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

12th Standard

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Physics

Time : 01:00:00 Hrs
Total Marks : 105
Multiple Choice Questions
15 x 1 = 15
1. In a hydrogen atom, the electron revolving in the fourth orbit, has angular momentum equal to

(a)

h

(b)

$\frac{h}{\pi}$

(c)

$\frac{4h}{\pi}$

(d)

$\frac{2h}{\pi}$

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. The fig. represents the observed intensity of X-rays emitted by an X-ray tube as a function of wavelength. The sharp peaks A and B denote

(a)

continuous spectrum

(b)

band spectrum

(c)

characteristic spectrum

(d)

4. 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

5. The energy of a photon of characteristic X-ray from a Coolidge tubes comes from

(a)

the KE of the electron of the target

(b)

the K.E of ions of the target

(c)

the K.E of the striking electron

(d)

an atomic transition in the target

6. In a discharge tube, the source of positive rays is

(a)

cathode

(b)

anode

(c)

gas atoms present in the discharge tube

(d)

fluorescent screen

7. J.J. Thomson discovered ________

(a)

protons

(b)

neutrons

(c)

electrons

(d)

quarks

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. Curie is a unit of

(a)

half life

(b)

(c)

(d)

10. In gamma ray emission from nucle

(a)

both the neutron number and the proton number change

(b)

there is no change in neutron number and the proton number change

(c)

only the neutron number changes

(d)

only the proton number changes

11. α - rays consist of α - particles which are ______ nuclei.

(a)

hydrogen

(b)

helium

(c)

heavy water

(d)

boron

12. β - rays are nothing but _______

(a)

protons

(b)

neutrons

(c)

electrons

(d)

helium

13. Pressurised heavy-water reactors in our country generally use _____ as fuel.

(a)

natural uranium oxide

(b)

(c)

titanium dioxide

(d)

plutonium

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. What is meant by excitation energy?

19. What is isobar? Give an example.

20. What is meant by radioactivity?

21. What is nuclear chain reaction?

22. What is the reason for using lighter nuclei as moderators?

23. What causes the sun to expand?

24. The ground state energy of hydrogen atom is -13.6 eV. What are k.E & P.E of the electron in this state?

25. Comment on the statement - "A nucleus contains to electrons and can eject them"

26. What is a reactor core?

27. 3 Marks
10 x 3 = 30
28. Explain the J.J. Thomson experiment to determine the specific charge of electron

29. Discuss the spectral series of hydrogen atom

30. Discuss the alpha decay process with example.

31. Explain the idea of carbon dating.

32. Explain in detail the four fundamental forces in nature

33. (a) Show that the ratio of velocity of an electron in the first Bohr orbit to the speed of light c is a dimensionless number
(b) Compute the velocity of electrons in ground state, first excited state and second excited state in Bohr atom model.

34. 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.

35. Write the properties of neutrino?

36. Show that the decay rate 'R' of a sample of a radionuclide is related to the number of radioactive nuclei 'N' at the same instant by the expression R = λN.

37. With respect to power generation, what are the relative advantages and disadvantages of fusion type and Fission type reactors?

38. 5 Marks
8 x 5 = 40
39. (a) A hydrogen atom is excited by radiation of wavelength 97.5 nm. Find the principal quantum number of the excited state.
(b) Show tliat the total number of lines in emission spectrum is $\frac { n(n-1) }{ 2 }$ and compute the total number of possible lines in emission spectrum

40. Explain the results of Rutherford α-particle scattering experiment.

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

42. For a radioactive material, half-life period is 600s. If initially there are 600 number of molecules, find the time taken for disintegration of 450 molecules and the rate of disintegration.

43. The half-life of radium is 1600 years. After how many years will one gram of the pure radium be reduced to one centigram?

44. Two nuclei P, Q have equal number of atoms at t = 0. Their half-lives are 3· hours and 9 hours respectively. Compare their rates of disintegration, after 18 hours from the start.

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. A power reactor develops energy at the rate of 30,000 kW. How many gram of 235U would be consumed daily? Assuming that on an average 200 MeV energy is released per fission.