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Instructors_Guide_Ch31

# Instructors_Guide_Ch31 - 31 Fundamentals of Circuits...

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31 Fundamentals of Circuits Recommended class days: 3 minimum, 4 preferred Background Information An extensive summary of research findings about students’ understanding of circuits was given in the “Physics Education Research” chapter of this guidebook. The primary references are Cohen et al. (1983), McDermott and Shaffer (1992), and Shaffer and McDermott (1992). From the research, we learn that: Students do not differentiate between the concepts of current, voltage, energy, and power. To them, it’s all just “electricity.” The situation is analogous to students’ use of the term “motion” to describe either velocity or acceleration. Students think almost exclusively about current, rarely or never about potential difference. Thus the majority of students, even after seeing experimental evidence that suggests otherwise, continue to believe that batteries are constant-current sources. Students cannot reason with the concept of potential difference, and they rarely invoke potential difference spontaneously. When potential differences arise, it is almost invariably in the context of Ohm’s law: V = IR . In these instances, students see a potential difference as an effect rather than a cause. Students do not use V loop = 0 to reason about circuits, and many have trouble following an argument that does so. Their understanding of potential difference is not so much a misconception as no conception. Students reason locally, not globally. In most circumstances, they do not see that changing one circuit component will affect the voltage and current at other points in the circuit. Students have no micro/macro understanding of circuits. They do not see any connection between macroscopic quantities, such as current or resistance, and their previous study of charges, forces, and fields. To students, circuits are a subject entirely independent of electrostatics. These conclusions are perhaps not surprising. Surveys at the University of Washington have found that less than 20% of students report even rudimentary experience with battery-and-bulb types of circuits. I’ve confirmed this in my own surveys of students, and I’ve also found that the percentage of women students with any previous experience is very close to zero. What is sur- prising is that students show essentially no improvement in their conceptual understanding of circuits following conventional instruction. Many of these students can successfully apply Kirchhoff’s laws to the analysis of circuits. This is an algorithmic procedure, and the ability to follow this procedure apparently does not imply that students understand the physical concepts or that they can reason about circuits. Indeed, several studies have found that students, when asked a question such as “What happens if I remove this light bulb,” immediately begin trying to apply formulas rather than reasoning.

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