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Saturday, 31 October

JEE Main 2021 · PhysicsMultiple choiceSingle correctEasyCalculation

JEE Main 26 February 2021, Shift 2, Physics Q1

Question 1 of 90 in this shift, Physics question 1 of 30, Section A.

A radioactive sample is undergoing α\alpha decay. At any time t1t_1, its activity is A and another time t2t_2, the activity is A5\frac{A}{5}. What is the average life time for the sample ?
  1. (1)ln⁡5t2−t1\frac{\ln 5}{t_2 - t_1}
  2. (2)ln⁡(t2+t1)2\frac{\ln (t_2 + t_1)}{2}
  3. (3)t2−t1ln⁡5\frac{t_2 - t_1}{\ln 5}Official answer
  4. (4)t1−t2ln⁡5\frac{t_1 - t_2}{\ln 5}

Official answer

Option 3

NTA final key.

Chapter
Nuclei
Topic
Radioactive Decay
Idea tested
Mean Life Calculation

Same idea in other shifts

Asked 2× in all
  1. 11 Apr 2023, Shift 1 · Q33Two radioactive elements A and B initially have same number of atoms. The half life of A is same as the average life of B. If λA\lambda_A…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.