Modelling electric circuits: Part 2

This post follows on from the last entry which explained how to set up the model and demonstrate current and potential difference in series circuit. In this entry 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 3Figure 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 in 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 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 4Figure 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 5Figure 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 6Figure 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.

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