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Activity - GCSE Physics Definition

Reviewed by: Ann Howell

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What is activity?

In GCSE Physics, activity is defined as the rate at which unstable nuclei in a source of radiation decay. Activity is measured in Becquerels (Bq), where 1 Becquerel is equivalent to one nucleus decaying each second.

A substance containing radioactive nuclei is a source of radiation. The rate at which a source of radiation decays is defined by its activity. When an isotope decays, the number of unstable nuclei remaining will decrease. As a result, the activity of that isotope will also decrease over time. This can be shown on a graph of activity against time for a decaying source.

Graph showing radioactive decay over time, with activity decreasing and levelling at background radiation level, marked by a red line and labelled.
Activity decreasing with time for a radioactive isotope

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Ann Howell

Reviewer: Ann Howell

Expertise: Physics Content Creator

Ann obtained her Maths and Physics degree from the University of Bath before completing her PGCE in Science and Maths teaching. She spent ten years teaching Maths and Physics to wonderful students from all around the world whilst living in China, Ethiopia and Nepal. Now based in beautiful Devon she is thrilled to be creating awesome Physics resources to make Physics more accessible and understandable for all students, no matter their schooling or background.

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