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Saturday, 31 October

JEE Main 2021 · PhysicsMultiple choiceSingle correctEasyCalculation

JEE Main 3 August 2021, Shift 2, Physics Q18

Question 18 of 90 in this shift, Physics question 18 of 30, Section A.

A radioactive isotope has a half life of T years. After how much time is its activity reduced to 6.25% of its original activity ?
  1. (1)2 T years
  2. (2)4 T yearsOfficial answer
  3. (3)1.5 T years
  4. (4)1 T year

Official answer

Option 2

NTA final key.

Chapter
Nuclei
Topic
Radioactive Decay
Idea tested
Half-Life Calculations

Same idea in other shifts

Asked 10× in all
  1. 8 Apr 2023, Shift 2 · Q44A radio active material is reduced to 1/8 of its original amount in 3 days. If 8×10−38\times10^{-3} kg of the material is left after 5 days…MediumSingle correct
  2. 10 Apr 2023, Shift 2 · Q44The half life of a radioactive substance is T. The time taken, for disintegrating 78\frac{7}{8}th part of its original mass will be:EasySingle correct
  3. 15 Apr 2023, Shift 1 · Q44The half-life of a radioactive nucleus is 5 years. The fraction of the original sample that would decay in 15 years is:EasySingle correct
  4. 27 Jul 2022, Shift 2 · Q48The activity of a radioactive material is 6.4×10−46.4\times10^{-4} curie. Its half life is 5 days. The activity will become 5×10−65\times10^{-6}…EasySingle correct
  5. 29 Jun 2022, Shift 1 · Q33The activity of a radioactive material is 2.56×10−32.56 \times 10^{-3} Ci. If the half life of the material is 5 days, after how many days the…EasySingle correct
  6. 3 Aug 2021, Shift 2 · Q25A sample undergoing beta decay, reduced to 116th\frac{1}{16}^{\mathrm{th}} of its initial mass in 48 years. Its half life period is…EasyNumerical value
  7. 27 Aug 2021, Shift 1 · Q19There are 101010^{10} radioactive nuclei in a given radioactive element. Its half-life time is 1 minute. How many nuclei will remain after…EasySingle correct
  8. 16 Mar 2021, Shift 2 · Q10Calculate the time interval between 33% decay and 67% decay if half-life of a substance is 20 minutes.EasySingle correct

Question text from the official JEE Main paper published by NTA; answer from the final answer key. Chapter, topic and idea tags, difficulty and skill are Lumi’s. Lumi analyses 108 of the 174 JEE Main shifts held from 2017 to 2026.