Thinking about what they bring – some ideas to build on: some not

The key challenge of this episode is for the pupils to come to understand how electric circuits work in terms of objects and ideas (such as charge and energy) which they can neither see nor directly experience. The pupils need to be able to explain and model the working of electric circuits in terms of charge, current, energy and resistance.

As youngsters enter secondary school and study electricity, they are already using a wide range of electrical appliances confidently, and very often extremely competently. They take for granted that these things must be switched on, cost money to run, can work from batteries or from being plugged in, can be dangerous and so on. Through these experiences of using electrical equipment, and from their work at primary school, your pupils will have developed some basic ideas about how electrical appliances work.

We asked a group of 11-year-olds about their understandings of how electrical appliances and electric circuits work. What they had to say makes for interesting listening!

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From these video clips and from research that has been carried out more widely, it seems that youngsters of this age typically have the following kinds of ideas about electric circuits.

Right Lines: Complete circuit; no gaps; battery stores energy; electric current is a flow; charge travels; add a battery for brighter bulb; extra battery gives more energy; battery runs out of energy

Wrong Track: Battery stores electricity; electricity from both ends of the battery ; electricity used up; battery runs out of charges; battery runs out of electric currents; shorter connecting wire needs less electricity.

Some of these ideas are consistent with a sound model of electricity and we refer to these as being along the right lines: complete circuit; no gaps; battery stores energy; charge travels; electric current is a flow; add a battery for brighter bulb; extra battery gives more energy; battery runs out of energy.

Others are not consistent with this view and see the pupils going down the wrong track: battery stores electricity; electricity from both ends of the battery; electricity used up; battery runs out of charges; battery runs out of electric currents; shorter connecting wire needs less electricity.

One obvious point is that pupils do arrive at secondary school with understandings of electric circuits, which are along the right lines and can therefore be built upon in subsequent teaching. Next we consider, in a little more detail, some of the key right lines and wrong track pairs for electric circuits.

Try diagnostic questions

Our experience suggests that there is not a big variation in the views that youngsters of this age bring to their science lessons.

Nevertheless, it is a good idea to probe the understandings of your pupils at the start of any lesson sequence or module, and so we have provided three diagnostic (think again!) questions, which you might use.

Support sheet

Here are some responses of 11-year-old pupils to the simple circuit think again questions.

Support sheet

The pupils had just started secondary school and had received no teaching on electricity in the secondary school. These are the verbatim responses of the pupils complete with one or two spelling mistakes!

Where are they before you notice them moving

Wrong Track: The charges all come out of the battery.

Right Lines: The charges originate in the circuit.

Tackling the challenge

Thinking about the learning

The incorrect idea here is that the charges all originate in the battery and flow out from the battery to form the electric current.

Thinking about the teaching

The charges originate in the circuit itself and are set in motion by the battery when the circuit is completed. They are simply parts of the atoms that make up the battery, wires and bulb. You might think about it as if the charges live in the wires.

When the light comes on

Wrong Track: The bulb cannot light until the energy is carried to it.

Wrong Track: The charges travel from the battery to bulb, and light it when they get there.

Right Lines: The circuit starts working as soon as you throw the switch. The charges start moving everywhere and electrical working lights the bulb.

When do the charges start to move?

Thinking about the learning

It's possible that this kind of thinking is encouraged in children by donation models that suggest that everything comes from the battery, and is then carried round to the bulb.

Thinking about the teaching

It would appear that when we turn on a switch the electric charges move immediately in all parts of the circuit and instantly light a bulb. At home, there's no noticeable delay between your finger activating the switch and the LED bulb glowing – even though the switch and bulb may be a long way apart.

Actually it does take a very short time for the charges to start moving. The electric field that sets the charges in motion takes a finite time to pass through the wires. The field propagates (moves) at approximately the speed of light: 300,000 kilometre /second, that is 300 millimetre in a thousand-millionth of a second.

Does this delay matter? An electrical signal would take a mere hundredth of a second to pass from the UK to the USA through cables under the Atlantic Ocean. However, a modern computer will time itself on a signal changing many millions of times a second. With signals that are fractions of a millionth of a second long, even small delays in signals travelling along wires must be taken into account when designing circuit boards.

The bulb in our circuit does not turn on immediately, but the delay is so short that it is only significant in situations far removed from school laboratory benches and domestic lighting.

All together, in every part of the circuit

Wrong Track: When the switch is closed, the charges leave the battery and move around the circuit.

Right Lines: When the switch is closed, the charges all around the circuit are set into motion together. When the circuit is completed, the charges start moving in all parts simultaneously. Those charges in the connecting wires, just before the bulb, move through the filament wire. The store in the battery starts to be depleted and the bulb comes on with no apparent delay. It is not a case of waiting for those charges, that have just left the battery to arrive at the bulb before the bulb lights. There is a continuous and steady flow of charges in all parts of the complete circuit.

