Many aspects of science are difficult for students to learn because they relate to objects or processes we cannot (easily) see or compete with ‘common-sense’ theories (misconceptions) that children already possess.
A recent study suggests that using analogies can help children form more valid conceptions. There are three stages to this process, according to the research paper:
1) give a range of examples that use the same general strategy but differ in superficial features.
2) ask systematic probing questions, such as those that encourage learners to generate examples of an idea or explanations of their own reasoning. Probing questions like these can help children to pay attention to important features of the source examples or to notice their own misconceptions.
3) provide children with implicit and explicit feedback. Both implicit feedback, gleaned by children as they interact with the tasks and adapt their approaches accordingly, and explicit feedback, offered by the instructor following children’s exploration of and/or solutions to the tasks, can facilitate subsequent generalization of the strategies.
Some further advice about generating and using science analogies is available here.
One aspect of physics that children (and non-specialists) frequently find difficult is the behaviour of current and voltage in circuits. Some teachers like the water pump analogy – though I’ve worked up a resource using a different framework (one I encountered as an ITT student and has stayed with me all these years). A range of elaborations on the analogy are included in the ppt file – though I would certainly break them up when teaching rather than overload working memory with the whole model at once.
A simple multiple choice quiz could be used to test whether the analogy is working – in terms of avoiding or correcting typical misconceptions. For example, using some of these bitesize quiz items before and after the presentation (of part) of the analogy.
One of the real opportunities of e-resources is the ability to animate scientific models so that students can see causal behaviour over time and experiment with those models as a simulation to confront misconceptions.
Evidence into Practice 
As I’m sure you are aware, there are numerous variations on this circuit analogy theme (with coal trucks delivering coal, and children carrying sweets the most common). Analogies are very useful in teaching but what I would add to your post is that, with circuits, you need to be very clear about where the analogy breaks down. The Petrol Difference = p.d. bit in your version is lovely but a limitation is that whilst the cell/petrol pump is clearly where energy is transferred from a chemical energy store to the charges/cars, your cars would transfer energy to the surroundings gradually as they drive round the circuit whereas real charges transfer their energy only in components with a significant resistance.
Coal trucks and pupil models don’t have this problem but, like your model, they fail to explain why, with one component all the energy is transferred in that component, but if another component is added in series, the charges ‘know’ to only give some of their energy to the first component and ‘save’ some for the second component. My view is that, in the end, everyone has to learn to apply Kirchoff’s Laws, and that analogies should be used to introduce current and p.d. and to show why current splits between parallel components and why p.d. does not, but with a really clear emphasis on using these to illustrate a bit of the idea but then coming back to the Law as the best way to reliably describe circuit behaviour.
However, if anyone can suggest an analogy that is easy to understand as well as accurately modelling current and p.d. in all simple circuits, then I’m all ears.
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Thanks for the comments. Agreed – all analogies fall down eventually and students need to be able to apply the genuine theories eventually. Indeed, it might make a good advanced activity to have students explain the weaknesses in an analogy.
Science is all about how we refine and reject ideas about the world based on observations and evidence that our ideas cannot adequately explain. There’s nothing wrong with an imperfect analogy, in my opinion, so long as it doesn’t reinforce major misconceptions and isn’t mistaken for ‘the truth’ at any stage.
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