Half-Life
Half-life is the time taken for half of the radioactive nuclei in a sample to decay.
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Half-life is the time taken for half of the radioactive nuclei in a sample to decay.
- Activity falls by half each half-life
- The decay is random but predictable overall
- Used in carbon dating
🧠 Quick quiz
Three questions. Score 2 or more to complete the topic.
What is Half-Life?
Within Physics, which covers forces, energy, waves, electricity, magnetism, light and space explained through motion and experiments, this topic explains Half-Life. Physics topics explain how things move, push, heat up and carry energy, so each one here pairs a definition with a live model you can change.
Check that you can explain Half-Life in your own words and give one example from everyday life. If you can do that, you have understood the core idea.
Understanding Half-Life explains how the physical world behaves, from sport and transport to electricity and medicine.
Half-Life in detail
The half-life of a radioactive isotope is the time taken for half the unstable nuclei in a sample to decay, or for the activity to fall to half. After one half-life, 50% remains; after two, 25%; after three, 12.5%.
Half-lives vary from fractions of a second to billions of years. A decay graph is an exponential curve, and half-life can be read off at any starting point. Short half-lives are chosen for medical tracers so patients are exposed briefly, and long half-lives, such as carbon-14 (5,730 years), allow archaeologists to date ancient remains.
How the interactive helps you understand Half-Life
Watch the population model in the interactive. Use any sliders or buttons it offers to change the values and see how the population model responds.
Before you change anything, predict the result. Then change one value at a time and check whether your prediction was right. This is a quick way to build a correct mental model of Half-Life.
Real-life uses and examples
- Use Half-Life to make sense of a practical and its results.
- Answer “explain” questions by linking Half-Life to a clear cause and effect.
- Predict what a ball, car or circuit will do with Half-Life before you do the calculation.
Common mistakes to avoid
- Mixing up Half-Life with a related quantity that has different units.
- Forgetting that some quantities, such as force or velocity, also have a direction.
- Learning the definition without testing it. Use the quiz and the interactive to check that you can apply the idea.
Frequently asked questions about Half-Life
What is Half-Life?
Half-life is the time taken for half of the radioactive nuclei in a sample to decay.
What are the most important points about Half-Life?
The main points are: Activity falls by half each half-life; The decay is random but predictable overall; Used in carbon dating.
Why is Half-Life useful?
Understanding Half-Life explains how the physical world behaves, from sport and transport to electricity and medicine.
How can I practise Half-Life?
Open the interactive at the top of this page and change the controls one at a time. Predict the result before each change, then take the three-question quiz. A score of two or more marks the topic as complete on this device.
Which topics are related to Half-Life?
Related topics in Physics include The Solar System, The Universe and the Big Bang, Rotational Motion and Torque.