I often use the idea of coal lorries to explain electric circuits. I say that the lorry collects coal at the battery and delivers it to components in the circuit such as bulbs. I like the model because it gets across the idea that charged particles like electrons carry the energy. They are not themselves the energy. When I started teaching it was common for students to think that electrons were used up in a bulb. This model gets round that idea. It helps to explain the sharing of voltage in a circuit by talking about the number of bags of coal delivered. Just don't mention the idea at a gathering of Physics teachers ... it caused a scene over dinner a couple of years ago! The weakness of the model is that it would suggest a time lag when you turn on a switch before the electrons arrive at the bulb. But actually electrons start moving at all points in the circuit at the same time. This can be show by having a long loop of rope and getting students to feed it through their hands. They are both models. Models help understanding but have weaknesses. Being able to identify and explain weaknesses really helps understanding, so I won't be giving up on coal lorries.
