Knowledge Organiser: Half-Life
Part of Half-Life · GCSE GCSE Physics revision
This topic summary covers Knowledge Organiser: Half-Life within Half-Life for GCSE Physics. Revise Half-Life in Atomic Structure for GCSE Physics with 13 exam-style questions and 23 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 13 of 13 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 13 of 13
Practice
13 questions
Recall
23 flashcards
Knowledge Organiser: Half-Life
Key Terms
- Half-life: Time for activity to halve
- Activity: Decays per second (Bq)
- Becquerel (Bq): 1 decay per second
- Count rate: Detections per unit time
Key Facts
- Half-life is constant for each isotope
- Decay is random but statistically predictable
- Decay curve is exponential — never reaches 0
- Unaffected by temperature or pressure
Calculation Method
- Start → ÷2 → ÷2 → ÷2...
- Count halvings = number of half-lives
- n half-lives: activity × (½)ⁿ
- From graph: draw construction lines
Applications by Half-Life
- Medical tracers: hours
- Smoke detectors: years
- Carbon dating: 5,730 years
- Long half-life = long-term hazard
Key Equations
- After n half-lives: remaining amount = initial × (½)ⁿ
- Activity (Bq) = number of decays per second
- Activity halves every half-life (same as count rate)
- Carbon-14 half-life = 5,730 years (used in carbon dating)
Common Mistakes
- Thinking the sample completely decays after a few half-lives: After each half-life, the activity halves — it never reaches exactly zero; the sample approaches (but never reaches) zero activity
- Confusing activity and count rate: Activity is decays per second (Bq) measured at the source; count rate is what a detector measures — count rate is always less than activity due to detector efficiency and distance
- Applying the wrong half-life for an application: Medical tracers need short half-lives (hours) to minimise patient dose; carbon dating uses 5,730 years; smoke detectors use americium-241 (~430 years)
- Reading the half-life incorrectly from a graph: Find the time for activity to halve (not to reach zero) — locate one point, halve the y-value, then read across to the x-axis to find the half-life
- Forgetting background radiation in practical measurements: Always subtract background count rate from measured count rate before using the data to determine half-life
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Practice Questions for Half-Life
What is the definition of half-life?
Explain what is meant by saying radioactive decay is 'random and spontaneous'.
Quick Recall Flashcards
13 questions on Half-Life — practise free
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