Adding a second identical bulb to a simple circuit, in series
Wrong Track: The second bulb will be brighter than the first because the current gets to it first after leaving the cell and so more energy will be shifted there.
Right Lines: The two bulbs will be of equal brightness because the current is the same through both of them and the cell potential difference is shared equally.
Thinking about the learning
The key insight here is that, when the second bulb is added to the loop, the current is reduced (and the charged particles move more slowly) in all parts of the circuit. This reduced current is the same in both bulbs and an identical potential difference is set up across each, with the bulbs being of equal brightness.
Thinking about the teaching
We suggest two approaches: the rope loop; and illustrate with numbers.
The rope loop model is very effective in helping to talk and think through what happens when the second bulb is added:
Instead of one person gripping the rope we now have two.
With the same pull/push from the
cell, the rope circulates more slowly due to the extra resistance.
The two people gripping the rope feel the same heating effect in their hands.
This heating effect felt by each person is less than when there was just one person gripping the rope.
Supposing the cell potential difference is 12 volt.
With one bulb, the full 12 volt potential difference is across the bulb: in other words, 12 joule of energy are shifted for each coulomb of charge passing.
With two bulbs, there is a 6 volt potential difference across each bulb: in other words, 6 joule of energy are shifted for each coulomb of charge passing.