WJEC GCSE Physics
WJEC GCSE Physics 2.8 Half-Life Exam Questions
Free WJEC GCSE Physics revision for 2.8 Half-Life: what you need to know, common mistakes and exam-style questions with answers. Covers random decay, modelling decay, decay curves, half-life, activity, carbon dating and uses of radioactive materials.
Practise 2.8 Half-Life with past papers
Once you have learnt this topic, find the 2.8 Half-Life questions in real WJEC GCSE Physics past papers and mark them with the mark scheme. Doing lots of real questions is the quickest way to learn how examiners ask about each idea.
StemUp mark scheme flashcards
GCSE Physics Triple Science Flashcards for 2027 Onwards
Every answer is written in mark scheme wording, so you learn exactly what examiners give marks for. Then test yourself with the questions on this page.
From £7.99View flashcards
About WJEC GCSE Physics 2.8 Half-Life
Half-life in Unit 2 deals with how quickly radioactive sources decay and how this decides their uses. You need to explain the random nature of decay, model it with dice, coins or spreadsheets, and find half-life from a decay curve. You must calculate how activity or the number of nuclei changes over several half-lives, including carbon dating. The uses of radioactive materials, such as medical tracers and cancer treatment, are linked to their half-life, penetration and biological effects. Determining the half-life of a model radioactive source, for example with dice, is the specified practical. Examiners often ask you to read a half-life from a graph and justify choosing a source.
What you need to know
- Random decay It is impossible to predict when a particular nucleus will decay, but for large numbers the rate of decay is predictable. Dice or coins model this: each throw, a fixed fraction 'decay'.
- Half-life Half-life is the time taken for the number of radioactive nuclei, or the activity, to halve. It is constant for a given isotope.
- Decay curves Plot activity against time; read the time for the activity to fall from any value to half that value. Repeat for different starting points and take a mean.
- Calculations After n half-lives, the activity is the original divided by 2^n. For example, after 3 half-lives, 1/8 remains.
- Carbon dating Living things contain a constant proportion of carbon-14. After death it decays with a half-life of about 5700 years, so the fraction remaining gives the age of the material.
- Uses Medical tracers use gamma emitters with short half-lives so they can be detected outside the body without staying radioactive for long. Radiotherapy uses gamma to kill cancer cells. Choose sources using half-life, penetration and ionisation.
Common mistakes in 2.8 Half-Life
- Thinking that after two half-lives all the substance has gone. After two half-lives one quarter remains.
- Reading the half-life as the time to reach half of zero or as half the total time. Find the time for the activity to fall from a value to half of it.
- Saying a long half-life is best for a medical tracer. A short half-life (hours) is used so the patient is not exposed for long, but long enough to complete the test.
- Forgetting to subtract background count before plotting decay data. Correct each reading first.
Try three 2.8 Half-Life exam-style questions
1. A source has an activity of 800 Bq and a half-life of 5 days. How long will it take for the activity to fall to 100 Bq? (2 marks)
Show the mark scheme answer
800 → 400 → 200 → 100 = 3 half-lives; 3 x 5 = 15 days.
2. A wooden object has 25% of the carbon-14 found in living wood. The half-life of carbon-14 is 5730 years. Estimate its age. (2 marks)
Show the mark scheme answer
25% = two half-lives; 2 x 5730 = 11 460 years.
3. Explain why a gamma emitter with a half-life of 6 hours is suitable as a medical tracer. (3 marks)
Show the mark scheme answer
Gamma penetrates the body so it can be detected outside; weakly ionising so causes less damage; 6 hours long enough for the test but decays quickly so the patient is not exposed for long.
2.8 Half-Life FAQs
What does WJEC GCSE Physics 2.8 Half-Life cover?
It covers the random nature of radioactive decay, modelling decay, decay curves and half-life, activity calculations, carbon dating, and the uses of radioactive materials as tracers and in treating cancer.
Which unit is Half-Life examined in?
It is examined in Unit 2: Forces, Space and Radioactivity, a 1 hour 45 minute written paper worth 45% of the GCSE, sat at foundation or higher tier. The Unit 3 practical assessment draws on the same practical skills.
How do I find half-life from a graph?
Choose a starting activity, halve it, read off both times and find the difference. Repeat from a different starting point and take an average to improve accuracy.
