#### 12th Standard Physics English Medium Atomic and nuclear Physics Reduced Syllabus Important Questions 2021

12th Standard

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Physics

Time : 01:00:00 Hrs
Total Marks : 105
Multiple Choice Questions
15 x 1 = 15
1. Suppose an alpha particle accelerated by a potential of V volt is allowed to collide with a nucleus whose atomic number is Z, then the distance of closest approach of alpha particle to the nucleus is

(a)

$14.4\frac { Z }{ V } \mathring { A }$

(b)

$14.4\frac { V }{ Z } \mathring { A }$

(c)

$1.44\frac { Z }{ V } \mathring { A }$

(d)

$1.44\frac { V }{ Z } \mathring { A }$

2. The electric potential between a proton and an electron is given by $V={ V }_{ 0 }in\left( \frac { r }{ { r }_{ 0 } } \right)$, where r0 is a constant. Assume that Bohr atom model is applicable of nth orbit rn with the principal quantum number n is

(a)

${ r }_{ n }\alpha \frac { 1 }{ n }$

(b)

${ r }_{ n }\alpha n$

(c)

${ r }_{ n }\alpha n\frac { 1 }{ { n }^{ 2 } }$

(d)

${ r }_{ n }\alpha { n }^{ 2 }$

3. An oil drop carrying a charge q has a mass m kg. It is falling freely in air with terminal speed v. The electric field required to make the drop move upwards with the same speed is

(a)

$\frac{mg}{q}$

(b)

$\frac{2mg}{q}$

(c)

$\frac{mgv}{q^2}$

(d)

$\frac{2mgv}{q}$

4. When hydrogen atom is in its first excited level, its radius is __________ of the Bohr radius.

(a)

twice

(b)

same

(c)

half

(d)

four times

5. J.J. Thomson's experiment demonstrated that

(a)

cathode rays are streams of negatively charged ions

(b)

all the mass of an atom is in the nucleus

(c)

specific charge of electrons is much greater than the protons.

(d)

e/m ratio changes when different gases is placed in the discharge tube

6. A beam of electron posses undeflected through mutually perpendicular electric and magnetic fields. If the electric field is switched off and the same magnetic field is maintained, the electrons move,

(a)

in a circular orbit

(b)

along a parabolic path

(c)

along a straight line

(d)

in an elliptical orbit

7. In Millikan's experiment an oil drop of mass 4.9 x 10-14 kg is balanced by applying potential difference of 2 kV between the two plates which are 2 mm apart. The charge of the drop is ________________

(a)

1.96 x 10-18C

(b)

1.602 x 10-19C

(c)

12 C

(d)

4.9 x 10-19C

8. The charge of an alpha-particle is _______________

(a)

same as that of a proton

(b)

twice as that of an electron

(c)

thrice as that of an proton

(d)

twice as that of a neutron

9. The orbits in which electrons supposed to be revolving according to Rutherford are ___________

(a)

square-shaped

(b)

circular

(c)

rectangular

(d)

linear

(a)

first

(b)

second

(c)

third

(d)

fourth

11. The __________ and ______ potentials are called the critical potentials of an atom.

(a)

chemical, magnetic

(b)

electric, magnetic

(c)

electric, electromagnetic

(d)

excitation, ionization

12. Elements having atomic number greater than ______ are radioactive.

(a)

48

(b)

68

(c)

88

(d)

83

13. Penetration power is the greatest in __________ rays.

(a)

alpha

(b)

beta

(c)

gamma

(d)

helium

14. The moderator used in nuclear reactor is

(a)

(b)

Boron oxide

(c)

Heavy water

(d)

Uranium

15. The explosion of hydrogen bomb is based on the principle of

(a)

uncontrolled fission reaction

(b)

nuclear fusion reaction

(c)

controlled fission

(d)

photo electric effect

16. 2 Marks
10 x 2 = 20
17. Give the results of Rutherford alpha scattering experiment.

18. Write down the postulates of Bohr atom model.

19. Define the ionization energy and ionization potential.

20. Define atomic mass unit u.

21. Show that nuclear density is almost constant for nuclei with Z > 10.

22. Whatis mass defect?

23. Calculate the energy equivalent of 1 atomic mass unit.

24. Give the symbolic representation of alpha decay, beta decay and gamma decay.

25. Define Roentgen.

26. Give the mass number and atomic number of elements on the right-hand side of the decay process $_{ 86 }^{ 220 }{ Rn }$ ⇾ Po + He.

