Using models to teach circuits

In my previous post, I explained why models are key to teaching for understanding as part of the CPA approach. Here I go into more detail about how I use models when teaching pupils about electric circuits, and I will also explain why I use them with primary school pupils.

One big challenge with circuits is that it can be tricky to take the step from pupils describing their observations to explaining them — which can only be done by considering what’s happening at a submicroscopic level.

What complicates things further is that most people, including plenty of teachers, have never stopped to think about what’s actually happening in the wires of a series or parallel circuit. This is in part because it’s happening at such a tiny scale that we can’t observe it directly in the classroom — which is where models come in.

A major flaw with the English National Curriculum

The English primary science curriculum refers to voltage but not current, and it doesn’t mention electrons or charges. This makes it very hard to explain anything at all!

English primary curriculum expectations for Y6

I understand the rationale behind not going into too much depth in Y6. However when we don’t provide children with an explanation for their observations, I have found that most children start forming their own — and, since the scientific explanation is not intuitive, children’s own explanations can be quite far off the mark (Read the classic book ‘Pupil as Scientist?’ by Ros Driver for more on this). Alternative conceptions that can form at this stage include:

• Electricity comes out of both ends of a cell
• Cells create electricity
• Cells store electricity
• When you connect a circuit, the electricity near the cell flows first then pushes the electricity next to it, so it moves too
• Electricity leaves one side of the cell and collects at the other
• The bulbs and other components use up electricity

When we don’t explain what is going on in a circuit, the word ‘electricity’ becomes an umbrella term for current, charge, voltage, energy… pretty much everything important. And yet, as I explain to the pupils and teachers I work with, ‘electricity’ doesn’t actually exist — electricity is a topic, like ‘optics’. When they use the word ‘electricity’ I ask them to be specific and tell me what they are actually talking about!

And so I wanted to find a way to explain what is happening in a circuit to children at the end of primary school.

Prior knowledge required

It’s worth noting the prior knowledge that pupils need before being introduced to these models, and which I refer to in lessons using these models:

• The atomic model and states of matter: solid materials, like copper wires, are made of very tiny atoms that are connected together, which is why they hold their shape. These atoms are in turn made of even smaller particles — some of which have a property called ‘electric charge’. I use the Scale 2 website to give pupils an idea of how tiny these particles are.
• Conductors: in some materials there are charged particles that can move freely (in the case of a solid, they can move between the fixed particles, a bit like children running through an obstacle course). We call these materials ‘electrical conductors’.
• Free electrons: in a metal wire these freely moving charged particles are electrons. They all have the same (negative) charge and therefore repel each other, a bit like two poles of a magnet that are the same. This means these freely moving electrons tend to spread out evenly along a wire, so they can be as far from each other as possible. I find it helpful to ask a group of pupils to act the roles of the fixed and freely moving particles in a wire as I explain this — it helps to illustrate what I am saying.
• Insulators: in some materials there aren’t any particles that can move freely. We call these materials ‘electrical insulators’.
• Current: is the rate of flow of charge, i.e. it tells us about how fast the electrons are moving / how many there are.

My two favourite circuit models

1. ‘Rope in a tube’

I start with the ‘rope in a tube’ model (I made this after using the ‘rope model’ and finding some pupils got confused about what the rope represented — they often thought it was the wire rather than the charged particles — and finding that bits of the rope would often get caught / too tight / too loose and cause problems). I put the rope in a clear plastic tube, then cut a hole in the middle to allow pupils to ‘add components’ by looping the rope around their fingers (see photo below).

Photos of the ‘rope in a tube’ model and close-up showing how ‘components’ can be added

When introducing a model to pupils, I start by spending some time discussing what each part of the model represents. In this case:

• Tube = plastic around the wire (an insulator)
• Rope = electrons in the wire, negatively charged particles that can move freely through the wire
• Hand moving the rope = cell / battery
• Fingers around which rope is looped = component, such as a bulb
• (Air inside tube = solid particles of the metal wire, I usually wait to introduce this when critiquing the model, but sometimes a pupil suggests it during the introduction)

It’s still not perfect — no model is — but I’ve found it particularly helpful for showing:

• Flow of charged particles (movement of rope) in one direction when a cell / battery (your hand moving the rope) is connected
• The charged particles (rope) are already in the wire (tube) before you connect the cell / battery (hand moves the rope)
• All the charged particles move together when the cell / battery is connected
• There is very little resistance (friction) in the wire
• When you add a component you increase resistance (friction) and therefore the charges flow more slowly

I have found two major benefits of this model over others:

1. I can lay my physical model out on the table and build a real circuit alongside / sitting inside it (ideally this would be set up under a visualiser, and you would ask for a couple of pupil lab assistants to help with the demo). This helps pupils make the link between their observations and the model. During my explanation and questioning, I keep making connections between the model, the circuit, and what’s going on in the wires — and pointing out limitations with the model.
2. Pupils observe friction increasing significantly when you add a ‘component’, which helps them understand the idea of resistance, and it then makes sense to them that adding components would reduce the current / ‘flow’ and make the bulb dimmer. It’s also very memorable — I can refer back to “remember when we added a component, what did you see and feel?”

I’ve also made a parallel circuit version — it assumes an equal division of current in both branches, but I think would make a good starting point for KS3.

Phet Colorado circuit simulator

Next, I use the online circuit construction lab from educational website Phet Colorado. It looks like a circuit, which helps pupils relate it to what they see in the classroom, and it also allows you to show:

• The charged particles (blue dots) are already in the wire before you connect the cell / battery
• Charged particles flow in one direction when a cell / battery is connected
• All the charged particles move together and the bulb lights at once when the cell / battery is connected
• When you add more components this increases resistance and so reduces the current
• When you add more cells the current increases and so the bulb gets brighter.
• Current and voltage readings with a virtual ammeter and voltmeter.

Screenshot of two series circuits from the Phet Colorado circuit lab

Critiquing the models

However useful a model is, it always has limitations. And discussing these limitations is another way to build pupils’ understanding.

For instance, if we take our ‘rope in a tube’ model, the rope represents the electrons, but in reality they are not all connected — they are all individual electrons, spread out through the wire. The Phet Colorado model vastly underestimates the number of electrons, and makes them seem much bigger than they are. Neither model shows us the fixed metal atoms, among which the electrons ‘flow’. In reality, the path of the electrons is more like the ricocheting of a ball in a pinball machine, drifting in one direction but very indirectly — a revelation that most pupils find fascinating and surprising (and often leads them to ask how fast the electrons are moving!).

As a class, we come to agree that these are all simplifications we can deal with, for the sake of having a model that helps us understand what we observe. I end by asking pupils to let me know if they can think of a better model — which is an excellent way to probe their understanding even further…