Scientists use models to help understand a process. Each part of the model represents something in real life.
A good model fits with the facts. Below are some models used to help understand electrical circuits.
Some questions are auto-marked; discussion questions ask you to compare your answer with a model answer.
The coal truck model
This model shows a coal mine and a power station, joined by a single-track railway. Coal trucks run along the track, delivering coal to the power station, then return empty to the mine to be refilled.
1. In the 'coal truck' model, what represents:
2. The moving trucks represent the current. When the truck speeds up, does the current increase or decrease?
3. Do you think there is anything in the model about potential difference? Explain your answer.
Not really. The model shows coal (energy) being carried by trucks (current), but it has no equivalent for potential difference — the energy transferred per coulomb of charge. It doesn't explain, for example, why two identical lamps in parallel have the same voltage across them even though the current splits between them.
Extension: Do you think this is a good model? Explain your answer.
It's a reasonable starting point for current as a flow rate and for energy being carried around a circuit, but it breaks down for potential difference, for parallel circuits, and for resistance (nothing slows the trucks down). Any answer that identifies a strength and a limitation is well justified.
The supermarket / shop model
A busy new supermarket is kept stocked with bread from a nearby bakery using a continuous chain of delivery vans.
4. In the 'supermarket' or 'shop' model, what represents:
5. The moving vans represent the current. When a van speeds up, does the current increase or decrease?
6. Do you think there is anything in the model about potential difference? Explain your answer.
As with the coal truck model, there's no clear equivalent of potential difference — just bread (energy) being moved by vans (current). It doesn't distinguish "how much energy per van" from "how many vans per second".
The hamster circuit model
A hamster runs on a wheel connected to a tube full of peas. When the hamster runs, the peas are pushed along the tube.
7. In the 'hamster' model, what represents:
This is genuinely tricky in this model — the peas represent both the current (Q8) and, loosely, the energy, since nothing else carries energy separately. That's actually a weakness of the model worth pointing out.
8. The peas represent the current. When the hamster speeds up, does the current increase or decrease?
9. Do you think there is anything in the model about potential difference? Explain your answer.
Not clearly — since the peas represent both current and energy, the model can't separate "energy per pea" from "peas per second", which is exactly what potential difference vs current does in a real circuit.
Extension: Do you think this is a good model? Explain your answer.
It's good for visualising current as continuous flow, but weaker than the coal truck/supermarket models for potential difference, since it doesn't separate the "carrier" from the "energy carried".
The electric circuit model
Compare the electric circuit diagram with the coal truck, supermarket and hamster models.
10. Which was the better model for you — the 'coal truck' model, the 'supermarket' model, or the 'hamster' model? Explain your answer.
There's no single correct choice — this is a judgement question. A strong answer names one model, gives a concrete reason (e.g. "the coal truck model made it clearest that the current doesn't get 'used up'"), and names at least one thing that model still doesn't capture (e.g. potential difference, resistance, parallel circuits).
