Modern Physics

Hot goIITian

Joined: 29 Mar 2012
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24 Apr 2012 22:09:17 IST
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Engineering Entrance , Medical Entrance , NEET , JEE Main , AIIMS , JEE Main & Advanced , Physics , Modern Physics

a radioactive isotope X of half life of 1.37 X 10^ 9 yrs decays to Y which is stable. a sample of rocks frm moon ws found to contain both X & Y in the ratio 1:7 . the age of rock is ? ans is 4.11 X 10^ 9 yrs .hw?



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Cool goIITian

Joined: 21 Apr 2012
Posts: 57
24 Apr 2012 22:42:58 IST
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thus t=3x1.37x10^9 yrs=4.11x10^9

edison's Avatar

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Joined: 19 Oct 2006
Posts: 7804
26 Apr 2012 10:49:24 IST
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 Using the expression

N_A = N_{A0}e^{-{\lambda}t} \,\!

λ = 0.693 / t1/2

or λ = 0.693 / (1.37 X 109)

or λ  = 5.05 x 10-10

NA / NA0 = 1/8 = exp (-5.05 x 10-10 t)

so t = 4.11 x 10-9 yrs




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