Webinar: Modelling Electric Current

22 August 2016
Tuesday 23 August, 8pm

Here’s the second of our 2016/17 webinar series:  Modelling Electric Current.

This session is run by our very own Dan Cottle, IOP Physics Network Coordinator in Birmingham.

Add your questions and comments below for your free copy of Physics Education on modelling electricity.

Got an idea for a physics webinar?  Let us know – and you could host the next one. 

PS – TalkPhysics webinars are © IOP and can be freely used in schools.  Please do get in touch if you’d like to modify or use them for commercial purposes.

  1. Mariana Salgueiro 3 years ago

    And if you would like to find out about how I put this circuit together or how I made the dough I will be writing a post which should be up by early October!

  2. Dan Cottle 3 years ago

    Hello everyone.
    Looking forward to discussions this evening!

  3. David Cotton 3 years ago

    Hi Dan, I remember your physics of fruit and sweets video, and am really excited to see what you have in store (that is energy speak) this time!

  4. Alessio Bernardelli 3 years ago

    Hi Dan,
    Looking forward to the discussions and to see the video.

  5. David Cotton 3 years ago

    Thanks Dan, a fine model. I like the use of the weights as they as you can, like you did show them moving with the collisions when necessary for explanation.

  6. One disadvantage of your model is that it ignores the relatively low electron drift velocity compared with the velocities of the electrons.

    • Alessio Bernardelli 3 years ago

      Interesting point. I had the reverse thought 🙂 as the marbles all move overall in the same direction down the ramp (which makes me think of the drift velocity of the free electrons). This also happens when the p.d. is applied, again as a good illustration of the drift movement of the electrons. What I thought the model could not show was the random movement of the free electrons, otherwise we should see them moving in random fashion even when the p.d. is zero, but with an overall zero net velocity.

  7. Dan Cottle 3 years ago

    If you have two sets of 100g masses with different diameters, you can show the difference it makes when the atoms vibrate and get hot.
    Just run the same number of marbles down with the small ones, then repeat with the large ones and time both. It takes longer with the large ones showing that they give a higher resistance.

  8. David Cotton 3 years ago

    Here is a similar version being made in some IOP SPN make and take workshops.


    I really like making comparisons between pd and height in the gravitational field case

  9. Aine Woods 3 years ago

    Thanks Dan – I think the webinar series is a great way of sharing ideas! I really like the idea of getting the students to compare this model to others and to encourage discussion about the limitations of different models. Please can I also get a copy of the Physics Education on modelling.

  10. Paul Bishop 3 years ago

    Does the model work to show the effect of wire thickness on current flow?

    • Jon Scaife 3 years ago

      Complicated question Paul. If there were two models side by side that were identical except for wire ‘width’ then the number of marbles arriving at the end per minute should depend on the width. But if a chicane were introduced into a single demo the ‘current’ should remain unchanged throughout the length of the ‘wire’, though the speed of marbles should, in theory, increase in the chicane. It would be interesting to see whether this could actually be modelled. Maybe Dan has done this.

    • David Cotton 3 years ago

      You could show how increasing current (more marbles) chenges as it flows down the same dimension wire or I guess you could make different width tracks with the same number of marbles

      • Dan Cottle 3 years ago

        I do model ‘chicanes’ sometimes. Normally when I use it to show a diode.

        You make a one way pass by making the masses into a funnel shape with a small gap allowing marbles to pass through in one direction only.

  11. David Ferguson 3 years ago

    One idea that jumps out: Does the idea of tilting the track work as an analogue of hall voltage?
    Thoughts please.

  12. Dan Cottle 3 years ago

    Christine – yes I agree. One nice thing though is that as soon as the pd is applied, marbles immediately flow but a single electron obviously takes a much longer time to traverse the length of the track so it can stimulate a discussion with an able class about that.

