GCSE Physics · Edexcel 1PH0 · Topic 6C
Half-life
Key points
- Activity is the number of nuclei decaying per second, in becquerels (Bq).
- Half-life is the time taken for half the unstable nuclei in a sample to decay, or for the activity to halve.
- Each isotope has its own half-life, from fractions of a second to billions of years.
- After half-lives the fraction remaining is . For example, after 3 half-lives remains.
- Find the half-life from a graph of activity (or count rate) against time: read off the time for the value to halve.
- Net decline questions (higher): find the amount lost, e.g. after 2 half-lives the activity has fallen by of its starting value.
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You cannot say when one radioactive nucleus will decay, but with billions of them the pattern is very predictable. Half-life captures that pattern and lets you calculate how activity falls over time.
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