Putting together thinking about two lamps
Thinking about the learning
The challenge for the pupils here is to be able to provide a full explanation as to why the two bulbs become equally dim:
The bulbs are now dimmer because adding the extra bulb increases the resistance and reduces the current everywhere in the whole circuit. As a result, less charge passes per second through each bulb, and each charged particle shifts less energy as it passes through each filament.
Thinking about the teaching
Once again (as in the previous episode), our advice is to start with observation and measurement (see what actually happens) and to account for these observations and measurements in terms of the electric circuit model and teaching model.
We advise this because sometimes pupils become quite fixed on the idea that the first bulb will
get all of the energy when two bulbs are connected in series. With this point in mind, we think that it is a good idea to start with practical observations and measurements. The fact of the matter is that the bulbs are equally dim. Selection and effective use of a good teaching model, together with consistent and careful modelling of reasoning with this model.
Expressed in terms of the electric circuit model:
Teacher: Fewer charged particles pass through a bulb each second, as the
push provided by the battery is the same, but the resistance present in the loop is greater. So less energy will be shifted by each bulb and, as a consequence, by the cell.