From one to two loops–adding a second identical bulb in parallel
Wrong Track: When the second bulb is added, there is extra resistance, the current goes down and the bulbs are both dimmer.
Right Lines: The two bulbs will be of equal, normal brightness because there is the same current in each loop (the same as in the original loop) and the potential difference across each lamp is equal to the potential difference across the cell.
Teaching responses to this challenge
Thinking about the learning
The key insight here is that, when the second bulb is added in parallel to the existing loop, this second loop provides an additional pathway along which charge can be set in motion and the total current in the cell is doubled.
Thinking about the teaching
We suggest two approaches: the rope loop; and illustrate with numbers.
Instead of one rope loop, we now have two, each containing a cell and a bulb.
With the same pull/push from the
cell the two rope loops circulate at the same rate as each loop has the same resistance.
Each person gripping each rope loop feels the same heating effect (rope circulating at same rate, with same slip force applied).
This heating effect felt by each person is the same as when there was just one person gripping the rope.
Suppose that the cell drives a current of 2 ampere in the original loop, containing a single bulb.
A second identical loop is added, again containing a single bulb.
The cell will drive a current of 2 ampere in this loop also.
The total current in the cell is now 4 ampere.