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.
We use the human model (although without the hats!). We give each pupil a piece of yellow paper which represents energy. They “collect” it at the cell and they deliver it to the bulb. This allows you to explain further concepts as well. For example if you double the voltage, pupils get two pieces of paper each. And you introduce resistance by placing chairs in the pupils way.
Thanks M – I like the human model for energy. In the past I have tried to explain voltage with students carrying smarties (1 smartie is 1V, 2 smarties is 2V), but students were often unclear whether they were supposed to be charge carriers (electrons) or unit charges (positive). I also use another model for voltage made of lego and string, but I’ll write that up another day! B
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