Half-Life and Decay Law MCQs
Half‑Life and Decay Law MCQs
Class: CBSE Class 12 | Subject: Physics
Section 8: Atoms and Nuclei
Topic: Half‑Life and Decay Law
Based on CBSE Board Examination Pattern
Section 8: Atoms and Nuclei
Topic: Half‑Life and Decay Law
Based on CBSE Board Examination Pattern
Instructions: These Multiple Choice Questions (MCQs) are designed strictly as per the NCERT syllabus, making them ideal for CBSE Class 12 Board Examination preparation.
1. Radioactive decay is:
Answer: A
Radioactive decay is a spontaneous nuclear process unaffected by physical or chemical conditions.
Radioactive decay is a spontaneous nuclear process unaffected by physical or chemical conditions.
2. Half‑life is the time in which:
Answer: C
Half‑life is the time required for half of the radioactive nuclei to disintegrate.
Half‑life is the time required for half of the radioactive nuclei to disintegrate.
3. Decay constant (λ) unit is:
Answer: B
Decay constant represents probability per unit time → unit s⁻¹.
Decay constant represents probability per unit time → unit s⁻¹.
4. Relation between half‑life and decay constant:
Answer: B
T½ = 0.693 / λ derived from exponential decay law.
T½ = 0.693 / λ derived from exponential decay law.
5. Decay law is expressed as:
Answer: A
Number of undecayed nuclei decreases exponentially with time.
Number of undecayed nuclei decreases exponentially with time.
6. Mean life τ relation:
Answer: A
Mean life is reciprocal of decay constant.
Mean life is reciprocal of decay constant.
7. If λ increases, half‑life:
Answer: B
T½ inversely proportional to λ.
T½ inversely proportional to λ.
8. Activity A equals:
Answer: A
Activity proportional to undecayed nuclei.
Activity proportional to undecayed nuclei.
9. SI unit of activity:
Answer: B
1 Bq = 1 decay/sec.
1 Bq = 1 decay/sec.
10. After 1 half‑life, remaining nuclei:
Answer: B
Definition of half‑life.
Definition of half‑life.
11. After 2 half‑lives remaining:
Answer: C
(1/2)² = 1/4.
(1/2)² = 1/4.
12. Decay constant depends on:
Answer: C
It is intrinsic nuclear property.
It is intrinsic nuclear property.
13. Graph of decay law:
Answer: C
Shows exponential decrease.
Shows exponential decrease.
14. Half‑life independent of:
Answer: D
Radioactivity unaffected by external factors.
Radioactivity unaffected by external factors.
15. If λ = 0.693 s⁻¹, T½:
Answer: B
T½ = 0.693/0.693 = 1 s.
T½ = 0.693/0.693 = 1 s.
16. Mean life greater than half‑life by factor:
Answer: B
τ = 1/λ = 1.44 T½.
τ = 1/λ = 1.44 T½.
17. Decay probability per unit time:
Answer: C
Defined as λ.
Defined as λ.
18. Radioactive decay is:
Answer: B
Individual decay unpredictable.
Individual decay unpredictable.
19. After 3 half‑lives remaining fraction:
Answer: C
(1/2)³ = 1/8.
(1/2)³ = 1/8.
20. Exponential decay indicates:
Answer: B
Rate decreases with time as nuclei reduce.
Rate decreases with time as nuclei reduce.