Answer :

a) The plot of binding energy per nucleon (BE/A) versus the mass number A is given below:

From the plot we note that

i) During nuclear fission

A heavy nucleus in the larger mass region ( A>200) breaks into two middle level nuclei, resulting in an increase in B.E/ nucleon. This results in a release of energy.


92U235 + n1 56Ba144 + 36Kr89 + 30n1 + energy

ii) During nuclear fusion

Light nuclei in the lower mass region (A<20 ) fuse to form a nucleus having higher B.E / nucleon. Hence Energy gets released.


1H2 + 1H2 2He4 + energy

b) Let the initial activity be Ao.

Let the time taken for the activity to reduce = t.

Half live is given T1/2 = 10years

If we take Ao = 100, then activity after time t will be

At = 3.125 (given)

We know the law of radioactivity decay is given as

At = Ao e-λt ….(i)

Where λ = decay constant.

Therefore, substituting the values of At Ao and λ in equation (i),

3.125 = 100e-0.0693t

Taking natural logarithm on both sides of above equation

the time taken for the activity to reduce to 3.125% is 50 years.

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