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Using The Rope Model of Electricity

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:

  1. 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).
  2. Choose one student only.
  3. 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).
  4. Explain that the rope is the electrons and the grip is the resistance.
  5. Pull the rope hand over hand through their (one, stationary) hand.
    • Question: What do you feel in your hand? 
    • Answer you want: Warmer
  6. 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.
  7. Pull harder, to make the rope move at the original speed again.
  8. Tell them to grip tighter with one hand, but not to tell you which hand that is.
    1. Question: what has happened to the rope speed?
    2. Answer you want: slower.
    3. Question: does the rope/the pd know which hand is gripping harder?
    4. Answer you want: no.
    5. Question: what is happening at the tighter hand?
    6. Answer you want: warmer.
    7. Explain how this is energy transfer
    8. Question: are the electrons in any way different before and after the hands?
    9. Answer you want: no.
    10. 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.
  9. 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.
  10. 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.

Good luck with this. Maybe report back in the forum?

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Somebody Out There Likes Us

Robin and I started with very limited ambitions, it wasn’t quite “more listeners than presenters” but not far off. We agreed to do episodes until the end of the academic year and then see where we stood. At that point we had not got much further than me telling Robin he and I should start a podcast and then months later him finding the statistic that about 50% of schools in the UK have one or zero physics teachers. We just wanted to try something, try to build a community and see what happened. I am a regular listener of The Cycling Podcast and modelled a lot of our approach on them.

As we started I had no idea about format, content, approach, social media, iTunes ratings, post production, how to talk to a microphone and had never considered a T-shirt or shop (or adding a forum) But I am top loaded with running web sites and general geekery, so I figured I would muddle through.

It was a revelation to me (physics teacher, but never an audiophile) to find that a microphone has preferred orientation. Listening back you can tell the episode where I worked out where to point my mouth when recording.  I found a great T-shirt supplier when looking for somewhere to buy a jokey shirt for Robin. They actually make the shirts on demand and provide a WordPress plugin, so one thing led to another and now… a shop! Just a bit of fun of course, as Robin says in Episode 9, this is nothing about money… But if it helps build a community then that’s brilliant.

As I went to sleep on the night before episode 1 I was not convinced I wouldn’t grab my phone at 4am and cancel it. Now, as we approach the recording of episode 10 I feel more comfortable hearing my own voice. I’ve also been dragged kicking and screaming in to social media and I feel a community is growing. (We have had a few emails thanking us for doing it and the interaction on Twitter has been fun). It has also been hugely enjoyable talking to different people and learning from them: I recorded another PIM this week which will most likely be in Episode 11. Please do continue to send us ideas. We love hearing them all. You can use the form below.

Finally, to get some 5* ratings in iTunes is pretty stunning to be honest, so thank you very much, listener(s!).

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Refutation Text for the Jelly Chair

I said in Episode 6 I would share what I came up with for the Jelly Chair lesson and refutation texts. I did put some thought in to this but think that there is room for improvement. I ended up writing a few sentences for the kids to do:

  • The ancient Greeks thought that arrows were pushed through the air by the god Apollo. We know this is wrong because…
  • Many people think that objects will slow down and come to rest if no resultant force acts upon them but really…
  • A typical GCSE level understanding of balanced forces allows a person to sit on a chair made of jelly. This is false because…
  • Galileo showed that two balls of different weights accelerate at the same rate when dropped.. Before this people thought heavier objects fell faster but this is wrong because…
  • Some people think the International Space Station has no forces acting upon it because it is in Space, the evidence that they are wrong is…
  • When a helicopter is hovering it is very common for people to think that the weight of the helicopter has an equal and opposite force that is the down-force from the rotors. This is wrong because…

I was particularly happy with the last one (though I have no idea if that is an actual misconception or not), and even happier when one of the students said something like:

The down-force is equal to the weight!

They had truly understood the situation, and was able to explain to their partner that the weight and down-force must be equal in magnitude and in the same direction as weight if the helicopter is hovering (not equal and opposite).

As I roamed the class and spoke to the students I was pretty confident the refutation sentences were having impact. The challenge is to use them again and again and embed them in my practice.

The podcast strikes again. Thank you Ben Rogers!

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