Atoms And Nuclei(12th Grade > Physics ) Questions and answers for exam Preparation

12th Grade > Physics

ATOMS AND NUCLEI MCQs

Atoms, Nuclei (12th Grade)

Total Questions : 60 | Page 3 of 6 pages

Question 21. What is the respective number of

α

and

β

particles emitted in the following radioactive decay

_{90} X^{200} \to_{80} Y^{168}

6 and 8
8 and 8
6 and 6
8 and 6

Discuss Question

Answer: Option D. -> 8 and 6
:
D

n_{α} = \frac{A - A^{'}}{4} = \frac{200 - 168}{4} = 8

n_{β} = 2 n_{α} - Z + Z^{'} = 2 \times 8 - 90 + 80 = 6

Question 22. The ratio of radii of nuclei

_{13}^{27}

Al and

_{52}^{125}

Te is approximately

6 : 10
13 : 52
40 : 177
14 : 7

Discuss Question

Answer: Option A. -> 6 : 10
:
A
By using

r \propto A^{\frac{1}{3}} \Rightarrow \frac{r_{1}}{r_{2}} = {(\frac{A_{1}}{A_{2}})}^{\frac{1}{3}} = {(\frac{27}{125})}^{\frac{1}{3}} = \frac{3}{5} = \frac{6}{10}

Question 23. In which radioactive disintegration, neutron dissociates into proton and electron

He++− emission
β− emission
γ− emission
Positron emission

Discuss Question

Answer: Option B. -> β− emission
:
B

β

- decay from nuclei based on this process only.

Question 24. The mass defect in a particular nuclear reaction is 0.3 grams. The amount of energy liberated in kilowatt hours is
(Velocity of light =

3 \times 10^{8}

m/s )

1.5×106
2.5×106
3×106
7.5×106

Discuss Question

Answer: Option D. -> 7.5×106
:
D

E = Δ m . c^{2} \Rightarrow E = \frac{0.3}{1000} \times (3 \times 10^{8})^{2} = 2.7 \times 10^{13} J

= \frac{2.7 \times 10^{13}}{3.6 \times 10^{6}} = 7.5 \times 10^{6} k W h

Question 25. If 20 gm of a radioactive substance due to radioactive decay reduces to 10 gm in 4 minutes, then in what time 80 gm of the same substance will reduce to 10 gm

In 8 minutes
In 12 minutes
In 16 minutes
In 20 minutes

Discuss Question

Answer: Option B. -> In 12 minutes
:
B
substance reduces to 10 gm (i.e. becomes half in 4 min). So

T_{\frac{1}{2}} = 4 m i n

. Again

M = M_{0} {(\frac{1}{2})}^{t / T_{1 / 2}}

\Rightarrow 10 = 80 {(\frac{1}{2})}^{\frac{t}{4}} \Rightarrow \frac{1}{8} = {(\frac{1}{2})}^{3} {(\frac{1}{2})}^{\frac{t}{4}} \Rightarrow t = 12 m i n

Question 26. The count rate of a Geiger- Muller counter for the radiation of a radioactive material of half life of 30 minutes decreases to

5 s^{- 1}

after 2 hours. The initial count rate was

25 s−1
80 s−1
625 s−1
20 s−1

Discuss Question

Answer: Option B. -> 80 s−1
:
B

A = A_{0} {(\frac{1}{2})}^{t / T_{1 / 2}} \Rightarrow 5 = A_{0} {(\frac{1}{2})}^{\frac{2 \times 60}{30}} = \frac{A_{0}}{16} \Rightarrow A_{0} = 80 s e c^{- 1}

Question 27. 16 gm sample of a radioactive element is taken from Bombay to Delhi in 2 hour and it was found that 1 gm of the element remained (undisintegrated). Half life of the element is

2 hour
1 hour
12 hour
14 hour

Discuss Question

Answer: Option C. -> 12 hour
:
C

N = N_{0} {(\frac{1}{2})}^{t / T_{1 / 2}} \Rightarrow 1 = 16 {(\frac{1}{2})}^{2 / T_{1 / 2}} \Rightarrow T_{\frac{1}{2}} = \frac{1}{2} h o u r

Question 28. A heavy nucleus at rest breaks into two fragments which fly off with velocities in the ratio 8 : 1. The ratio of radii of the fragments is

1 : 2
1 : 4
4 : 1
2 : 1

Discuss Question

Answer: Option A. -> 1 : 2
:
A