A radioactive source has a half-life of 80s. How long will it take for 7/8 of the source to decay?
Correct answer: C. 240 sec
- A. 10 sec
- B. 70 sec
- C. 240 sec
- D. 640 sec
Explanation
Correct option is C. Given that the half life is 80 sec ; Then lambda (λ) = 0.693/t1/2 =0.693/80 If ⅞th of the source has decade this means that only ⅛th is left. Using the formula N =No e -(λ)t N/No = e- (0.689/80)t 1/8= e-(0.689/80)t In (⅛) = -(O.689/80) Then t = In (⅛) /- (0.689/80) Therefore; t = 240 sec As this is a numerical thus only one option is correct.
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