  13. Sarah Stevens 3 years ago

    Hi, I’m interested to see how you might use this at A level. Especially drift velocity? I will use it for resistivity. Thanks for the idea 🙂

  14. Paul Nugent 3 years ago

    Well done Dan
    Most models have limitations-but not so many with what you have demonstrated

    looking forward to reading more

    • Dan Cottle 3 years ago

      Thanks! – I do think the students need to be pretty secure in their understanding that electrons are everywhere in the wires before the current starts to flow and not do not ‘start’ in the battery though.

  15. Anna Rothwell 3 years ago

    This model is fantastic, I can see so many discussions taking place with my classes. I am in school tomorrow and can’t wait to build one. Thank you for sharing this idea. I would like a copy of the paper please.

  16. Jon Scaife 3 years ago

    Nice demo. A modification might be to drill holes in the board (or use pegboard) and use something that bends a bit – springs maybe – in place of the masses. (I have vaguely in mind some kind of old pub bar game!) This would illustrate transformation of energy in the system: work done by the ‘battery’ into internal energy in the ‘wire’.

  17. Dan Cottle 3 years ago

    Aine – thanks! Hope you enjoy reading the paper.

    David – it what way?

    Paul – yes for wire thickness. You can blutack meter rulers down the length of the track in the middle to halve the thickness or set up identical runway on one side to double.

  18. David Cotton 3 years ago

    @christineabdelmoutaleb Like all good models the discussion with students is often where the good ideas take shape. I agree with you that it is important to point out the idea of the drift of electrons. This after all was the way Drude calculated conductivities with a model based on a gas of free electrons flowing through the positive metallic ions.

  19. Dan Cottle 3 years ago

    Sarah – drift velocity I think is hard because there is actually only a very small net flow of electrons giving rise to the current. A lot of the motion of the electrons inside the metal would still be considered to be random in direction. This model is a simplification – as are all models!

    Jon – yes that would be a permanent solution. I choose to do it in the way shown because I often get the students to build it with me which I think helps to cement the memory of it for them.

  20. David Cotton 3 years ago

    @sarahstevens You can also link to A level gas laws when expanding on the Drude model.
    Also a good model for explaining how increasing pd increases current flow which increases temperature and hence resistance.
    This could be done like Dan’s wiggling of the weights from KE of electrons(marbles)

  21. Dan Cottle 3 years ago

    David – yes I think it is really interesting that essentially applying the model of an ideal gas to the electrons in a wire can result in a way of thinking about current that can predict what actually happens because of course electrons are NOT an ideal gas…

    Because of this though the model works especially well if you can make the size difference between the ‘electrons’ and the ‘ions’ as large as possible. I just use marbles and masses because they are convenient. In the paper results are compared from using very small ball bearings with the same size masses. I was thinking that a further investigation into this might suit an interested 6th form student with an EPQ to write…

    • David Cotton 3 years ago

      What an excellent idea for an EPQ. There is real scope to investigate how the model’s flaws are things like the interaction between electrons and the need for a different statistical model (Fermi-Dirac)

      • Dan Cottle 3 years ago

        Yes – it goes off from the curriculum in a way that would allow the student to do some higher level research and also bring their own ideas in.

        Just waiting for the right student to come along!

  22. Carol Hyatt 3 years ago

    Thankyou for sharing, it is a great idea which would allow me to build on from children walking round a class being electrons bumping into carefully placed ion chairs to the same children measuring the height of the track, measuring the number of marbles passing a given point per second, (photographic evidence! ) and drawing graphs. I like this model the best of all the ones I have seen. It explains the basics and has the potential to do so much more! 🙂

  23. Paul Bishop 3 years ago

    Thank you Dan. In order show different resistivity/conductivity would I change the size or number of masses?

    • David Cotton 3 years ago

      I would initially say that for the students we are targeting that it is all about the space in between the ions (masses) that is important.
      This is because we are going to show how heating and hence jostling of the masses increases resistance

    • Dan Cottle 3 years ago

      I think changing the structure works quite well. This is because the ‘size’ of the ions of similar metals does not change that much compared to the size difference between the ion and an electron. Students could think about any types of metallic structure they may have learned in Chemistry e.g. FCC, BCC etc. They could then think about the way that ions can be arranged closer packed or more loosely packed and the effect this will likely have on the current flow. Arranging the masses on the track in different regular patterns simulates this quite well.

