Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How to:

• build on the helpful ideas present
• avoid injecting unhelpful lines of thinking
• construct a model of flow, relating this to current
• build on the idea that something is used up
• draw out, and challenge wrong track thinking

Teacher Tip: These are all related to findings about children's ideas from research. The teaching activities will provide some suggestions. So will colleagues, near and far.

Focusing on the physics:

Representing–noticing–recording. How to:

• deal with a circuit loop as a single linked system
• focus on the steady-flow state, not on transients
• work practical experiences in with theoretical descriptions
• develop a toolkit to support pupils' reasoning about circuits

Teacher Tip: Connecting what is experienced with what is written and drawn is essential to making sense of the connections between the theoretical world of physics and the lived-in world of the children. Don't forget to exemplify this action.

Where are they before you notice them moving?

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

Right Lines: The charge originates in the circuit.

Tackling the challenge

Thinking about the learning

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

Thinking about the teaching

The charges – the charged particles – 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 say that the charged particles 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 charged particles start moving everywhere and electrical working lights the bulb.

When do the charged particles 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 electrically charged particles move immediately in all parts of the circuit and instantly light a bulb. Even if we connect all the wires available in the laboratory, to make a big circuit, the light bulb still appears to react immediately.

Actually it does take a very short time for the electrons to start moving. The electric field that sets the electrons 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 bench experiments.

Think about what your words might be taken to imply

You need to be careful with your choice of words in introducing and talking about these basic electric circuit ideas. It is very easy to suggest incorrect ideas and have your pupils moving down the wrong track!

For example, a recently published teaching scheme states that pupils should learn:

that a cell/battery provides an electric current, which travels round the circuit.

We don't necessarily believe that the writers of this teaching scheme don't understand electric circuits! We do, however, recognise that what is written is ambiguous at best. We do think it's an unhelpful way of writing things.

To some it might suggest that the electric current originates in the battery. You will come across similar statements elsewhere.

Try to avoid them in your own teaching! It would be better to say that:

Teacher: The battery drives the charged particles round the circuit.

Teacher Tip: Always work in such a way that you make it obvious that you think the charged particles are already distributed around the electrical loop. The battery simply makes them drift along.

Electricity is not a word to use often

In everyday speech the word electricity is very commonly used. Thus you might overhear statements such as:

it runs off electricity, and we need to pay the electricity bill.

This sort of expression inevitably carries over into school and so pupils might say:

Emma: Electricity flows round the circuit.

Here you should encourage the pupils to be more precise in their use of terms: What do you mean when you say that electricity flows? What is it that actually moves round the circuit?

You should also model the careful use of language (as well as other expressive media), to encourage accurate and precise thinking.

Teacher: That's right! The charge moves round the circuit: that is an electric current.

Teacher Tip: There aren't many situations where simply banning the use of a particular word is likely to help pupils come to understand and be able to use new physics ideas… but this might be one of those situations! A teaching colleague reckons that the only time he allows his pupils to use the word electricity is in writing the title in their books at the start of the new topic!

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 charged particles all around the circuit are set into motion together. When the circuit is completed, the charged particles start moving in all parts simultaneously. Those charged particles in the connecting wires, just before the bulb, move through the filament wire. Energy is shifted and the bulb comes on with no apparent delay. It is not a case of waiting for those charged particles that have just left the battery to arrive at the bulb before the bulb lights. There is a continuous and steady flow of charged particles 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 charged particles 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 provides the energy for the circuit and sets the charged particles in motion. However, it is important to reinforce the idea to pupils that once a circuit is completed, the charged particles start moving in all parts simultaneously.

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

Current measures flow of charge

Wrong Track: The current measures how fast the charges are moving.

Right Lines: The current measures how much charge passes each second.

Current is more than just speed of charged particles

Thinking about the learning

The idea that the electric current is a measure of how much charge passes per unit time can be quite challenging for pupils. It is common for pupils to mix up how much charge passes with how fast the charged particles are moving. Some pupils refer to the electric current being faster or slower when changes are made to a circuit.

Mixing up these two ideas is understandable. For example, if we add a second cell to a circuit with one bulb, the current increases because the charged particles in the circuit move round more quickly (this circuit is considered in detail in episode 02). Nevertheless, we must be clear in stating that the electric current is measured in terms of the amount of charge passing.

