Modelling electric circuits by @Mr_Gillett

Activity for Electric Circuits

I have found this method of teaching electricity extremely useful teaching students from year 7 right through to year 12. This has also been an extremely popular and useful activity when training teachers. I have successfully used this method in outstanding, good and requires improvement schools, as well as with student teachers and teachers who have years of experience.

The model helps us to visualise and explain the major points of electricity. When answering questions after the modelling activity, students should be encouraged to use the model to help think about/visualise what is happening. It is not intended to be used in full as a one-off activity but built up over time, or partly used, to help develop understanding.

This model has been developed over the last 5 years, often in response to great questions, people ask when running the sessions. So please do ask questions and challenge them (there are weaknesses!).

Here is a video of the method in action using buzzer sounds, ammeters counting, and a voltmeter from the Teach First Summer Institute 2014.

Resources needed

* A length of string or rope (about 6 meters should do)

* Laminated cards of circuit symbols (cell, switch, ammeter x 3, resistor, voltmeter, bulb x 3, buzzer x 3)

Part 1: Electrons and Current

First, lay out the rope in a rectangle on the floor.

Get students to represent electrons. Ask them to stand on the wire. Tell them they all have negative charge, so how should they stand? They should work out that they are repulsed by each other (!!) since like charges repel, and so should spread out evenly on the wire.

Highlight at this point that everything about current can be explained by the force of repulsion on electrons.

Have a student holding the cell symbol join the circuit (an issue with the model is that they have to stand ½ a step behind the wire so ‘electrons’ do not collide with them). Ask them which side is positive and which side is negative. The electron nearest the positive will be attracted towards the cell whilst the electron closest to the negative side will be repulsed. As soon as these electrons move, the others are pushed by repulsion to move as well. This is shown in the model as the ‘electrons’ (students) needing to keep the space between them even.

Add in the ‘switch’ (a students to open/close the circuit). When the switch says “switch closed” the electrons move around (from the negative side of the cell to the positive) in a continuous loop. Practice this basic form first, making sure than the electrons keep an even gap as they move. The switch stops the flow of electrons by yelling “switch open”.

Part 2: Current in a series circuit

The circuit model set up in part 1 is a series circuit. Students know this because the electrons only have one possible route (there are no ‘decisions’ to make as they move around).

Position your 3 ammeters around the circuit as shown in Figure 1. In this model, an ammeter works by raising their hand and counting electrons every time an ‘electron’ walks by them, “one electron, two electrons, three electron….” This should be done loudly (and theatrically) so the whole group can hear. Trial or demo this with one ammeter.

Run this with all three ammeters at the same time with the three people (ammeters) counting out loud. What is the result? You should find that all 3 record the same number of electrons passing. This proves the first circuit law: current is the same at all points in a series circuit. (Note: If your numbers are slightly out, this is fine as ammeters will sometimes get a slightly different reading and are worth highlighting as an experimental note. If your numbers are wildly out trying again and keep the electron gaps constant).

Figure 1

Figure 1: Series circuit with 3 ammeters

Part 3: Potential difference in series circuits

The potential difference is extremely difficult to articulate, and there are more misconceptions about this area than any others in electricity. Although this model is far from perfect, it equips students to be able to answer GCSE and KS5 questions without introducing major misconceptions. (Trying to avoid describing p.d. as ‘push’ or ‘energy’).

The analogy here is made between gravitational potential energy and potential difference. Students can grasp the idea that a high object has more gravitational potential energy than a low object. The model uses the arm of the ‘electrons’ to indicate their potential. When passing the cell, they gain potential and so lift their arm up fully. To do this, you need some resistance in the circuit, so add a resistor opposite the cell. When they pass the resistor, the electrons lose their potential and so their arms go down by their sides.

Before ‘switching on’ this circuit, ask the students to put their arms into the position they should be in when the circuit turns on. This leads to an interesting discussion about why a light turns on immediately when you flick a switch. In fact, electrons move very slowly in a wire, but because they feel the effect of all other electrons in the wire (due to like charges repelling) the effect is near-instantaneous. So, what should happen is all students between the negative side of the cell and the resistor raise their arm fully, and those between the resistor and the positive side of the cell have their arms down as zero/lowest potential.

This is a good point to explain to the students that they need to have ‘zero potential’ left when they return to the cell.

Figure 2

Figure 2: Series circuit with 3 voltmeter positions. Note: I always draw voltmeters in a different colour so that there is a clear distinction between the circuit and the voltmeter. Voltmeters do not contribute to the circuit as they have high resistance so no current will flow through them. I also only use one student as a voltmeter and move them between the 3 different locations shown so students can talk through the process as they do it.

Using the voltmeter:

To measure potential difference, we use a voltmeter. Give a student the voltmeter symbol, and ask them to measure the potential difference at different points in the circuit. Stop the circuit, but ask the ‘electrons’ to keep their arms in their potential positions. The ‘voltmeter’ can measure the difference in potential by connecting their left arm to one electron and their right to another. Students like to use the ‘spud’ connector (see video). If this is done across the resistor they will notice a ‘potential difference’ – a drop from full to none, and an equal gain across the cell.

