⏳ Counting Down the Atoms
⏳ Counting Down the Atoms
Physics glossary
- What is half-life?
- Half-life is the time taken for half the unstable nuclei in a radioactive sample to decay, or the time for the activity of a radioactive source to fall to half its original value.
Half-life is the time taken for the number of radioactive nuclei (or the activity of a sample) to halve. Each radioactive isotope has a fixed, characteristic half-life that cannot be changed by any physical or chemical process.
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🧠 Memory Aids
Half-life calculation method: Use the "halving chain" — write out each step:
Start → ÷2 → ÷2 → ÷2 → ...
Count how many times you had to divide to get to your answer. That's how many half-lives have passed.
Remembering activity unit: "BecQuerel = Becquerel's Quantity per second" — he discovered radioactivity, so the unit is named after him. 1 Bq = 1 decay/second.
Graph reading tip: Always draw your horizontal and vertical construction lines on graph questions — examiners look for this evidence of method and award marks for it even if you misread the value slightly.
Now try it yourself
Quiz · Question 1 of 15
What is the definition of half-life?
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This topic in real past papers
Every real exam question we've found on half-life, with a full worked answer.