It has recently been suggested that pedagogical content knowledge (PCK) has a concept-specific nature, beyond the topic-specific level of PCK. This paper reports a case study of three pre-service teachers' reported PCK about three fundamental concepts within the topic of electrostatics, namely electrostatic force, electric field and electric field strength. The aim of the study was to compare the quality of the PCK of each of the individual participants across these fundamental concepts. Data was collected using a content representation tool and a lesson planning form prescribed by the participants' teacher training institution. A topic-specific PCK model was adopted as the framework for this study. The model asserts that the content of a particular topic is transformed for instruction through five components, namely learners' prior knowledge, curricular saliency, what is difficult to teach, representations including analogies and conceptual teaching strategies. Guided by the model, we designed a rubric to assess the concept-specific PCK of the participants on a four-point scale. The results of the study indicated that the PCK of the participants varied across the fundamental concepts of electrostatics, with each participant reporting better PCK for a different concept. The results of the study imply that describing PCK at concept level is appropriate and recommend that PCK should be strengthened at concept level during teacher training to ensure that it develops across all of the concepts within a topic.