Encouraging thinking about movement everywhere in the loop

Thinking about the learning

The wrong tracks statement suggests the same incorrect idea from an earlier challenge that the charges all originate in the battery and flow out from the battery to form the electric current.

Thinking about the teaching

When talking to classes about how electric circuits work, it is natural to start with the battery, which is essential to making things happen everywhere in the loop. However, it is important to reinforce the idea that once a circuit is completed, the charges start moving in all parts simultaneously.

So, rather than pointing with one finger to trace the path of the charges as they leaves one side of the battery, it is helpful to gesture with both hands together, showing the charges simultaneously moving in opposite sides of the circuit loop. Don't always place one of the gesturing hands over the battery.

All together, in every part of the circuit

Wrong Track: It's either a conductor or an insulator, and that's that.

Right Lines: Some materials conduct electricity well, some very badly. Humans are somewhere in the middle: that's why you have to be careful with electric circuits.

Encouraging thinking about movement everywhere in the loop

Thinking about the learning

Absolute distinctions make the clear categorisation, but may store up difficulties for the future. If children happen to test themselves as a material to go into circuit they will notice that the bulb does not light up and therefore conclude that what they are made of is an insulator. this decision would not serve them well if they were later to come into contact with a live wire in the home, connected to the mains supply. Conductor versus insulator is not a hard and fast category.

Thinking about the teaching

Materials which we call insulators to conduct electricity much more poorly than materials which we call conductors. However it is not the case that insulators absolutely do not conduct electricity: they might only allow very small electrical current in exchange for a very large battery. So what might be thought of as an insulator when tested with a small battery could turn out to be a conductor when tested with a much larger battery.

The rope loop

You'll have to settle on a way of sharing a line of thinking about what goes on in electrical loops. We think that the rope loop provides a powerful mechanical analogue of the electrical loop. It's tangible, manipulable, and the physical quantities map well onto the electrical quantities. It's a way of thinking that can carry the children far into thier future studies. SO it's good to have in the back of your mind whilst exploring loops with children.

Imagine a loop of rope being held lightly by a child and a teacher. The teacher sets the rope in motion by pulling hand over hand, using her hands to make the motion as smooth as possible (a steady rope current). The rope everywhere in the loop moves at this steady rate. Then the child increases their grip (so impeding the passage of the rope: providing resistance). This reduces the flow of rope everywhere in the loop.

TeacherTip{ Here's how the model works. electric circuit model  →  rope loop teaching model The battery sets charged particles in motion around the whole circuit.  →  The teacher sets the rope loop in motion. Energy is shifted where charged particles meet resistance in the circuit.  →  Energy is shifted by working where the child grips the rope, so providing a frictional force (slip, not grip). }

Reflecting on teaching models

Thinking about the teaching

All ways of thinking about electrical loops have their strengths and weaknesses and it is important to be aware of what these are, and how they'll help or impede children as they try and come to terms with electric circuits. We don't think that all (teaching) models are of equal value to learners. We suggest consistently having the rope loop model in the back of your mind, given some of its advantages. You'll have to decide on one (and we think it should be one – or else the children need a very deep understanding of electrical loops to be able to select from amongst the models available to them, to apply the situation at hand), and we think you should be prepared to justify your choice.

We think it is not good to teach electric circuits as a collection of to-be-memorised rules, as this undermines pupil's confidence in their ability to make sense of the phenomena.

When using a teaching analogy in class, we find it very helpful to talk about it as a picture and to avoid calling it a model. This helps distinguish between helpful pictures (teaching analogies) and models: things you can reason with and that have predictive power. Both the electric circuit model (the scientific model the children will come to use) and the rope model are worthy of that name.

We suggest you aspire to develop a model, with which you can engage in exploratory reasoning with the children. Then use the model consistently. We don't recommend flipping around amongst models( it's a bit like this; it's a bit like that, because in such a case the short time that they you'll have to explore circuits, there's insufficient time to acquire confidence in many models. We certainly don't suggest that this is a good area for children to be asked to discriminate amongst models – there are far too many conceptual tripwires in this topic (and some models guide children into the trip zone).

One other common model involves energy being given to the charges as they leave the battery, which is in turn given to the bulbs. Energy may be modelled as loaves of bread given to bread vans (so the vans are the charges) or sweets given to children. All of these might be grouped as donation models.

A significant weakness of the donation analogies is that it paints the picture of charges collecting energy only in the battery and giving out energy in the bulb. As detailed earlier, this is not the case. The physical reasoning is wrong, even if the sums done later exhibit similar structure. The analogy does misrepresent the physics – think back to lessons of the big circuit, and is likely to reinforce many of the wrong tracks identified in this topic.

