Modelling electric circuits: Part 1

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 it (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 electron, 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 is worth highlighting as an experimental note. If you numbers are wildly out try again and keep the electron gaps constant).

Figure 1

Figure 1: Series circuit with 3 ammeters

Part 3: Potential difference in series circuits

Potential difference is extremely difficult to articulate, and there are more misconceptions around 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.

In next weeks blog the model will be extended to cover current and potential difference in parallel circuits, using buzzers and bulbs, and how it can be extended. Click here for part 2

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