GCSE Physics Revision — Half-Life
Revise Half-Life for GCSE Physics with a topic explanation, worked example and common mistakes. Check the board notes for specification differences.
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What is Half-Life?
The half-life of a radioactive isotope is the time it takes for the number of unstable nuclei in a sample to halve, or the time it takes for the activity (the number of decays per second) of the sample to fall to half its initial value. It is a constant value for a given isotope and is not affected by external factors.
Board notes: A Higher Tier topic for all major GCSE Physics boards (AQA, Edexcel, OCR). You will be expected to perform calculations and interpret decay graphs.
Step-by-step explanationWorked examples
Worked example
A radioactive sample has an initial activity of 800 Bq. Its half-life is 10 minutes. What will its activity be after 30 minutes? Solution: 30 minutes is three half-lives (30/10 = 3). After 1 half-life: 400 Bq. After 2 half-lives: 200 Bq. After 3 half-lives: 100 Bq.
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Common mistakes
- 1Thinking that after two half-lives, the activity will be zero. It will be a quarter of the original activity (half of a half).
- 2Calculating the half-life incorrectly from a graph. You must find the time taken for the activity to fall to half of its starting value.
- 3Confusing half-life with the lifetime of a single nucleus. Half-life is a statistical measure for a large number of nuclei.
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Frequently asked questions
How is half-life used in radioactive dating?
Carbon-14 is a radioactive isotope with a half-life of about 5730 years. By measuring the amount of Carbon-14 remaining in an ancient organic object, scientists can determine its age.
Why are isotopes with short half-lives used in medical tracers?
Isotopes with short half-lives are used so that they decay quickly and do not remain in the patient's body for a long time, minimizing the radiation dose and potential harm.