define activity of a radioactive material and write its si units draw graph for activity vs time ?

Activity of a radioactive substance is the number of disintegration taking place in the given sample per second.

Its SI unit is Becquerel (Bq) . 1 Bq is one disintegration per second.

Mathematically, activity (A) is defined as

Where N = number of atoms present at an instant that has not decayed.