Question
Define the activity of a given radioactive substance. Write its S.I. unit.
✓
Answer
The rate of decay of a radioactive substance is called activity of that substance. It is negative of the rate of decay of the radioactive substance. Activity, $\text{R} = \frac{\text{N}' - \text{N}}{\text{t}' - \text{t}}$ $ = - \frac{\Delta\text{N}}{\Delta\text{t}}$ $ \DeclareMathOperator*{\median}{\text{lim}} \median_{\Delta\text{t}\rightarrow0}\bigg(-\frac{\Delta\text{N}}{\Delta\text{t}}\bigg) = -\frac{\text{dN}}{\text{dt}}$ S.I. unit of activity 
- becquerel (Bq).
- decay per second.
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