With a big electric current, many charged particles pass each second.

With a small electric current, fewer charged particles pass each second.

Thinking about the teaching

A useful approach to getting over the idea of measuring electric current is to encourage the pupils to picture what is happening in the wires of the circuit – so starting with a teaching model.

You might start with the rope loop referring to the amount of rope that passes by each second. Then move on to the idea of the number of charged particles passing per second. You might well explicitly connect the length of rope passing each second to the quantity of charge passing in each second.

In introducing the ammeter, emphasise that its job is to measure how many charged particles pass through that point in the circuit each second. In such a way, talking about the ammeter helps to clarify and reinforce the concept of electric current.

You could almost imagine that there is a little helper inside the ammeter. The little helper is armed with a stop watch in one hand and a counting stick in the other. As the charged particles pass through the meter the little helper has the job of counting the amount of charge passing each second, and this is the current reading.

When the pupils start measuring electric currents in ampere, take every opportunity to talk through what is meant by the readings that they take.

Teacher: So, the current here is 5 ampere and the current there is also 5 ampere. Anybody – what's going on with the charged particles in the circuit?

Paula: The amount of charge passing through both points each second is the same.

Teacher: Excellent! Same charge passing everywhere, per second.

The battery gives a current

Wrong Track: The current flowing in the circuit is 2 ampere. The battery is giving a current of 2 ampere.

Right Lines: The current in the circuit is 2 ampere. This rate of flow of charge is fixed by choosing the battery and the lamp, so the voltage and the resistance.

The circuit as a system

Thinking about the learning

Having been introduced to the idea of measuring electric currents, some pupils see the battery as providing a fixed current. This incorrect line of thinking may be linked to the earlier wrong track idea that the current originates in the battery.

Thinking about the teaching

You should always talk so that the pupils see the circuit as a whole system with the charge being set in motion by the battery and any resistance in the circuit acting to impede that motion. So do try to avoid any discussion of the current moving from place of place, or splitting.

Choosing a teaching approach

Thinking about the teaching

Planning to teach about the connection between charge flow and current leads to a necessary choice of teaching approach, which involves either:

Starting with measurement: The pupils are told that ammeters measure electric currents and that they are to use an ammeter to find out what they can about current values at different points in circuits; or

Starting with the electric circuit model/teaching model: The concept of electric current, as charge passing per second, is introduced (drawing on the teaching model) and the pupils are instructed to use this idea to make predictions of electric current values at different points in circuits.

Starting with measurements, in this particular context, does not make much sense to us, since the pupils are being asked to make measurements of currents when they have no idea of what an electric current is.

The approach which we suggest, therefore, is to:

• Talk through the electric circuit model, and teaching model, with the pupils
• Demonstrate how to use an ammeter to measure an electric current
• Encourage pupils to use the model to make predictions of the value of the electric current at different points around various circuits
• Get pupils to check their predictions through practical measurements with an ammeter.

In this way, the practical measurements are used to confirm the developing electric circuit model, helping to make it seem plausible and fruitful for the pupils.

A very practical advantage of this approach is that the pupils set about making measurements of current with the expectation (hopefully!) that the values of the current will be the same. If the instruments indicate slight differences, these are likely to be accepted as being just about the same.

Current does one thing: energy another

Wrong Track: The electric current gets used up in the bulb.

Right Lines: Energy is shifted by the action of the bulb. As the charged particles first pass through the bulb, they encounter resistance in the filament, and energy is shifted to the thermal store of the filament as it warms up and starts to glow. The charged particles continue around the circuit. Energy, and not the charged particles, is shifted by the bulb and dissipated.

Two kinds of flow

Thinking about the learning

The incorrect idea here is that when the charged particles pass through the bulb, they make the bulb light and so get used up.

Thinking about the teaching

There are two kinds of flow: electric charge and energy. The Physics narrative explains the difference. You should refer to both kinds explicitly. You may even bring the wrong tracks thinking in explicitly, so as to focus attention on the issue:

Teacher: So, in last year's class, they thought that the charged particles got tired as they went through the bulb, and got used up. What do you think of that?

Current is used

Wrong Track: The electric current gets used up in the bulb to make it work.

