| Lesson Plan |
| Grade: |
Date: 25/02/2026 |
| Subject: Physics |
| Lesson Topic: understand the exponential nature of radioactive decay, and sketch and use the relationship x = x0e–λt, where x could represent activity, number of undecayed nuclei or received count rate |
Learning Objective/s:
- Describe the exponential decay law and define each variable.
- Derive the relationship between decay constant, half‑life and mean life.
- Apply x = x₀e⁻ˡᵃᵐᵇ𝑑𝑎 t to calculate remaining activity, nuclei number or count rate.
- Sketch linear and semi‑log decay graphs and interpret the slope as ‑λ.
- Identify common mistakes and ensure unit consistency when solving decay problems.
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Materials Needed:
- Projector or interactive whiteboard
- Slide deck covering derivation and examples
- Handout with decay‑law worksheet
- Graph paper or digital plotting tool
- Calculator or spreadsheet software
- Sample problem set (e.g., ⁶⁰Co activity calculation)
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Introduction:
Begin with a short video of a Geiger‑counter click rate decreasing as a source decays, prompting students to describe the pattern they see. Connect this observation to prior learning on half‑life and exponential functions. State that by the end of the lesson they will model decay mathematically, sketch appropriate graphs, and solve related quantitative problems.
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Lesson Structure:
- Do‑now (5') – Quick written question: “Define half‑life and explain its significance.”
- Mini‑lecture (10') – Derive the exponential law from dN/dt = ‑λN, highlighting each symbol.
- Guided practice (12') – Work through the ⁶⁰Co activity example together, completing the worksheet step‑by‑step.
- Graphing activity (8') – Students plot x versus t on linear and semi‑log axes, identify the straight‑line slope (‑λ) and mark half‑life.
- Independent problem solving (10') – Students solve 2–3 practice problems (solve for λ, t, or x) and check answers with peers.
- Quick check (5') – Exit ticket: write one sentence describing how to rearrange the decay equation to find the elapsed time.
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Conclusion:
Recap the key steps for using the decay equation and how a semi‑log plot confirms exponential behaviour. Collect exit tickets, emphasise the importance of unit consistency, and assign the worksheet as homework for further practice with conversions and graph interpretation.
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