Answer the following questions about radioactive decay and activity:
(i) Define the activity of a radioactive sample and state its SI unit.
(ii) Using the law of radioactive decay, describe qualitatively how the activity of a pure sample changes with time. No numerical calculation is required.
(iii) Two radioactive nuclides P and Q initially contain the same number of nuclei, but their decay constants satisfy λP>λQ. Without using any calculations, explain which sample will have the greater activity at (a) t=0, (b) a time equal to one half-life of P, and (c) a very long time. Justify your answers in each case.