CBSE Class 12 · 2010 · Modern PhysicsShort Answer

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(i) Define 'activity' of a radioactive material and write its S.I. unit.
(ii) Plot a graph showing the variation of activity of a given radioactive sample with time.

Answer: $R = λ N$ (Bq); $R = R_{0} e^{- λ t}$ (exponential decay).

Solution

(i) Activity is the rate of disintegration of a radioactive sample,

R = - \frac{d N}{d t} = λ N

. Its S.I. unit is the becquerel (Bq), where

1 Bq = 1

disintegration per second.
(ii) Since

R = R_{0} e^{- λ t}

, the activity falls off exponentially with time from its initial value

R_{0}

(see graph).

R = λ N = R_{0} e^{- λ t} (unit: Bq)

CBSE Class 12 Board · 2010 · Subjective

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