Half Life: GCSE Physics Definition

Half Life: GCSE Physics Definition

What is Half Life?

In GCSE Physics, the half-life of a radioactive sample is the time it takes for the number of nuclei in the sample to decrease by half. For a slightly different, but equivalent wording: half-life is the time taken for half the undecayed nuclei to decay. An alternative definition is that half-life is the time taken for the activity of a source to decay by half.

The exact definition you need to learn depends on which of the GCSE Physics exam boards you will take your exam with. With each half-life that passes, the amount of undecayed nuclei remaining halves. This can be represented using a graph of activity against time. This type of graph also makes it easier to calculate the half-life of a sample.

Radioactive decay is spontaneous and random, so it is impossible to know when a particular unstable nucleus will decay. The half-life of an isotope is the rate at which the activity of a sample decreases. Different isotopes have different half-lives and half-lives can vary from a fraction of a second to billions of years in length. An isotope with a short half-life contains nuclei that decay very quickly. The most unstable nuclei have the shortest half-lives. 

Half Life Revision Resources to Ace Your Exams

Learn more about half-life and the requirements for your exam board using the relevant revision notes such as those in AQA GCSE Physics. Once you are ready you can use our specific exam practice questions and past papers, such as these ones for AQA.

Explore all the Save My Exams GCSE Physics resources