In this paper, we examined teachers’ understanding of electrical concepts such as power, current and
potential difference based on how these concepts were applied to understand the relative brightness
seen in bulbs of different wattage under different connections—series or parallel. From the
responses of teachers to a concept question, we identified common lines of reasoning and the
associated conceptual difficulties. To support the explanation of the concept question, we set up
relevant circuits and made measurements of the circuits. We discuss the temperature dependence of
the resistance of the light bulb which although critical for in depth understanding of the relative
brightness, was often omitted in the teacher responses. Lastly, we share insights and strategies to
elicit and confront students’ thinking and to help them resolve, extend and apply their thinking
with regard to the related electrical concepts using various light bulb activities.