The rope loop
You'll have to settle on a way of sharing a line of thinking about what goes on in electrical loops. We think that the rope loop provides a powerful mechanical analogue of the electrical loop. It's tangible, manipulable, and the physical quantities map well onto the electrical quantities. It's a way of thinking that can carry the children far into thier future studies. SO it's good to have in the back of your mind whilst exploring loops with children.
Imagine a loop of rope being held lightly by a child and a teacher. The teacher sets the rope in motion by pulling hand over hand, using her hands to make the motion as smooth as possible (a steady
rope current). The rope everywhere in the loop moves at this steady rate. Then the child increases their grip (so impeding the passage of the rope: providing resistance). This reduces the flow of rope everywhere in the loop.
Here's how the model works.
electric circuit model → rope loop teaching model
The battery sets charged particles in motion around the whole circuit. → The teacher sets the rope loop in motion.
Energy is shifted where charged particles meet resistance in the circuit. → Energy is shifted by working where the child grips the rope, so providing a frictional force (slip, not grip).
Reflecting on teaching models
Thinking about the teaching
All ways of thinking about electrical loops have their strengths and weaknesses and it is important to be aware of what these are, and how they'll help or impede children as they try and come to terms with electric circuits. We don't think that all (teaching) models are of equal value to learners. We suggest consistently having the rope loop model in the back of your mind, given some of its advantages. You'll have to decide on one (and we think it should be one–or else the children need a very deep understanding of electrical loops to be able to select from amongst the models available to them, to apply the situation at hand), and we think you should be prepared to justify your choice.
We think it is not good to teach electric circuits as a collection of to-be-memorised rules, as this undermines pupil's confidence in their ability to make sense of the phenomena.
When using a teaching analogy in class, we find it very helpful to talk about it as a picture and to avoid calling it a model. This helps distinguish between helpful pictures (teaching analogies) and models: things you can reason with and that have predictive power. Both the electric circuit model (the scientific model the children will come to use) and the rope model are worthy of that name.
We suggest you aspire to develop a model, with which you can engage in exploratory reasoning with the children. Then use the model consistently. We don't recommend flipping around amongst models(
it's a bit like this;
it's a bit like that, because in such a case the short time that they you'll have to explore circuits, there's insufficient time to acquire confidence in many models. We certainly don't suggest that this is a good area for children to be asked to discriminate amongst models–there are far too many conceptual tripwires in this topic (and some models guide children into the trip zone).
One other common model involves energy being
given to the
charges as they leave the battery, which is in turn
given to the bulbs. Energy may be modelled as loaves of bread given to bread vans (so the vans are the charges) or sweets given to children. All of these might be grouped as
A significant weakness of the donation analogies is that it paints the picture of charges
collecting energy only in the battery and
giving out energy in the bulb. As detailed earlier, this is not the case. The physical reasoning is wrong, even if the sums done later exhibit similar structure. The analogy does misrepresent the physics–think back to lessons of the big circuit, and is likely to reinforce many of the wrong tracks identified in this topic.
A further weakness of donation analogies is the reliance on ad-hoc rules. For example, using a supermarket picture, in moving from one to two bakeries, it may be plausible to suggest that each van collects twice the amount of energy, but it is not so clear as to why the vans also move round at twice the rate. For the correct working of the analogy it is essential to recognise that changes to the amount of bread carried per van and the rate at which the vans move round cannot occur independently. Similarly, in adding an extra supermarket it is necessary to accept that the bread is shared between the supermarkets and that the loop of vans is slowed down.
Whilst reasoning securely and fluently with a model will help guide childrens' enquiries, we don't think it a good idea to do the modelling explicitly. That can come later.