      The number of electrons per ion (or marbles per mass) gives the most effect in the model though (unless you really cram the masses together quite tightly) e.g. 1 per mass, 2 per mass or 3 per mass.

      • Keith Jones 3 years ago

        It’s a nice demo Dan. thanks. I always struggle to relate free electron density to conductivity (the numbers for e.g. copper and aluminium don’t always make sense; bigger n doesn’t necessarily mean bigger conductivity)

  24. Dan Cottle 3 years ago

    For me it is all about the discussion as many have also stated!

    Another useful way to structure the discussion is to have a long piece of think bare wire e.g. nichrome, set up for the students to see alongside the trolley track. Using a sturdy lab power pack to apply a pd to this making a large current flow causes it to heat up and glow red. The students can be challenged to explain what is happening in terms of the model.

    You could do a before and after type activity. Starter activity to explain what they see in the real wire before they see the model and then reflect on it after the model to see how it helps them to think about what is happening.

  25. Melissa Finlay 3 years ago

    This is great! Thank you for sharing. I will definitely be using this, along with the rope model too. It makes the more difficult concepts so clear. I like the above idea of using springs too as it could show the transfer of kinetic energy.
    Thanks again, and please could I request a copy of the paper?

  26. Carol Hyatt 3 years ago

    I have just looked at the you tube video, a great idea for STEM club!

    • Dan Cottle 3 years ago

      Yes it is – you could combine it with some burning of wire wool by passing a current through for example. Is it just my STEM club kids who seem to think it is not science unless there is fire involved….!

  27. David Cotton 3 years ago

    Here is something you can build to accompany this model
    It is a model light bulb based on a pencil lead


  28. David Cotton 3 years ago

    I will take some pictures and post later.
    It is just two crocodile clips holding a pencil lead in an inverted glass that is in water.
    The initial heat burns off the oxygen, then it lights for a while. Once the oxygen is burnt off no new arrives as the whole thing is in water at the base.
    I can’t quite remember pd, but it was on a 0 – 12 Volt power pack

  29. David Cotton 3 years ago

    Resistance wire behind thermochromic paper is a nice way to show heating effects of current


  30. Dan Cottle 3 years ago

    Modelling energy transfer in A.C. circuit is also possible.

    Two students, one holds each side of the track and they slowly lift each end up and down without letting any marbles drop off the ends. Collisions occur and there is resistance, energy is transferred but no electron moves off the track.

  31. David Ferguson 3 years ago

    Another idea albeit a little limited:
    pivot the track at the centre and this should be able to show a.c.

  32. David Cotton 3 years ago

    Excellent idea Dan/David great minds think alike.
    I am never sure students really understand the backwards and forwards of AC.
    They will now!

  33. Carol Hyatt 3 years ago

    Many thanks! I love that students can benefit from people that know more than I do, that you save me valuable time, sharing things that work and that I can watch the Webinairs at a time to suit me .(And go back and look at things again when I have forgotten them!) A great learning resource. Many thanks again.

  34. Robert Bell 3 years ago

    Some excellent ideas. I agree that the class discussion/plenary is very important here. Perhaps suitable lengths of high carbon spring wire fixed to a second board underneath and passed centrally through large holes in the top board to the masses could produce suitable vibrations. I certainly think that it would work with GCSE students and you would get a lot of mileage out of the discussion. It is interesting how a suitable and well thought out model can bring the Physics to life. A good exercise is to mount different circuits of lamps suspended between bus-bars on an inclined surface to simulate pd. The inclined surface seems to bring the idea of current flow to life and students will immediately begin to recognise why the current flow changes downhill as the paths split through different branches.

  35. Jill Noakes 3 years ago

    I do like this for exploring factors affecting resistance and resistivity. Nicely adaptable for different groups too. Thanks Dan.

