Half-Life and Activity
Activity is the number of decays each second, measured in becquerels, and it falls by half in every half-life. Reading a half-life off a decay curve turns an unpredictable single nucleus into a reliable count for a large sample.
What a learner can do afterwards
- Reads a half-life from a graph of activity against time
- Calculates the fraction or the count rate left after a whole number of half-lives
- Explains that a single nucleus decays at random while a large sample follows a steady pattern
1 · Read
Activity counts the decays each second, measured in becquerels, where 1 Bq means one decay per second. As unstable nuclei pop, fewer remain, so the activity falls over time. The half-life is the time for the activity to halve, and it is a fixed fingerprint of each isotope.
Read a half-life straight off an activity graph by finding where the curve drops to half its start value and reading across to the time axis. After each whole half-life the fraction left halves again. One half, one quarter, one eighth, then one sixteenth after four.
Priya watches a counter fall from 800 to 100 Bq. She lists the drops: 800 to 400, then 200, then 100. That is three halvings, so three half-lives have passed. If one half-life is 5 minutes, the whole fall took 15 minutes.
Single nuclei are surprises, since no one can predict when one will pop. A sample holds trillions of them, and the crowd average smooths into a steady curve. Half-life describes the crowd, never the individual.
Activity halves each half-life, from a crowd no single atom can predict.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.