A further weakness of donation analogies is the reliance on ad-hoc rules. For example, using a supermarket picture, in moving from one to two bakeries, it may be plausible to suggest that each van collects twice the amount of energy, but it is not so clear as to why the vans also move round at twice the rate. For the correct working of the analogy it is essential to recognise that changes to the amount of bread carried per van and the rate at which the vans move round cannot occur independently. Similarly, in adding an extra supermarket it is necessary to accept that the bread is shared between the supermarkets and that the loop of vans is slowed down.

Whilst reasoning securely and fluently with a model will help guide childrens' enquiries, we don't think it a good idea to do the modelling explicitly. That can come later.

Adding a bulb prevents things happening

Wrong Track: The first bulb grabs the current, so that there's none left for the second.

Wrong Track: The first bulb slows down the current, and the second one slows it even more.

Right Lines: The current is the same everywhere in the loop. The bulbs both act together to reduce the flow. There really isn't a first and second.

One more bulb, reduced current

Thinking about the learning

The fact that adding a second identical bulb in series results in the two bulbs being dimmer makes intuitive good sense to many. Their underlying reasoning is that the single source (or battery) is now being shared between the two bulbs. This idea needs both development and some refining, so that it increases in precision.

The key idea is that adding an extra bulb introduces more resistance to the circuit, and this has the effect of reducing the flow of charge in every element in the circuit. The number of charges passing any point in the circuit is reduced; in other words, the current is reduced.

Thinking about the teaching

The rope loop teaching model is very useful in exploring the idea that the electric current goes down when a second bulb is added to the circuit. A discussion for later might go like this:

Teacher: OK! Now I want both Julia and Anita to loosely hold the rope. I'm pulling the rope round with the same force and it's obvious that Anita's extra grip or resistance has slowed down the whole of the rope loop.

Having talked through the teaching model, attention is returned to the electric circuit model:

Teacher: When a second bulb is added, extra resistance is introduced to the circuit. This has the effect of slowing down the flow of charges all around the circuit. In other words the current is reduced.

It is worth emphasising that it is the filament of the bulb that offers the resistance to the flow of charge:

Teacher: The filament of the bulb is made from very thin tungsten wire. The filament is designed so that it is difficult for the charges to pass through and, as they do, they interact with the tungsten atoms and the thin wire heats up until the filament glows white-hot. This is rather different from what happens in the connecting leads. These are made from relatively thick lengths of copper wire that have a very low resistance and so very little heating occurs in the connecting leads.

For now, it's enough that the explorations of circuits don't put tripwires in place, to catch the unwary later.

More batteries

The fact that adding a second battery in series results in the bulb being brighter makes intuitive good sense to many. The reasoning is that the double source (or battery) is now supplying just one consumer (or bulb) and therefore gives a bigger effect.

A more detailed explanation is, however, more demanding and is based on two effects which occur simultaneously:

With an extra battery, the positive terminal of the battery becomes more positively charged and the negative plate becomes more negatively charged, and this exerts a bigger force on the charges.

As a result:

• More charges pass through the filament each second.
• Each charge depletes the store of the battery more during its passage.

In other words, adding a battery both increases the current and increases the effect of each charge passing.

One current set by the resistance

Wrong Track: The current is smaller in the filament of the bulb because of the very high resistance, but then speeds up and gets bigger through the conducting wires.

Right Lines: The extra resistance at a single place in the circuit has the effect of reducing the current around the whole circuit. The same amount of charge passes per second through each and every part of the circuit.

The current is reduced in the whole circuit

Thinking about the learning

A further important point to emphasise is that when additional resistance is introduced to a circuit in one place, the current is reduced everywhere in the whole circuit. This can be the source of confusion for pupils.

Thinking about the teaching

It may be helpful to think of the resistance as providing the rate-determining step for the whole circuit. The rope loop is useful in establising this. As further resistance (more pupils gripping the rope) is added to the circuit, the movement of the whole rope loop is slowed down. All parts of the rope loop move around at the same speed.

It is worth exploring the effect on the current of a full range of resistance.

Teacher: What would happen to the current if more and more bulbs were added to the circuit?

Quite simply, as more bulbs are added, the resistance in the circuit increases, the charge moves around more slowly and the current gets smaller. With an infinite resistance, the current would fall to zero. We can demonstrate this simply by making a gap in the circuit.

Teacher: What would happen to the current if all of the bulbs were removed from the circuit and we were left with a connecting lead from one side of the battery to the other?

This is called a short circuit and is to be avoided! Because there is very little resistance in the circuit (apart from that provided by the battery itself), a very large current results with the charged particles moving around very quickly. It is likely that the battery will be damaged, or the wire become so hot that it will burn you or even melt!