Use of Analogies

Analogies can be useful but if not well-chosen they can give incorrect information to children. It is important then that if we as teachers describe what electricity is like, that we use a useful analogy.

1. The speeding dots view of electric current is perhaps the least inaccurate portrayal of circuit behaviour under different circumstances (e.g. different batteries, numbers of bulbs etc.) and it has been used by the University of Colorado for their circuit animations (http://phet.colorado.edu/index.php). The concept of resistance is only introduced to give the students an idea about how to control the current flowing in a circuit – in reality, this is not at all how modern dimmer switches work, so better to avoid that topic specifically.

2. The water analogy may be helpful.

Each bulb offers a resistance to the flow of electrons. It is like having a single hosepipe with water flowing. Each bulb is like a pinch in the hose, which reduces the flow. The more bulbs are present in a series circuit, the more pinches in the hose, the smaller the current that can flow, so the dimmer the bulbs light.

In a parallel circuit, think of a hose pipe with a T-connector to 2 pipes, then reconnected back to a single pipe. Because there are 2 paths for the water to flow, it comes out twice as fast and then slows down through each branch and then speeds up when the branches join again. The parallel resistors offer a smaller resistance to flow than the series resistors because the flow can branch, however, the hosepipe analogy isn’t as convincing as for the series case. This is always the problem with analogies!

In terms of the battery/mains supply, there is a need to resist all temptation to introduce the concept of ‘voltage’. Also, never ever describe the battery as the ‘source’ of electrons. The battery just makes the conditions right for the electrons in the wires to flow. Going back to the water analogy, it is the height difference that allows the water to flow in the waterfall. The water was there already!

To develop the model of what is going on in circuits requires some very sophisticated thinking. Conceptually most appropriate perhaps is in terms of the electron flow as permitting the transfer of energy, stored in chemicals in the battery, to the light bulb, where it is transformed to light energy, or to the buzzer, where it is transformed into sound energy.