  36. Alessio Bernardelli 3 years ago

    Excellent work Dan. Highlights of this model for me are:

    – p.d. modelled as lifting the ramp and analogy to gravitational potential – this sort of thing and ways of comparing similar phenomena from different topics is very relevant to the new GCSE specifications and the idea that where there are differences in a system there is the potential for a change to occur. So, when the conditions are right (e.g. you lift the ramp high enough) the change will happen (energy discussion is also relevant here)

    – All electrons start moving along the ramp when the p.d. is applied – really important point that students need to grasp (as you point out in the video)

    – Simple and clear model, very visual and with plenty of opportunity for adaptation – the summary table at the end that spelled out what each part of the model represented was excellent and just what learners need, so really great idea to model that for the teachers watching

    Thanks for another superb video and an even greater step up from your last one, which was also awesome. How will we ever compete with this one now 😉

  37. Heather Evans 3 years ago

    As many people have already mentioned, the model will be a powerful vehicle for discussion. It is even better because of its limitations; students will learn more by identifying these than if it was a “perfect” model. In my experience, the concept of potential difference is much harder to get across than that of current or resistance. Do you have any suggestions regarding how to explain that (after demonstrating that the raising of the ramp is the model’s way of creating a p.d.)? Thanks

  38. Robert Essex 3 years ago

    A copy of the paper for me too please. Thanks.

  39. Matt Perks 3 years ago

    This is an excellent teaching model, thank you. With all models, I think the critical thing as a teacher is to be aware of limitations – a point made very clearly – and to make sure that children are able to make the links between the model and the real thing. The matching list on the board in the video is critical to the success of the model as a teaching tool and I would suggest that most classes will benefit from either producing that list from discussion, or from a matching exercise (card sort, or matching a mixed up list).

    I think this model shows really well:
    the relationship between p.d. and current, and will help children with understanding p.d.
    the presence of free electrons throughout the wire and therefore the (near) instantaneous current through the whole wire
    heating of the metal lattice due to the current

    I think this model may be quite good for:
    the relationship between the cross-sectional area of the wire and its resistance (but care is needed because as the width of the track is changed the number of marbles needs to change in proportion – perhaps running two tracks side-by-side and doubling cross-sectional area is best)
    the relationship between length and resistance (but changing the length of the track changes it’s angle if p.d. is the same; this is correctly modelling reality but I think a lot of children will feel that changing the angle is changing the p.d. I think this will need very carefully structured explanation/questioning for most classes).

    I think pitfalls to be aware of are:
    Obviously, the electrons not being continuously replaced. I think this just needs pointing out to children as a limitation of the model.
    The can of worms that is energy. At a macro level there is no electrical store of energy, just an electrical pathway, so having a kinetic store of energy in the electrons could be a problematical idea. It comes out in the wash I think because transfer between energy stores works at the micro level but becomes a transfer pathway at the macro level. This is just that idea of start and end points – you can only sensibly have a start and end point before and after a single electron-lattice interaction at a micro level – but almost all of us are still vulnerable on energy ideas I think.
    The lack of random motion of the electrons before (and during) the current flow. Those electrons have average speeds of 10^6 m/s both with and without a current and a drift velocity of less than 1 mm/s when the p.d. is applied.
    If you try to show a change in resistance due to a change in material (like a filament lamp or resistor in a circuit) by changing the width of the track or the arrangement of the masses, the electrons will either slow down, or keep to the same speed. In reality, drift velocity is higher if a change in material causes a change in resistance because the number of free electrons is lower. I don’t think the model can show this correctly.

    Finally, in the past I have preferred to avoid the idea of hotter ions moving more and ‘getting in the way’ more since (and I feel the lack of a physics degree here) I think there is a complicated situation, with the average speed of the random motion of the electrons increasing, the available energy levels of the ions changing to make collisions more likely, and the behaviour of the lattice changing too. Maybe, though, it’s just another model with the complicated reality well-represented by the simpler idea of the ions ‘getting in the way’ more, which would be larger masses in the model in the video. Thank you for making me think about this again.

