Day 22: Circuit puzzles

In Intro Physics we’ve started working on circuits using a modified version of the CASTLE materials and students encountered a question about which bulb in a two bulbs in series circuit would light first. This led to a lot of interesting conversations—some students swore they saw one lightbulb light before the other. Then one student suggested we try lengthening the wire and see if we could then tell which bulb lights first. Since this was another good chance to try out the high speed settings on my phone, I went ahead and created these two videos.

My colleague took this even further—he has his kids assume that the electrical activity in the wires was moving at near the speed of light, and then had them calculate how much wire you would need for there to be a 1/240 s difference in time.


About John Burk

The ramblings of a physics teacher.
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2 Responses to Day 22: Circuit puzzles

  1. Lauren says:

    In my physics class this semester we started electricity and magnetism, and we learned that in a wire the current is formed from the moving charge carriers, but that drift velocity of the charge carriers is actually very small. The textbook (Serway and Jewett: Physics for Scientists and Engineers with Modern Physics, 9th edition) offered an explanation that the electric field set up by the potential difference travels through the conductor at near light speed, so all the electrons start moving right away, including those in the filament of the lightbulb, thus the light seems to turn on almost immediately. My question is that when your colleague’s students calculated how much wire would be needed to make a difference, would it matter if they just calculated that the electrons were moving at near light speed as compared to calculating and taking into consideration the drift speed of the electrons?

    • John Burk says:

      You are right. The actual electrons in the wire are moving very slowly (on the order of 1 m/s) while the “push” (potential difference) travels at near the speed of light. For the purposes of this class, and at this point in the study, we just wanted the students to see that whatever was moving in the wire was moving very quickly, so quickly that increasing the path difference by a factor of 1000 or so made no perceptible difference in the time of travel.

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