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What a difference a line makes

May 2, 2013

A student was reviewing how to connect a multimeter to measure current in parallel circuits today. In lab, the student was given some question with pictures of a battery, resistors, and a multimeter and asked to draw in wires to show the multimeter connected to measure current at various points in the circuit. The student was struggling to understand which resistor the multimeter was measuring the current through, particularly when there are multiple wires connected to the same port on the multimeter. Below is an example of one of the sketches we made.

Screen shot 2013-05-01 at 8.51.18 PM

The student wasn’t sure how to understand that the setup above will measure the current through R2 and not through R1. We talked for a bit using phrases like “R2 is connected in series with the meter”, “current travels through the meter to R2” and “the battery and R2 are connected through the meter”. The student continued to struggle and without thinking much of it I drew in a small line connecting A and COM saying that these jacks are connected together inside the multimeter.

Screen shot 2013-05-01 at 8.51.28 PM

The student looked at the sketch for a bit* and it suddenly clicked. She explained that she had been focusing on where the wires plugged into the meter and hadn’t thought about how current gets from the A to the COM jacks. Adding the line between A and COM allowed the student to “see” the entire path and to think in terms of current loops. Adding one tiny line completely shifted the way the student was thinking about the question and the tools she was drawing on to construct her understanding.

Part of the student’s difficulty in this case was her inability to operationalize or to cue the idea of a complete circuit to answer questions. The Washington PER group has a recent paper on this topic.

*By coincidence, right after I added this line another student asked a question causing me to shift my attention, giving the original student a minute to reflect on the diagram without me saying anything.

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