Current only slows down at the resistor – by analogy with water flow
Keith S. Taber
Students commonly think that resistance in a circuit has local effects, and in part that is because forming a mental model of what is going on in circuits is very difficult. Often models and analogies can be useful. However when an analogy is used in teaching there is also the potential for it to mislead.
Amy was a participant in the Understanding Science Project. Amy (when in Y10) told me she had been taught to use a water flow analogy for electric current. However, because her visualisation of what happens in water circuits was incorrect, she used the analogy to inform an alternative conception about circuits:
Do you have any kind of imagined sort of idea, any little mental models, about what (the flow of electricity round the circuit) might look like? Do you have a way of imagining that?
Erm, yeah, we've been taught the water tank and pipe running round it. … just imagine the water like flowing through a pipe, and obviously like, if the pipe becomes smaller a one point, erm, the water flow has to slow down, and that's meant to represent the resistance of something.
So, so if I had my water, er, tank and I had a series of pipes, they'd be water flowing through the pipes, and if I had a narrower pipe at one point, what happens then?
The water would have to slow down.
So would it slow down just as it goes through the narrow pipe, or would it slow down all the way round?
Erm – just through that part.
(Amy does not appreciate the implications of conservation of mass {that is, the continuity principle} here – at steady state there cannot be a greater mass flow at different points in the circuit).
…
And so how do you imagine that's got to do with resistance, how does that help you understand resistance?
…well resistance, it slows the current down, but then erm, once it passes a resistor or something it, the current is free to flow through the wire again
Analogies can be very useful teaching tools, but when using them it is important to check that the students already understand the features of the analogue that are meant to be helpful. It is also important to ensure that they understand which features are meant to be mapped onto the target system they are learning about, and which are not relevant.
Analogies are only useful when the learner has a good understand of the analogue. In this case, as Amy did not appreciate that the water flow throughout the system would be limited by the constriction, she could not use that as a useful analogy for why a resistor influences current flow at all points in a series circuit. This is an example of where a teaching model meant to support learning, which actually misleads the learner. That is, for Amy, with her flawed understanding of fluid flow, the teaching model acted as a pedagogic learning impediment – a type of grounded learning impediment.
