B.Tech. (I SEM.)
PHY 106
B.Tech. (FIRST SEMESTER) EXAMINATION, DECEMBER/JANUARY- 2020
(PHYSICS)
PHY 106— Modern Physics
Maximum Marks – 60
Attempt six questions in all, selecting not more than Two Questions from each section.
Section A
1. (a) The mass of a neutron, mn = 10 -27 kg is trapped inside a nucleus of a cubical
box of size 10-14 m with impenetrable walls. Calculate the energy of neutron
in Mev.
(b) A hydrogen atom is 0.53 Å in radius. Use the uncertainty principle to estimate
the minimum energy an electron can have in this atom. Also find the
uncertainty in the velocity of an 𝛂 particle when it is located within 5 × 10-8
cm.
5+5=10Marks
2. (a) A rocket of mass 1.40 x 105 kg has a relativistic momentum, the magnitude of
which is 3.15x1013 kg.m /s. How fast is the rocket traveling?
(b) A Klingon spacecraft has a speed of 0.75 c with respect to the earth. The
Klingons measure 37.0 hours for the time interval between two events on
the earth. What value for the time interval would they measure if their ship
had a speed of 0.94 c with respect to the Earth?
5+5=10Marks
3. (a) What do you understand by ‘time Dilation’ On the basis of Lorentz
Transformations, discuss the variation of time with velocity according to the
special theory of relativity. Explain why a moving clock appears to run slow?
(b) Using Heisenberg uncertainty principle, show that electron cannot exist inside
the nucleus.
5+5=10Marks
Section B
4. (a) What is the effect of periodic potential on the energy in a metal? Explain it on
the basis of Kronig-Penney model and explain the formation of energy bands.
(b) For an intrinsic semiconductor having band gap 0.7 eV. Calculate the density
of holes and electrons at room temperature (= 27C).
5+5=10Marks
5. (a)How you conclude that the all superconductors are diamagnetic but all
diamagnetic materials are not superconductors.
(b)Determine the transition temperature and critical field at 4.2 K for a given
specimen of a superconductor if the critical fields are 1.41 x 105 and 4.205 x 105
amp/m at 14.1 K and 12.9 K, respectively.
10Marks
6. (a) What is the effect of an external magnetic field on the superconducting state of
a material? Discuss the A.C and D.C Josephson Effect
(b)What do you mean by BCS Theory? Discuss the cooper pairs.
5+5=10Marks
Section C
7. (a) The binding energy per particle in MeV as a function of the mass number of a
nucleus has been represented in the following histogram. A nucleus having
mass number A =140 fissions into two nuclei of equal masses. How much
energy will be released in this process?
(b)
249Cf
undergoes alpha decay.
(i) Write the reaction equation.
(ii) Find the energy released in the decay.
5+5=10Marks
8. (a) Carbon tetrachloride contains 74 electrons in its molecule. Its relative permittivity
is 2.26 and density 1.68 × 103 kg/m3. If the field acting on the liquid is 5 × 106 N/C,
what is its electronic polarizability, polarization and average electron
displacement?
(b) A solid contains 5 × 1028 atoms/m3 each with a polarisability of 2×10–40 F m2.
Assuming that the internal field is given by the Lorentz formula. Calculate the ratio
of internal field to the external field. 0 = 8.854 × 10–12 Fm–1.
5+5=10Marks
9. (a) An isotropic material of relative permittivity r is placed normal to a uniform external
electric field with an electric displacement vector of magnitude 6  10-4 C/m2 . If
the volume of the slab is 0.7 m3 and magnitude of polarization is 5  10-4 C/m2 .
Find the value of r and total dipole moment of the slab.
(b)Briefly discuss different dielectric polarization mechanisms. Also specify
temperature dependence in each case. (b) How many alpha and beta particles are
emitted when uranium 92U 238 decay to lead 82Pb206.
5+5=10Marks
----------------------------------