Gas station without pumps

2016 April 29

Miswiring errors

Filed under: Circuits course — gasstationwithoutpumps @ 15:25
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In yesterday’s post, Revised microphone pre-amp lab too long, I wrote about problems in this week’s lab, and one of the items seems to have resonated with at least one other instructor:

a surprisingly large number connected both nodes for a resistor to the same end of a resistor, leaving the other end unconnected.  I’ve not seen that mistake before, so I don’t know what triggered it.

CCPhysicist commented

I’ve seen that error (connecting two wires to the same end of a resistor) before, more than once, but I also don’t have a clue why they do it. It is worst if the resistors are in a box where they can see the connectors but not the resistors (even when they see the resistor symbol between the connectors), but also happens with loose resistors. Now my students have the excuse that we start doing those labs before we get to DC circuits in lecture, so I assume it means they have no idea that current flows through things and that switches break a circuit, but I have no idea why they get to college without any experience related to the basic concept of electric current. Maybe whatever misconceptions they have about current are stubborn enough to survive a semester of physics.

As for why you got many instances of that error, I’d suspect “authoritative ignorance” syndrome. Others were following someone who talks a good game but doesn’t know the play. Can happen just by one person looking at what another is doing, without any actual bad mentoring taking place.

I don’t think that “authoritative ignorance” was the problem here, as the students making the error were in both sections and they made it in different places in the circuit. I responded with my best guess at what was happening:

My conjecture is that students aren’t using a misconception of current—they aren’t thinking about the function of the resistor at all. They just have the idea “connect up the resistor to A and B”. Having a wire between point A and the resistor and between point B and the resistor satisfies that objective, even though it doesn’t mean anything if the resistor is not between A and B

I discussed this with the class today, and suggested that they change their mental language and think of connecting a resistor between two nodes, rather than to two other components. I also talked about switching their thinking from “components connected by wires” to the dual graph, nodes connected by components, and assigning a color to each node.

Color coding each node makes it much easier to notice incorrect connections (two different colors connected together), though it doesn’t help with noticing missing connections.  For that, I recommend that students check each component to make sure every node is there, and every node to make sure it has the right number of components.

CCPhysicist commented

Perhaps I will work on introducing the concept that labs like most of our circuit labs are about discovering the function of everything we use (meters as well, because they are part of the circuit, and even the wires themselves), and discourage the use of words like “to” instead of “through”. After all, the two wires in your example actually do carry current “to” and “from” the resistor!

I insist in the weekly design reports that students not use “voltage through” or “current across”, but always “voltage across” and “current through” to talk about V and I for a component.  I don’t think that this help much with their understanding, though, as the misunderstandings about voltage always being a difference are still common, and students still routinely apply Ohm’s Law to voltages and currents measured in different places.

Any problem that involves a voltage, a current, and a resistance causes many of them to invoke V=IR, even when the voltage and current are unrelated or related in something other than a simple resistance.  (For example, when chosing a DC bias resistor for an electret microphone, we have a non-linear I-vs-V relationship for the mic, and generally have a voltage drop across the resistor that needs to be added to the voltage drop across the microphone to get the power-supply voltage, but students will take any of the voltages (the mic voltage, the voltage across the resistor, or the power-supply voltage) to get the resistance of the bias resistor, when only one of the voltages is appropriate.

My labs are not about “discovering the function of everything we use”, but about learning how to design circuits with imperfect parts. (That’s one difference between a physics lab and an engineering lab.) I’m trying to give the students tool skills: both mental tools and physical tools.  The notion of having multiple models for something and using the simplest one you can get away with is one of the skills I’m trying to get them to develop.  The extremely simple models used in intro physics courses are often not good enough for practical use and developing better models from first principles is too hard, so we do a lot of measuring and empirical fitting.  (The loudspeaker modeling lab is a good example, where we go through 4 different models of the loudspeaker: R, L+R, L+R+RLC, semi-inductor+R+RLC.  Sometimes the simple model of the loudspeaker as being 4Ω is adequate, and sometimes we’ll use the full complexity of the non-linear model.)

There are a lot of learning experiences that are generally unavailable with simulations (like the problem of measuring voltages in voltage dividers made of 4.7MΩ resistors when your meter has a 10MΩ input impedance, or the problem of clipping when using high gain in an op amp, because of input voltage offset errors).  Students are much more likely to remember to design around input offset voltages if they have observed an unexpected output voltage offset and tried to figure out what caused it, than if they are simply guided to do designs that have low gain without knowing why (or allowed to do large-gain designs without realizing that they wouldn’t work reliably, as I have often done myself, even though I theoretically know better).

 

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