Fundamentals of Circuits
Recommended class days:
3 minimum, 4 preferred
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:
. In these instances, students see a potential difference as an
than a cause. Students do not use
0 to reason about circuits, and many have trouble
following an argument that does so. Their understanding of potential difference is not so much
• 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
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
prising is that students show essentially no improvement in their conceptual understanding of
circuits following conventional instruction.
Many of these students
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
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.
In addition to the findings described in the “Physics Education Research” chapter, other