HOWTO: The Rope Model of Electricity

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Thomas W-P
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HOWTO: The Rope Model of Electricity

Post by Thomas W-P » Tue Dec 18, 2018 9:51 pm

In the next podcast we inevitably talk about the rope model. I tried it a few times in the past and hated it 😝. It was only in making the podcast that I finally understood how to do it and how good it is. Not knowing how to do it is as much a function of my comfort with the donation model as it is my being alone in my school with no other physicist to talk to. But talking to Robin and Stuart about electricity really got me thinking.

When I did the rope model it didn’t work because I had quite a few kids involved in the demo. I found it really hard to make the rope run smoothly through their hands (not least because the rope had a huge knot in it!) and it was a very static sort of demo. There was no fluidity at all, no flow. I had visions of attaching pulleys to the walls of the room to make it work (I like big scale). I chatted to Stuart about this and he was able to tell me a way to do it that worked. It works so well, and is so easy to do. I was bowled over.

My recipe for an effective rope model demo:
  • Have a rope that is in a 3-4m loop where the join is as smooth as possible. (Cutting and melting together a rope is ideal).
  • Choose one student only.
  • Hold the loop and pass the other end to the student. Tell them to grip it lightly (they don’t want rope burns) in one hand with the rope passing vertically down through their fist (this is the detail I never understood - one student, one hand).
  • Explain that the rope is the electrons and the grip is the resistance.
  • Pull the rope hand over hand through their (one, stationary) hand.
    • Question: What do you feel in your hand?
    • Answer you want: Warmer
  • Tell them to add another hand, holding with the same light grip. The rope will get harder to pull, make this obvious, it will naturally run more slowly.
    • Question: why is the rope moving more slowly?
    • Answer you want: because there is more resistance.
    • Question: how can I increase the current to make it move as fast as before?
    • Answer you want: Pull harder.
    • Explain that this means more energy being delivered by the cell to the rope, or greater pd.
  • Pull harder, to make the rope move at the original speed again.
  • Tell them to grip tighter with one hand, but not to tell you which hand that is.
    • Question: what has happened to the rope speed?
    • Answer you want: slower.
    • Question: does the rope/the pd know which hand is gripping harder?
    • Answer you want: no.
    • Question: what is happening at the tighter hand?
    • Answer you want: warmer.
    • Explain how this is energy transfer
    • Question: are the electrons in any way different before and after the hands?
    • Answer you want: no.
    • Remind them that electrons just go where they are pushed/pulled by the pd, they aren’t changed, they don’t make a choice, they just go where they go.
  • Get another rope, get them to hold one loop in each hand, but you pull them together. You can model parallel with this but I wouldn’t go too far as the model does tend to break down a bit.
  • You can show AC nicely too. Remove one loop then tell them to hold their fist horizontally, not vertically. Now grip the loop in both your hands and pull it backwards and forwards. The hand gets warm just the same.

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