the nuclear force (a residual effect of the strong force).
Q. No. 24 What is the nuclear shell model used for?
In nuclear physics and nuclear chemistry, the nuclear shell model is a model of the atomic nucleus which uses the Pauli exclusion principle to describe the structure of the nucleus in terms of energy levels.
Q. No. 25 What is collective model?
Collective model, also called unified model, description of atomic nuclei that incorporates aspects of both the shell nuclear model and the liquid-drop model to explain certain magnetic and electric properties that neither of the two separately can explain.
Q. No. 26 What is optical model?
Optical model, in physics, description of atomic nuclei as similar to cloudy crystal balls in that, when struck by a beam of particles, they partially absorb the beam, partially scatter it, and partially transmit it in a way analogous to the behavior of light.
3.27 Why – decays occur?
Answer: The attractive nuclear force between nucleons is of short range. The binding energy of a heavy nucleus is related to mass number A AS,
B, E A
The coulomb repulsive electrostatic force between protons is of long range and
EC ∝ Z(Z-1)
For nuclei containing nucleons greater than 190 , the short range nuclear force is unable to counterbalance the repulsive electrostatic force between protons, decay occurs in such nuclei to increase their stability by reducing their size.
3.28 When thorium decays by emitting a beta particle ( an electron), what is the atomic number of the resulting nucleus ? What happens to its atomic mass?
Answer: When a nucleus undergoes beta decay , one of its neutrons changes into proton as it emits an electron. Therefore, the atomic number increases by one. The atomic number will be 91. Although the fleeing electron carries a tiny bit of mass away with it , the atomic mass of the atom does not change.
3.29 What is evidence for the existence of neutrinos? Why was their existence postulated?
Answer : The inverse – decay process provides a method of establishing the actual existence of neutrinos. The existence of neutrino was postulated to explain the contradiction in the laws of conservation of energy and momentum in the continuous energy spectrum of the
-particles and their end point energy.
3.27 Why – decays occur?
Answer: The attractive nuclear force between nucleons is of short range. The binding energy of a heavy nucleus is related to mass number A AS,
B, E A
The coulomb repulsive electrostatic force between protons is of long range and
EC ∝ Z(Z-1)
For nuclei containing nucleons greater than 190 , the short range nuclear force is unable to counterbalance the repulsive electrostatic force between protons, decay occurs in such nuclei to increase their stability by reducing their size.
3.28 When thorium decays by emitting a beta particle ( an electron), what is the atomic number of the resulting nucleus ? What happens to its atomic mass?
Answer: When a nucleus undergoes beta decay , one of its neutrons changes into proton as it emits an electron. Therefore, the atomic number increases by one. The atomic number will be 91. Although the fleeing electron carries a tiny bit of mass away with it , the atomic mass of the atom does not change.
3.29 What is evidence for the existence of neutrinos? Why was their existence postulated?
Answer : The inverse – decay process provides a method of establishing the actual existence of neutrinos. The existence of neutrino was postulated to explain the contradiction in the laws of conservation of energy and momentum in the continuous energy spectrum of the
-particles and their end point energy.
3.30 What is difference between electron capture and positron emission?
| Electron capture | Positron Capture |
| In electron capture an orbital electron is captured by the nucleus but there is no particle emission. | In positron emission a positron is given out by the parent nuclide |
| The atomic number Z decreases by one in electron capture and the proton changes to a neutron by capturing an electron. | The atomic number Z decreases by one in positron emission and a nuclear proton changes to neutron by emission of a positron. |
| The condition for electron capture is that the mass of the parent atom must be greater than the mass of the daughter atom. | The condition for positron emission is that the mass of the parent atom must be greater than the sum of the masses of the daughter atoms and electrons. |
3.31. Why are there more 6C14 in new bones than there is in old bones of the same mass?
Answer : Carbon 14 changes to Nitrogen-14 with a half life of 5,730 years so the amount of carbon-14 present in a substance is reduced over time.
3.32 A sample of a particular radioisotope is placed near a Geiger counter, which is observed to register 160 counts per minute. Eight hours later the detector counts at a rate of 10 counts per minute. What is the half-life of the material?
Answer : The half-life is 2hours. If one cuts 160 in half then we will have 80.
$$\frac{1}{2} of 80 = 40$$
$$\frac{1}{2} of 40 = 20$$
$$\frac{1}{2} of 20 = 10$$
We repeated this process four times. Four half lives have elapsed. Eight hours divided by 4 , equal 2 hours.
3.33 How is the internal conversion different from photoelectric effect?
Answer : In internal conversion the excited nucleus transfer its surplus energy directly to one of its orbital electrons so that it is ejected from the atom with a kinetic energy,
E=Ee – φ
Here Ee is the available excitation energy (i.e. energy given out by the excited nucleus in passing from a higher energy level to a lower energy level) and φ is the binding energy of the ejected electron in it shell of origin.
In photoelectric effect , when a high energy photon strikes a metal from outside , it ejects a free electron from the metal known as photoelectron. If u is the frequency of the incident photon ad Kmax is maximum kinetic energy of the electron, then
H= Kmax + W
where W is the work function.
Thus in internal conversion energy is supplied from within the nucleus and the ejected electrons are orbital electrons of the atom . In photoelectric effect energy is supplied from an outside in the form of a high energy photon and the electron ejected is a free electron in the metal.
3.34 In bombarding atomic nuclei with proton bullets, why must the proton be accelerated to high energies if they are to make contact with target nuclei?
Answer: Atomic nuclei are positively charged . The proton bullets are positively charged. They will be repelled away from each other by the electromagnetic force.
3.35 Why are alpha and beta rays deflected in opposite directions in a magnetic field ? Why aren’t gamma rays deflected?
Answer: Alpha rays consist of positively charged helium nuclei. Beta rays consist of negatively charged electrons. Gamma rays are uncharged photons of light. A magnetic field will apply a forced to a moving charged particle. Positively charged particles are accelerated in one direction and negative charged particles are accelerated in the opposite direction. Because gamma rays are not charged, they are unaffected by the magnetic field.
3.36. Which type of radiations results in the least change in atomic mass? The least change in atomic number ?
Answer: Gamma radiation. There is no change in atomic number and mass number because a gamma ray is a photon of light .
3.37 Which type of radiation results in the greatest change in atomic mass? Atomic number?
Answer: Alpha radiation. The resulting nucleus will be missing two protons and two neutrons. The atomic mass will be four times less than the original and the atomic number will be two times fewer than the original.
3.38 When beta emission occurs, what change takes place in an atomic nucleus?
Answer: Beta emission occurs when a neutron emits an electron. The neutron changes into proton in the process. The atomic nucleus now has one more proton that before the emission and thus is now an atom of a different element.
