Lesson Plan

• Describe the concepts of activity and decay constant, including their units.
• Explain the relationship A = λN and how it follows from exponential decay.
• Apply A = λN to calculate activity or the number of undecayed nuclei in given situations.
• Convert between decay constant and half‑life using t½ = ln 2 / λ.
• Interpret and solve typical A‑level radioactivity problems while checking units.

• Projector and screen
• Whiteboard and markers
• Printed worksheet with sample calculations
• Scientific calculators (or calculator app)
• Handout of key equations and unit table
• Diagram of a decay chain (displayed on screen)
• Clicker/online quiz tool for exit ticket

Begin with a quick poll: “If a sample’s half‑life is short, how ‘active’ do you think it is?” Connect this to prior knowledge of exponential decay and half‑life, then state that today’s success criteria are to define activity and decay constant, use A = λN, and solve related problems.

1. Do‑now (5 '): Students answer a short question on half‑life vs. activity on a sticky‑note.
2. Mini‑lecture (10 '): Define activity and decay constant, present A = λN and derive it from N(t) = N₀e⁻ˡᵗ.
3. Guided example (10 '): Work through Sample Calculation 1 together, emphasising unit handling.
4. Pair work (10 '): Students solve Sample Calculation 2 and a conversion between λ and t½, checking answers with a partner.
5. Class discussion (5 '): Highlight common misconceptions (e.g., sign of dN/dt) and clarify.
6. Exit ticket (5 '): Quick online quiz: one problem requiring A = λN and one conceptual question.

Summarise that activity is directly proportional to both λ and N, and that half‑life and λ are inversely related. Collect the exit tickets to gauge understanding, and assign homework: complete the additional worksheet problems involving A = λN and half‑life conversions.