This topic summary covers Knowledge Organiser: Half-Life within Half-Life for GCSE Physics. Revise Half-Life in Atomic Structure for GCSE Physics with 15 exam-style questions and 23 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. 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.
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
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
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.
Why is radioactive decay described as random?
Radioactive decay is random because we cannot predict when any individual nucleus will decay. We can only predict the probability of decay and the average behaviour of large numbers of nuclei.