Activity of a Radioactive Source
Lesson notes
Overview
Activity falls exponentially: A = λN. This topic is rated advanced and is number 214 of 227 in Physics. Deeper models, formal results and harder reasoning.
What you are seeing
The panel above shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.
In depth
The activity of a source, the number of decays per second, is A = λN, measured in becquerels. Since N falls exponentially, so does the activity: A = A₀e^(−λt). A source with a short half-life is intensely active but dies away quickly, while a long-lived source emits slowly for ages. Medical isotopes are chosen with half-lives of hours to days so that they are active long enough to image but do not linger in the body.
Reading the relationship
The expression "A = A₀e^(−λt)" is a compact summary of the idea. It uses the symbols A, A₀e, λ, t; name what each one stands for before you start. Notice that it contains an exponential, which changes by the same factor for every equal step. Then use the sliders to see how changing one quantity changes the others.
A guided exploration
1. Set every slider to its middle value and note the shape of the curve. 2. Move one slider at a time and describe whether the curve shifts, stretches or turns. 3. Find the value where the behaviour changes sharply, then connect it to the formula.
Try this
Try to explain why the pattern holds, not just what it is. Look for limiting cases and what happens at the edges.
Common pitfalls
Check units and directions first. Many mistakes come from mixing up a scalar with a vector, or from using a formula outside the conditions where it holds.
How it connects
It builds on “Quantum Tunnelling Probability”, the topic before it in Physics. Once you are comfortable, continue with “Radioactive Chain Equilibrium”.
Why it matters
Physics explains how the world moves, heats, shines and interacts, and the same principles power technology around you.
Where you might use it
- Revising “Activity of a Radioactive Source” before a class, exam or certification
- Designing vehicles, bridges and devices where motion, forces or waves must be predicted
- Understanding how sensors, lasers and communication links actually work
- Explaining everyday phenomena such as sound, light, heat and orbits
Frequently asked questions
What is Activity of a Radioactive Source?
Activity falls exponentially: A = λN. It is an advanced topic in Physics.
How do I use the Activity of a Radioactive Source simulation?
The panel at the top shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. Move the sliders one at a time, then press Space to pause the animation and look closer.
What does the formula mean?
The expression A = A₀e^(−λt) packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.
Who is this topic for?
Deeper models, formal results and harder reasoning. If the quiz feels hard, reread the lesson notes and try the previous topic first.
What should I learn before this?
Start with “Quantum Tunnelling Probability”, the previous topic in Physics. Each topic in a subject builds on the ones before it.
How do I track my progress?
Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.