| Lesson Plan |
| Grade: |
Date: 25/02/2026 |
| Subject: Physics |
| Lesson Topic: understand the effects of the internal resistance of a source of e.m.f. on the terminal potential difference |
Learning Objective/s:
- Describe how internal resistance influences the terminal potential difference of a source.
- Derive and interpret the linear relation V = ℰ – rI, identifying slope and intercept.
- Determine the internal resistance experimentally by plotting V against I.
- Analyse common sources of error and propose methods to minimise them.
- Apply the technique to calculate ℰ and r from given data sets.
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Materials Needed:
- Cell or battery with unknown internal resistance
- Variable resistor or set of known resistors (load R)
- Ammeter (low internal resistance)
- Voltmeter (high internal resistance)
- Connecting wires and crocodile clips
- Graph paper or spreadsheet software for V‑I plot
- Worksheet with data table and analysis questions
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Introduction:
Begin with a quick demonstration: connect a battery to a lamp and ask why the brightness changes when additional devices are added. Recall Ohm’s law and the concepts of e.m.f. and internal resistance from previous lessons. Today students will investigate how internal resistance affects terminal voltage and will learn to determine r experimentally.
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Lesson Structure:
- Do‑now (5') – Short problem on terminal voltage with a given r; discuss answer.
- Mini‑lecture (10') – Present V = ℰ – rI, explain slope = –r and intercept = ℰ, highlight error sources.
- Practical set‑up (15') – In groups assemble the circuit, vary R, record V and I for several load values.
- Data analysis (10') – Plot V versus I, determine gradient and intercept, calculate r and ℰ.
- Concept check (5') – Quick quiz (Kahoot or handout) on interpreting the V‑I graph and errors.
- Extension discussion (5') – How temperature, state of charge, or age affect internal resistance; link to further investigation ideas.
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Conclusion:
Recap that terminal voltage decreases linearly with current, the slope of the V‑I graph gives the internal resistance and the intercept gives the e.m.f. Students complete an exit ticket stating one practical implication of internal resistance. For homework they design a brief experiment to test how temperature influences r.
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