NEET IIT JEE NCERT Class11,CBSE Modern physics MCQs,PDFs,Tests

Modern Physics Mock Test-4


A radioactive nucleus undergoes a series of decay according to the scheme
\( A\xrightarrow{α} A_1\xrightarrow{β} A_2\xrightarrow{α}A_3\xrightarrow{γ}A_4\)
If the mass number and atomic nubmer of A are 180 and 72 respectively, then what are these number for A4?

[A] 172 and 69
[B] 174 and 70
[C] 176 and 69
[D] 176 and 70





An archaeologist analyses the wood in a prehistoric structure and finds that C14 (Half-life = 5700 years) to C12 is only one-third of that found in the cells [A] 5700 yr
[B] 2850 yr
[C] 1 1,400 yr
[D] 22,800yr





Unit of radioactivity is Rutherford. Its value is

[A] 3.7×1010 disintegration/s
[B] 3.7×106 disintegration/s
[C] 1.0×1010 disintegration/s
[D] 1.0×106 disintegration/s





The counting rate observed from a radioactive source at t = 0 was 1600 count/s and at t=8 s it was 100 count/s. The counting rate observed, as count/s at t = 6 s, will be

[A] 400
[B] 300
[C] 200
[D] 150





In the given nuclear reaction A, B, C, D, E represents
92U238\(\xrightarrow{α}\)BThA\(\xrightarrow{β} \) DPaC\(\xrightarrow{E}\)92U234

[A] A =234, B = 90, C = 234, D=91, E =β
[B] A = 234, B – 90, C = 238, D = 94, E =α
[C] A = 238, B = 93, C = 234, D = 91, E = β
[D] A = 234, B = 90, C = 234, D = 93, E =α





1 curie is equal to

[A] 3×1010 disintegration/s
[B] 3.7×107 disintegration/s
[C] 5×107 disintegration/s
[D] 3.7×1010 disintegration/s





The half-life of a sample of a radioactive substance is 1 h. If 8×1010 atoms are present at t=0, then the number of atoms decayed in the duration t=2h to t=4h will be

[A] 2×1010
[B] 1.5×1010
[C] zero
[D] infinity





Threshold frequency for a metal is 1015 Hz. Light of λ= 4000Å falls on its surface. Which of the following statements is correct?

[A] No photoelectric emission takes place
[B] Photoelectron come out with zero speed
[C] Photoelectron come out with 103 m/s speed
[D] Photoelectron come out with 105 m/s speed





Half-life of a radioactive substance is 20 min. Difference between points of time when it is 33% disintegrated and 67% disintegrated is approximately

[A] 10 min
[B] 20 min
[C] 30 min
[D] 40 min





A and B are two radioactive substances whose half-lives are 1 and 2 years respectively. Initially 10 g of A and 1 g of B is taken. The time (approximate) after which they will have same quantit [A] 6.62 yr
[B] 5 yr
[C] 3.2yr
[D] 7 yr





Half-life of a radioactive substance is 20 min. The time between 20% and 80% decay will be

[A] 20 min
[B] 40 min
[C] 30 min
[D] 25 min





After 280 days, the activity of a radioactive sample is 6000 dps. The activity reduces to 3000 dps after another 140 days. The initial activity of the sample in dps is

[A] 6000
[B] 9000
[C] 3000
[D] 24000





If 10% of radioactive material decays in 5 days, then the amount of the original material left after 20 days is approximately

[A] 60%
[B] 65%
[C] 70%
[D] 75%





In a sample of radioactive material, what fraction of the initial number of active nuclei will remain undisintegrated after half of a half-life of the sample?

[A] \( \sqrt{2}-1\)
[B] \( \frac{1}{\sqrt{2}}\)

[C] \( \frac{1}{2\sqrt{2}}\)

[D] \( \frac{1}{4}\)






90% of a radioactive sample is left undecayed after time t has elapsed. What percentage of the initial sample will decay in a total time 21?

[A] 20%
[B] 19%
[C] 40%
[D] 38%





In a radioactive sample the fraction of initial number of radioactive nuclei, which remains undecayed after n mean lives is

[A] \( \frac{1}{e^n}\)
[B] en
[C] \( (1-\frac{1}{e^n})\)
[D] \( (\frac{1}{e-1})^n\)





A stationary radioactive nucleus of mass 210 units disintegrates into an alpha particle of mass 4 units and residual nucleus of mass 206 units. If the kinetic energy of the alpha particle is E, [A] \( \frac{2}{105} \)E

[B] \( \frac{2}{103} \)E

[C] \( \frac{103}{105} \)E

[D] \( \frac{103}{2} \)E






A freshly prepared radioactive source of half-life 2 h emits radiation of intensity which is 64 times the permissible safe level. The minimum time after which it would be possible to work safel [A] 128h
[B] 24 h
[C] 6h
[D] 12 h





An atom of mass number A and atomic number Z emits successively a γ-ray, a β-particle, and α-particle. The mass number and the atomic number of the end product are

[A] A – 4, Z – 4
[B] A, Z + 1
[C] A – 4, Z + 2
[D] A – 4, Z – 1





The dacay constant of a radioactive substance is λ Its half-life and mean-life, respectively are

[A] \( \frac{1}{λ} \) and loge2

[B] \( \frac{log_e2}{λ}\) and \( \frac{1}{λ}\)

[C] \( 2log_e2 and \frac{1}{λ} \)

[D] \( \frac{1}{λ}\) and \( \frac{1}{λ}\)






A β-particle is emitted by a radioactive nucleus at the time of conversion of a

[A] neutron into a proton
[B] proton into a neutron
[C] nucleon into energy
[D] positron into energy





Atomic mass number of an element is 232 and its atomic number is 90. The end product if this radioactive element is an isotope of lead (82Pb208). The number of alpha and b [A] α = 3 and β = 3
[B] α = 6 and β = 4
[C] α = 6 and β = 2
[D] α = 1 and β = 6





The decay constant of a radioactive element is defined as the reciprocal of the time interval after which the number of atoms of the radioactive element falls to nearly

[A] 50% of its original number
[B] 36.8% of its original number
[C] 63.2% of its original number
[D] 75% of its original number