Physics II

starmoonsun SG

1)It is known that one gram of carbon from a living plant which contains a definite proportion of carbon-14 decays at a rate of 15 disintegrations per minute. a specimen which was formed from a living plant a long time ago shows 15 idsintegration every 4 minutes from one gram of carbon present in it.

given that the count rate from carbon-14 begins to fall only when tree dies, estimate the age of the specimen. (the half life of carbon-14 is 5000 years)

ans = 10 000 years.

2) N atoms of isotopes phosphorus-32 undergo radioactive decay. it decays to a non radioactive sulphur-32. calculate in terms of N, the number of sulphur-32 atoms remaining at the end of the 60 days, given that the half life of the isotope is 15 days.
ans = N/16

unclebutcher SG

living plant:15 /4 min
one half life:

15 disintegrations per 2 mins now

another half life:

15 disintegrations per minute

thus goes thru 2 half lives. so 5000*2=10000years

2.)Let no. of 32P atoms be N.
after 15 days
N/2
after 30 days
(1/2)(N/2)= N/4
after 45 days
(1/2)(N/4)= N/8
after 60 days
(1/2)(N/ Cool = N/16