Current – An Argument for Many Models

Current is a nice simple word; it means the rate of flow of charge. So you need to explain rate to get a proper understanding. And there are two parts to rate: the velocity of the charge, and the amount of charge per unit volume. So you need to explain what the amount of charge per unit volume is….

This is where models come in. And I recommend laying the models on thick for current. The classic model is the flowing water model (water is the charge, flowing through pipes. You can demonstrate the ‘rate of flow’ by filling a beaker with a narrow jet of fast-flowing water or a wide jet of slower water).

The Institute of Physics favours the rope model (the rope is the charge, passing round the circuit. You can easily increase the current by increasing the speed, but it isn’t so simple to increase the charge per unit volume idea – fatter rope?) The advantage of this model is the idea that current doesn’t get used up or lost.

The third model commonly used is the human model of charge moving round the circuit. Move the desks to make a circuit. Get a student to wear a light bulb hat and another to wear a cell hat. The rest of the students are charges, moving round the circuit (the light bulb lighting up and the cell pushing the children round).

Each of these models has strengths and weaknesses, but isn’t that the point? By comparing the models, students develop a critical understanding of the properties of electricity denied by the use of a single model. Be explicit that the models don’t all illustrate the same thing. Don’t oversimplify for the sake of understanding, or if you do, don’t do it for long. If you don’t reintroduce the complexity, their understanding will be impoverished.