Potential difference (voltage) law in series circuits:

To find this law, you need two (identical) resistors in the circuit. As they need to have zero potential left when they return to the cell, electrons should have a drop in potential of half across each resistor (so the arm goes from top to shoulder height at the first resistor, and then from shoulder height to low at the second.). Again, all ‘electrons’ should get their arms into the correct positions immediately when the circuit is turned on – the electrons get their potential through the force of repulsion between them and the other electrons.

To find the law, you need to stop the circuit and ask the electrons to keep their arms in their ‘potential positions’. Challenge the voltmeter (I often use an able student) to measure the potential at different points and work out the rule. They should work out that the electrons gain a full arm movement, and then lose half across each resistor. The rule is that the gain in potential difference across the cell is equal to the loss in potential throughout the circuit (voltage is split up in a series circuit). Make sure the student who is the voltmeter talks through what they are doing, so all can follow the thought process to find the potential difference rule.

We continue as the model is extended to parallel circuits.

Part 4: Current in a parallel circuit

Set out the rope as a parallel circuit. Ask the electrons to stand on the wire in a suitable way. They should spread out evenly across the wire. Add in the cell so that the electrons will move. I tend to not give instructions here, and see if students can work it out. What will happen is chaos, as electrons are not sure which of the two routes they should move around. Stop the circuit here, and highlight that this is the difference between series and parallel circuits – electrons have different routes they can follow.

How do electrons know which way to go? Well, when paused, go to the electron that is next to approach the junction. You should find the electron that was just in front of them and went in one direction at the junction is the closest electron (see figure 3). This means that due to like charges repelling the electron will go in the opposite direction. This lead to electrons alternating so one will go left, one right, one left etc. This is the simplest form (and only applies if the resistance in both directions is equal). Practice this briefly so they get the idea of alternating who goes which direction.

Figure 3

Figure 3: The electron approaching the junction is closest to the one branching off, and so feels a stronger repulsive force. In this case, the electron approaching the junction will go straight on.

Now to measure current in parallel circuits. You will need 3 ammeters, with one near the cell, and one on each ‘branch’ of the circuit. Run the circuit with the ammeters loudly counting the electrons that pass. This will really clearly show that more electrons pass the ammeter near the cell (1) than those on the branches (2 and 3) (see figure 4) You should be able to determine that the current at point 1 is equal to point 2 added to point 3, ie that current is split up into a parallel circuit. I find that when answering questions about these later, asking students to visualise the electrons walking around the circuit helps them to answer current questions. In fact, I have found that students who had the role of the electron are more likely to answer current questions correctly. This tells us that the model works for understanding the topic and that you should rotate the student roles.

Figure 4

Figure 4: Parallel circuit with 3 ammeters. The amount recorded today will be the sum of ammeter 2 and ammeter 3.

What happens if the resistors are not equal?

To be clear modelling this, I will arrange one ‘branch’ with two resistors, and the other with one. With the information that ‘current is the same as students – more likely to take the easier option’ you can say that electrons are twice as likely to go the way of one resistor, and so 2 will go that way, the 3rd the other route (two resistors). (Note: at this point, the two resistors on the branch are in series together, and so potential difference is split between the two).

Part 5: Potential difference in a parallel circuit

For this, set up the circuit as in figure 5. Challenge students to work out what should happen in terms of the potential (if they need a hint, remind them that electrons must return to the cell with zero potential left. They should work out that in the case of parallel, they must lose all their potential at the resistor. Checking with the voltmeter will show that they lose the same amount as they gain (full arm) at each of the resistors. This tells us that potential difference (voltage) is the same in parallel circuits.

Figure 5

Figure 5: Three voltmeter locations to prove the potential difference in parallel law.

Part 6: Buzzers and bulbs

To make this activity memorable, I often use buzzers or bulbs instead of resistors. In the case of the buzzer, ask them to make a buzzing noise each time an electron passes and loses potential (you can also use animal noises or other comedy buzzers – see the video above for what happens when a primary school teacher is a buzzer!). For a bulb, ask them to ‘shine’, which could be a star jump, lifting up arms or something else. Leaving this up to students to remember can lead to hilarious results, and definitely makes it a memorable activity.

Part 7: The challenge circuit

To check their understanding, arrange the wires and components as shown in figure 6. This is the most challenging type of circuit students will meet in GCSE, and they are likely to find it easy to understand if the previous steps have been understood.

There are plenty of extensions and adaptations to this activity. I will often give a circuit diagram to higher ability year 11 or year 12 students and ask them to lead the activity.

Figure 6

Figure 6: The challenge circuit

I hope the model described here helps with understanding electric circuits and gives you some ideas on how to go about teaching it. Please let me know if you can think of weaknesses, improvements and other ideas.

This is a re-blog post originally posted by Mark Gillett and published with kind permission.

The original posts can be found here and here.

You can read further posts via Mark by Clicking Here

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