Three Identical Bulbs, One Ammeter, and 24 Students Who Already 'Know' the Answer

The model behind the prediction. Students are running a 'sequential consumption' model: current is treated as a substance carried by the wire that gets deposited in each component, like fuel being used. It is a rational generalisation from everyday language ('using up the battery') and from fairy lights where a break anywhere kills the whole chain.

Why the demonstration fails on its own:

• The evidence is perceptually ambiguous. Three incandescent bulbs can differ slightly in manufacture, and room lighting varies across the board. 'The first still looks brighter' is a rationalisation the evidence cannot block.
• The students already predicted before observing, and nothing recorded that prediction. Without a written commitment they can quietly adjust and feel no conflict.
• Their prior belief is well-rehearsed and observation is weak and single-shot; under those conditions prior belief wins - this is the standard outcome, not a sign of a weak class.

What is actually needed: a test that is a number, not a judgement of brightness, and a situation where the consumption model makes a firm wrong prediction the students themselves have stated.

1 (3 min) - Commit the prediction in writing. A three-column table on the board: I before bulb 1 / I between 1 and 2 / I after bulb 3. Every student writes their predicted values in their notebook. This is what was missing last lesson.

2 (4 min) - Measure in three places. Insert the ammeter before bulb 1, then between 1 and 2, then after bulb 3. Students read the same value each time and record it. Then physically move the battery to the other end of the loop and repeat: readings and brightness are unchanged, which disposes of 'position matters'.

3 (3 min) - The removal test. Unscrew bulb 2 and replace it with a plain wire. The remaining two bulbs brighten, and the ammeter reading rises.

Elicit the inference and write it on the board: if current were used up, removing a bulb could not make the first bulb brighter - but it does. The consumption model predicts no change; the observation contradicts it. The refutation comes from their own prediction, not from the teacher's assertion.

4 (5 min) - Exit ticket. One sentence: what does the ammeter say about current at three points, and what did the removal test prove?

Note on the correct model at this level: charge is not consumed; current is the same at every point in a series loop, while energy per charge falls across each bulb.

Item. A 6 V battery, three identical bulbs and an ammeter are connected in series. The ammeter placed between the battery and bulb 1 reads 0.3 A. (a) What will an ammeter read between bulb 2 and bulb 3? Give a reason. (1 + 1 marks) (b) Bulb 2 is removed and replaced by a plain wire. State what happens to the brightness of bulb 1 and to the ammeter reading, and explain why. (1 mark)

Marking. (a) 0.3 A, with a reason referring to the same path / no charge being lost. (b) Bulb 1 becomes brighter and the reading rises; total resistance has fallen.

What the responses reveal.

• Misconception held: (a) a smaller value, or 'almost 0.3 A because some is used before bulb 3'; (b) either 'no change - the first bulb already got its share' or 'bulb 1 goes dimmer because more bulbs means more current'. Both are the consumption model speaking.
• Model correct: 0.3 A in (a) with a path-based reason; brightening plus a rising reading in (b).

Why part (b) is essential. Part (a) alone can be answered by pattern-matching 'series = same current' from a formula sheet. Part (b) requires a causal model, and the misconception makes a firm, opposite prediction.