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S6 Physics: Radioactivity and Half-life – Practice Paper 1 (Basics, 10 Questions)
S6 Physics: Radioactivity and Half-life – Practice Paper 1 (Basics, 10 Questions)
Paper one builds the basic routine: divide the elapsed time by the half-life to count half-lives, then halve repeatedly. It covers nucleus number, mass and activity, plus reverse questions that recover time or half-life from a ratio.
Quick answer Answers: 1000 nuclei, 30 g, 120 Bq, 1/16 which is 6.25%, 36 years, 20 days, 6 days, 15 days, one quarter, and 1/8 which is 12.5%.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 25 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: S6 Physics: Radioactivity and Half-life |
| Level | Basics |
02 Questions (10 items)
- FoundationA radioactive sample starts with 8000 undecayed nuclei and has a half-life of 5 years. How many undecayed nuclei remain after 15 years?
- FoundationA 240 g radioactive sample has a half-life of 8 days. What mass remains after 24 days?
- FoundationA sample has an activity of 960 Bq and a half-life of 6 hours. What is its activity after 18 hours?
- IntermediateA sample has passed through 4 half-lives. What fraction of it is undecayed? Give the answer as a fraction and a percentage.
- IntermediateA nuclide has a half-life of 12 years. How long does its activity take to fall to one eighth of the original value?
- IntermediateOnly 1/32 of a sample remains and its half-life is 4 days. How many days have passed?
- ChallengeThe activity of a sample falls from 960 Bq to 60 Bq in 24 days. Find the half-life of the nuclide.
- ChallengeA 512 g radioactive sample decays to 16 g. The half-life is 3 days. How many days have passed?
- ChallengeSamples A and B both start at 100 g. A has a half-life of 10 years and B has 20 years. After 40 years, what fraction of B is A?
- Error redoA nuclide has a half-life of 5 years. What fraction is undecayed after 15 years? (Common error: answering 0)
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 15 / 5 = 3 half-lives, so 8000 / 2 / 2 / 2 = 1000 nuclei remain. | Divide the elapsed time by the half-life to find the number of half-lives, then halve that many times. |
| 2 | 24 / 8 = 3 half-lives, so 240 / 2 / 2 / 2 = 30 g remain. | The mass of the undecayed nuclide falls in the same proportion as the number of undecayed nuclei. |
| 3 | 18 / 6 = 3 half-lives, so 960 / 8 = 120 Bq. | Activity also halves with every half-life, and three halvings means dividing by 8. |
| 4 | (1/2)^4 = 1/16 = 0.0625, that is 6.25%. | Each half-life multiplies the amount by 1/2, so four half-lives leave 1/16 rather than 1/4. |
| 5 | 1/8 = (1/2)^3, which is 3 half-lives, so 12 x 3 = 36 years. | Write the target fraction as a power of one half; the exponent gives the number of half-lives. |
| 6 | 1/32 = (1/2)^5, which is 5 half-lives, so 4 x 5 = 20 days. | One thirty-second is the result of five halvings, not thirty-two half-lives. |
| 7 | 960 / 60 = 16 = 2^4, which is 4 half-lives, so 24 / 4 = 6 days. | First find how many times the activity has dropped, then express that factor as a power of two. |
| 8 | 512 / 16 = 32 = 2^5, which is 5 half-lives, so 3 x 5 = 15 days. | A mass ratio of 32 corresponds to five halvings; treating 32 as the number of half-lives is the usual slip. |
| 9 | A has passed 4 half-lives, leaving 100 / 16 = 6.25 g; B has passed 2 half-lives, leaving 100 / 4 = 25 g. 6.25 / 25 = 1/4, so A is one quarter of B. | The two half-lives differ, so count the half-lives separately instead of assuming the same number of halvings. |
| 10 | 15 / 5 = 3 half-lives, so (1/2)^3 = 1/8 = 0.125; 12.5% is still undecayed. | Answering 0 treats the half-life as the time for complete decay; it is only the time to halve, so decay never fully finishes. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
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