Day 144: What is the Bike Light Circuit?

Conceptual Physics: So far we’ve looked at what’s inside the bike light generator, how it produces an alternating current, and how a diode can be used to convert alternating current into direct current in order to recharge a battery. But we have not yet looked at the actual bike light circuit — how are the generator, headlight, and taillight wired together?

What’s confusing is that there’s just one wire that runs from the generator to the headlight (gray wire, pictured above) and another one that runs from the generator to the taillight (black wire, pictured above). This contradicts what we know about complete circuits. So where is the return wire from the light back to the generator?

To figure this out, we looked a disassembled headlight and taillight. I asked the students how we could wire one of the lights to a battery pack so we could “find” the return wire:


As you can see in the picture, the metal light mounting bracket is the return. On the bike, the light bracket is connected to the bike frame. The bike frame is “return wire” that connects back to the generator. The generator is also connected to the bike frame via a mounting bracket. Then we sketched the complete current-carrying path for the headlight and taillight:


We also noticed that when the headlight was disconnected from the generator, the taillight still lit up, and vice-versa. Students were then challenged to design and test (using PhET) a circuit with a battery and two bulbs that behaved the same way.

College-Prep Physics: Work day to tie up loose ends on WebAssign and lab work.

AP Physics C: Faraday’s Law problems.



About Frank Noschese

HS Physics Teacher constantly questioning my teaching.

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