The energy is shared equally between the two bulbs
Teaching Guidance for 11-14
Two identical bulbs
Wrong Track: The first bulb must get all of the energy because the charges don't know that there is a second bulb.
Right Lines: Equal quantities of energy are shifted by both bulbs as the currents in both are identical, and the resistance of both are identical. The energy shifted by the battery is is equal to the energy shifted by both bulbs.
Using the rope loop model
Thinking about the learning
Problems can arise for those pupils who think of the charges starting out from the battery and shifting all of their energy in the first bulb. We'd suggest that you undermine this by not using any model that suggests that charges leave the battery having had energy donated to them.
Thinking about the teaching
The idea to get over here is that the battery is working equally on the charges in both bulbs. There is the same current in both and both have the same resistance. There is no
first bulb and
second bulb. As the energy shifted from the store of the battery is equal to the sum of the energies shifted at each bulb, and these energies are equal, then the energy shifted by each bulb will be half that shifted by the battery. But this conclusion is a consequence, not a determining rule.
Once again, the rope loop can be very helpful in talking through the idea that with two bulbs there is a simultaneous and equal shifting of energy at the two sites:
Teacher: So, with both Julia and Anita holding the rope, both of them can feel their hands warming up. In fact, if they both grip the rope equally they will be equally warmed.
Teacher: Equal grip leads to equal (slipping) frictional force on each. What else could it be? There is the same amount of rope going through both Julia's and Anita's hands. So with no differences between them, what other result could there be than this?
If you encounter particular reluctance, reinforce the idea by making the rope move in the opposite direction; the
position order in the circuit is not what is important.