Right Lines: Current is the same in each element and energy is shifted. The current in the wire after a bulb has the same value as the current in the wire before the bulb because the same number of charged particles enter and leave the bulb each second. What goes in must come out! Charge is conserved. However, as each charge passes through the bulb, energy is shifted by the filament as it glows.

Current the same everywhere

Thinking about the learning

The incorrect idea here is that the current after the bulb is less than the current before the bulb, because some of the current gets used up to make the bulb work.

It is clear to most pupils that something must get used up when a battery is connected to a bulb and the bulb lights up. The key learning challenge is for pupils to come to understand that the electric charge is conserved whilst energy is shifted by the circuit.

Thinking about the teaching

To communicate the idea that electric current is the same everywhere in the circuit, it is helpful to make practical measurements of electric current and to relate these to the electric circuit model and to the teaching model. So there are three facets to think about.

Constructing an electric circuit model

Thinking about the learning

The pupils need to be able to picture what is going on as energy is shifted from battery to surroundings as the charged particles move around the circuit. That is, they need something to reason with that allows them to develop expectations about the behaviours of circuits that they've not yet met.

Teacher Tip: Develop, and make consist use of, a teaching model to support learning about intangible entities such as electrical current and energy.

Thinking about the teaching

Because you are dealing with objects and ideas that cannot be seen, electric circuits present an interesting teaching challenge.

We can think of two possible starting points for introducing the electric circuit model.

• A Direct approach: Develop an account based on a formal model of an electrical circuit.
• Using a teaching model: First introduce a teaching model for the electric circuit as a starting point for developing the formal electric circuit model.

The advantage of using a teaching model is that it can provides a familiar, manipulable, tangible starting point for the pupils. The disadvantage is that an ill-chosen model may reinforce some wrong tracks, and not nudge pupils thinking down the right lines.

Share tools for thinking with

Thinking about the teaching

Very often, analogies are drawn upon in teaching about electric circuits in an opportunist way, with the teacher perhaps briefly referring to the electric current as being, for example:

• Like the flow of water down a pipe.
• Like peas passing down a tube.
• Like pupils running down a corridor.

We believe that this can be confusing and recommend a more systematic approach in which a teaching model is carefully introduced.

Teacher Tip: Make systematic use of a teaching model.

1. Start with the electric circuit.

   Teacher: This is what happens when the circuit is completed: the bulb lights. How can we explain why this happens?

2. Introduce the teaching model.

   Teacher: Let's think about something quite different: a loop of rope.

3. Make links between model and circuit.

   Teacher: In what way is the rope loop similar to the electric circuit?

4. Introduce the formal electric circuit model (through the teaching model).

   Teacher: We don't have rope moving around the circuit, but we do have charge.

5. Continue to use the teaching model as needed.

   Teacher: So, why does the brightness of the bulb increase? You might want to go back to the rope loop in talking through your explanation.

The rope loop

You'll have to settle on a way of sharing a line of thinking about what goes on inside a circuit. 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.

Imagine a loop of rope being held lightly by a pupil 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 pupil 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 pupil grips the rope, so providing a frictional force (slip, not grip). }

Teacher Tip: Build and maintain this pair of essential links between the electric circuit model and the rope loop teaching model.

Using the rope loop to tackle teaching challenges

The teaching model allows the teacher to start talking through the electric circuit ideas in terms of familiar and concrete objects:

Teacher: So, I pass the rope from one hand to the other and the whole loop moves around over your fingers.

This familiar picture helps to bring meaning to the idea that:

Teacher: The battery sets the charged particles in motion in all parts of the circuit.

The teaching model helps the pupils in two ways:

• It helps the pupils to visualise what is going on in the circuit.
• It provides the pupils with a set of simple ideas to think with.

With these points in mind, you should encourage the pupils to talk and think, with both the teaching model and developing electric circuit model, wherever possible.

The rope loop can be used to address directly some of the key learning challenges introduced earlier.

Teacher: Now it's obvious that the rope isn't coming from me. I'm just making it move around. In just the same way, the charged particles don't come from the battery. It just makes them move around the circuit.

Teacher: Look everybody! As soon as I set the rope moving here, it starts moving all around the loop. In just the same way, as soon as the circuit is completed, the charged particles start moving in all parts of the circuit.

