The Bulb That Uses Up Current
Class XII Physics, 37 students in a government school in Hisar. After a unit on current electricity, a 20-item test showed 74% correct on items requiring formula substitution such as V = IR and P = VI, but only 26% on conceptual items: why a bulb lights instantly when switched on, why the filament glows but the connecting wires do not, and why an ammeter is connected in series and has low resistance. In short interviews, 21 students said current is used up in a bulb so less returns to the battery; 14 said the bulb lights because electrons from the switch reach it. The laboratory has four multimeters, six bulbs and 40-minute periods. The teacher has five periods before the practical examination.
1. Diagnose the misconception and explain why formula success can coexist with conceptual failure.
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The pattern is a case of formula success without a mental model. Students have learned to match V = IR to numbers, but 21 of 37 explain a bulb as consuming current, and 14 think the switch sends electrons to the bulb. Both are substance or local models: current is treated like water used up, and the circuit is not seen as a closed loop with a source field. Formula substitution can be done by pattern recognition, finding the two known quantities and choosing the equation, without any model of what current, potential difference or resistance are. The filament item exposes this: if current is used up, the wire after the bulb should have less current, which contradicts an ammeter reading. The teacher should not simply re-explain; students need to be confronted with the contradiction and then build the energy-transfer model in which power is dissipated in the filament, not current consumed.
2. Design a predict-observe-explain sequence using the available equipment to confront and revise the misconception.
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Use a predict-observe-explain cycle with the four multimeters and six bulbs. Set up a series circuit with two identical bulbs and two ammeters, one before and one after the first bulb. Ask students to predict the two readings and the brightness. Many will predict a smaller reading after the bulb. Observe: the readings are equal and both bulbs are equally bright. This creates conflict. Then measure voltage across each bulb and the battery; students see that energy is transferred, not current used. Rotate four stations so that 37 students use four multimeters: station one for series bulbs, station two for filament versus wire using a low-power bulb, station three for ammeter and voltmeter placement, and station four for a short prediction quiz. Each station has a role sheet and a recorder. Finish with a whole-class explanation and a bridging question about what changes in a parallel circuit.
3. Propose an assessment that tests model-based understanding rather than only numerical substitution.
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Assess the model, not only the calculation. Use two-tier items: the first tier asks for the answer, the second asks for the reason, for example current is the same because charge is conserved in a series loop versus current is used up. Include a circuit diagram with ammeters in two positions and ask students to compare readings and justify. Add a transfer item: predict what happens to brightness when a second bulb is added in series, and explain using energy. Use a short oral interview for eight to ten students who scored high on formulas but low on concepts; this reveals whether the model has changed. A rubric can score correct prediction, accurate observation record and a written explanation that mentions conservation of charge and energy transfer. Avoid grading only the numerical answer, otherwise the same pattern will persist.