27. 3 Marks
10 x 3 = 30
28. Discuss the Millikan's oil drop experiment to determine the charge of an electron.

29. Derive the energy expression for hydrogen atom using Bohr atom model.

30. Explain the variation of average binding energy with the mass number by graph and discuss its features.

31. Discuss the alpha decay process with example.

32. Discuss the properties of neutrino and its role in beta decay.

33. Discuss the process of nuclear fission and its properties.

34. Find the
(i) angular momentum
(ii) velocity of the electron in the 5th orbit of hydrogen atom.
(h = 6.6 × 10–34 Js, m = 9.1 × 10–31 kg)

35. Suppose the energy of a hydrogen–like atom is given as En$\frac { 54.4 }{ { n }^{ 2 } }$ eV where n ∈ N. Calculate the following
(a) Sketch the energy levels for this atom and compute its atomic number
(b) If the atom is in ground state, compute its first excitation potential and also its ionization potential.
(c) When a photon with energy 42 eV and another photon with energy 56 eV are made to collide with this atom, does this atom absorb these photons?
(d) Determine the radius of its first Bohr orbit.
(e) Calculate the kinetic and potential energies in the ground state.

36. Calculate the density of the nucleus with mass number A.

37. Digine atomic mass unit. Find its energy equivalent in MeV

38. 5 Marks
8 x 5 = 40
39. In the Bohr atom model, the frequency of transitions is given by the following expression $v=Rc\left( \frac { 1 }{ { n }^{ 2 } } -\frac { 1 }{ { m }^{ 2 } } \right)$, where n < m, Consider the following transitions:

 Transitions m➝n 1 3➝2 2 2➝1 3 3➝1

S how that the frequency of these transitions obey sum rule (which is known as Ritz combination principle)

40. Calculate the radius of the earth if the density of the earth is equal to the density of the nucleus. [mass of earth 5.97x1024 kg].

41. Show that the mass of radium $(_{ 88 }^{ 226 }{ Ra })$ with an activity of 1 curie is almost a gram.
Given T1/2 = 1600 years.

42. (a) Calculate the disintegration energy when stationary $_{ 92 }^{ 232 }{ U }$ nucleus decays to thorium $_{ 90 }^{ 228 }{ Th }$ with the emission of α particle. The atomic masses are of $_{ 92 }^{ 232 }{ U }$ = 232.03756 u, $_{ 90 }^{ 228 }{ Th }$ = 228.028741u and $_{ 2 }^{ 4 }{ He }$ = 4.002603 u
(b) Calculate kinetic energies of $_{ 90 }^{ 228 }{ Th }$ and α-particle and their ratio.

43. What are the drawbacks of Rutherford atom model?

44. Write the application of alpha decay in smoke detectors.

45. In a fusion reactor, the reaction occurs in two stages.
(i) Two deuterium $(_{ 1 }^{ 2 }{ D) }$ nuclei fuse to form $(_{ 1 }^{ 3 }{ T) }$ nucleus with a proton as .product.
(ii) A tritium nucleus fuses with another deuterium nucleus to form a  helium ($_{ 2 }^{ 4 }{ He }$) nucleus with neutron as another product.
Find (a) the energy released in each stage.
(b) the energy released in the combined reaction per deuterium and
(c) what percentage of the mass energy of the initial deuterium is released?
Given :
$_1^{ 2 }{ D}$ = 2.014102 u, $_{ 1 }^{ 3 }{ T }$ = 3.016049 u; $_{ 2 }^{ 4 }{ He }$ = 4.002603 u, $_{ 1 }^{ 1 }{ H }$ = 1.007825; $_0^{ 1 }{ n }$ = 1.008665 u, Take 1u = 931 MeV

46. Consider the case of bombardment of 235U nucleus with a thermal neutron. The fission products are 95Mo and 139La and two neutrons. Calculate the energy released. (Rest masses of the nuclides: 235U = 235.0439 u, $_{ 0 }^{ 1 }{ n }$ = 1.0087 u, 95Mo= 94.9058 u, 139La= 138.9061 u, Take 1 u = 931 MeV.)