IIT JEE , AIEEE Forum - Physics , Modern Physics - A very conceptuyal modern physics doubt rates assured

rakesh61

A sample of radio active element has a mass of 10 kg at an instant t = 0

The approxim,ate mass of this element in the sample afetr 2 mean lives is

Ans 1.35 g

Please give detailed solution

bhartibhanushali

rakesh bhai
what do we mean by 2 mean lives??

feynmann

After each mean life the wt decreases to 1/e of its previous wt .

Hence mass after 2 mean lives = 10 kg / e^2 = 1.35 kg

johri_anshuman

@feynmann
pls give the exact definition of a mean life...

priyesh

mean life means the time in which 63 percent(approx) has decayed or we can say 37 percent i.e 1/e of the sample is left

so N = No e^-

t

for t = 1/

N = No/e so mean life is 1/

after two mean lives sample left undecayed is No/e^2 = 1.35 kg