  40. Gerry Blake 3 years ago

    There are various good electric circuit models, such as the rope (force) model and the ball (gravity) model that Dan has shown. I also like the straw (pressure) model.
    Blow into the straw harder for more pd. Pinch the straw or tape two straws in series for more resistance.
    Pressure difference (p.d.) is analogous to voltage. Pa = N/m2 = Nm/m3 = J/m3. The p.d. makes the air a pathway for transferring energy.
    The volume of air flowing through per second is analogous to current.
    Power (= p.d. x flow rate). J/m3 x m3/s = J/s = W.

  41. Philippa Wallington 3 years ago

    Thanks for sharing it is a great starter for a level. I also think you may be able to link it to I=nAve and have two different thicknesses of tracks in series to get some key points across. Please could I have paper too?

  42. Keivan Sarrafan 3 years ago

    I would appreciate a copy of the paper. Thanks.

  43. Ruth Brett 3 years ago

    Looks great. I’m going to give this a go if I can! Would also appreciate a copy of the paper, thanks.

  44. Thomas Chapman 3 years ago

    Thanks Dan. Model complements one that can be made to simply model electric circuit that was recently demonstrated by Richard Grimmer, also a network coordinator. His model was built using ‘coroplast’ sheets to build a circuit and also used marbles as free electrons and gravitational potential energy for pd.

  45. Angus Gregson 3 years ago

    Best thing about this model compared to others is, as you alluded to, that at heart it explains a circuit working as electrons transferring their potential energy to other forms. Models that don’t start with this inevitably hit the buffers as you push them further. Younger students (I’ve tried it from Y5 up) don’t have a problem with the idea of gravity acting by a field – “things” feel a force – and didn’t seem to have difficulty transferring that idea to a circuit. Maybe we should introduce circuit work through electric fields? I did this with my Y7 this year and their feel for circuits was a lot better than usual.
    Extensions? Blu-tak marbles to masses (and suggest araldite would be better) = insulators. Thin sliver of blu-tak to attach marbles = semiconductors – electrons don’t flow initially as you apply pd but give the whole thing some energy (shake it = heating) and more charge carriers are released to flow.

  46. Chenai Gwenzi 3 years ago

    Thanks for sharing Dan. I now have a great starting point for my A level students. Can I have a paper copy please.

  47. Jo Hands 3 years ago

    HI – Thanks for sharing – great for my GCSE pupils. Please can I have a copy of the modelling electricity article? Also – did you show the potato and straw activity at the IOP event in June this year at Exeter? If so, please can you send me some info on that – ie the physics behind it etc? Many thanks

  48. Peter Sharp 3 years ago

    Hi – thanks for the ideas Dan. Just an idea on diodes – might a step not be better? Possibly a good puzzle for older students would be adding components into the model….

  49. Allysmith 3 years ago

    HI. Fantastic video and explanations. Would like to use this with my HNC/D Electrical Engineering classes 🙂

    Thanks a lot.


  50. Martyn Long 3 years ago

    Hi all – sorry II am really late to this discussion but watching this has only just got to the top of my to do list. Pleased I did it is a great model and will be really useful; for A level discussion especially. Something I have really struggled to explain is the increasing length increasing resistance (from the point of view of more ions in the way overall but there are more de-localised electrons in a longer wire and the number density of charge carriers is the same throughout the wire so each section of wire has the same ratio of conduction electrons to obstacles?). With this model if you populate the runway with the de-localised electrons throughout at the start and then lift it and count the number of electrons arriving per second the average number arriving per second will go down as the runway gets longer. The electron number density is the same and hence average number of marbles per cm length of track is the same but on average less marbles will pass each point per second because of the reduced gradient? Is is analogous to the strength of the ‘electric field’ decreasing even though the pd is the same because of the longer length? The volts per meter is less? – just like parallel plate fields but applied to a wire?

    Am I making any sense???


    Martyn Long

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