Teacher: The same amount of rope returns to me as leaves me. The rope doesn't get used up, or disappear, on the way round. In just the same way, the current doesn't get used up on the way around the circuit.

We suggest that you read back through the challenges faced in this thread, and then match them up to the descriptions above.

Thinking more about electrical working

There are clear and direct parallels between the rope loop model and the formal electric circuit model set out in the Physics Narrative.

With the rope loop the teacher works on the rope to pull it through, a length at a time. At the same time, energy is shifted elsewhere in the loop due to the frictional force of a hand gripping the rope. The pulling force applied by the teacher acts to keep the rope moving, whilst the frictional force applied by the pupil acts to resist the motion of the rope. Indeed we can say that the rope does work on the hand to warm it up. In exactly the same way we can say that the battery works on the loop of charged particles to keep them in motion, whilst at the same time the charged particles work on the ionic lattice in the bulb to heat it up.

Teacher Tip: With the rope loop energy is shifted through mechanical working whilst with the electric circuit we are dealing with electrical working (more detail on working as a pathway for shifting energy in the SPT: Energy topic).

Use those wrong tracks

You might make the wrong track pupil ideas an explicit part of your classroom discussions by introducing them to the lesson yourself:

Teacher: Now then, in one of my other classes one of the boys said that the charged particles all come out of the battery when the circuit is switched on. Was he right? What do you think about that?

Teacher Tip: Use this kind of approach to bring out the possible wrong track step in learning so that it can be made explicit and challenged, rather than just left lurking in pupils' thoughts! Part of the trick of building expertise in any area of teaching is to be able to anticipate pupils' questions and problems.

Forces on charged particles drifting around electrical loops

Wrong Track: The concentration of charged particles on the battery repels and attracts the charges around the circuit.

Right Lines: It's the force where the drifting charged particles are that changes the motion of the particles. So you need to see how the charged particles are arranged around the circuit by the battery, and how this arrangement of charged particles affects the moving charged particles.

Dealing with circuit loops – issues that may be triggered

Thinking about the learning

Here it's possible to mislead by acting and talking as if the battery acts direct on the charged particles at each point in the circuit, or even that the concentrations of charged particles on the battery act directly on the moving charged particles at any point. You need to be clear that the charged particles are acted on by forces at their location. These local forces do exist as a result of the repulsion between the flowing charged particles and other charged particles. But by far the most significant charged particle concentrations for the individual moving charged particles are those close to that moving charged particle. These distributions of charged particles are set up by the concentration of charged particles in the battery. So we think that this is another place where it's important to emphasise systematic thinking. It's all about the whole electrical loop.

Imagine the kind of difficulties you might get in to if you try and explain using the repulsion and attraction between the charged particles on the battery terminals and the drifting charged particles, using what you know about the forces between charged objetcs (from the SPT: Forces topic). A circuit laid out on the benchtop will work perfectly well with the wires snaking back and forth. At some points the force acting on the drifting charged particles from the concentration of charged particles on the battery will seems to be driving the charged particles in just the wrong direction – perhaps even out of the wire.

Thinking about the teaching

Here again we'd suggest not always starting at the battery when discussing the flow of charged particles around any selected loop. The important thing is to emphasise the complete loop, acting as a unit, or system, and to avoid leading children into using patterns of simple sequential reasoning.

You'll have to pay attention to the distribution of charged particles around the loop when there is a current in the loop in order to generate a good account of the forces on the drifting charged particles.

A rather convincing explanation is presented in the Physics Narrative, but that is for teachers, and probably not for children at this age.

There's a good chance you could improve your teaching if you were to:

Try these

• thinking about electrical loops
• emphasising the simultaneous movement of charge, everywhere in the loop
• using model to mean a toolkit for thinking with that is capable of generating predictions
• reasoning about electrical loops as complete systems

Teacher Tip: Work through the Physics Narrative to find these lines of thinking worked out and then look in the Teaching Approaches for some examples of activities.

Avoid these

• not challenging imprecise language, reasoning, diagramming
• using sequential models, such as donation models
• electrical energy

Teacher Tip: These difficulties are distilled from: the research findings; the practice of well-connected teachers with expertise; issues intrinsic to representing the physics well.