Today, we did some peer instruction of conceptual circuits questions as a review before the multiple choice portion of the exam. Due to short periods tomorrow, I moved the multiple choice portion of the circuits exam to today. The question that generated the best discussion was this one:
Listening to the justifications, I realized that the values specified in the question have the potential to leave students with a misunderstanding. Some groups discussed how after the switch is flipped the two capacitors have to end up with the same charge. They didn’t appreciate that, while their answer was correct, the two capacitors only have the same charge because they have the same capacitance. The correct justification is that the two capacitors must have the same potential difference after the switch is flipped and current has stopped flowing. This realization is a result of the peer instruction discussions that I listen to. If I simply looked at answers, I wouldn’t have realized that students didn’t correctly understand the answer to this question. I’ll change this question for next time!