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.
More to read
31.5 Half-Life and Activity
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 against time 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, and so on. Four half lives leave one sixteenth of the start.
21.3 Radioactive Decay
Decay math runs on doubling and halving. If you know the half life and the whole number of half lives that pass, divide by two that many times to get the fraction left. A sample after three half lives keeps one eighth of its nuclei, and its count rate falls by the same factor.
The same halving logic dates ancient objects. Carbon 14 forms in the air and enters living things, then ticks down with its long half life after death. Measuring what fraction remains tells scientists how many half lives have passed since the organism died.
31.1 Nuclear Radioactivity
Single nuclei are drama queens: each one pops at a random moment nobody can predict. But a sample holds trillions of them, and with numbers that huge the randomness smooths into a steady pattern. That is why the decay curve is glass smooth even though every single pop is a surprise.
This split between random individuals and reliable crowds is the heart of the topic. You can never say when one nucleus will go, yet you can bet the lab budget on the count rate of a mole of them. Half life describes the crowd, never the individual.
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2 · Watch
En anglaisTake 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.